INTRODUCTION. When on board H.M.S. 'Beagle,' as naturalist, I was much struck
with certain facts in the distribution of the inhabitants of South America, and
in the geological relations of the present to the past inhabitants of that
continent. These facts seemed to me to throw some light on the origin of
species--that mystery of mysteries, as it has been called by one of our greatest
philosophers. On my return home, it occurred to me, in 1837, that something
might perhaps be made out on this question by patiently accumulating and
reflecting on all sorts of facts which could possibly have any bearing on it.
After five years' work I allowed myself to speculate on the subject, and drew up
some short notes; these I enlarged in 1844 into a sketch of the conclusions,
which then seemed to me probable: from that period to the present day I have
steadily pursued the same object. I hope that I may be excused for entering on
these personal details, as I give them to show that I have not been hasty in
coming to a decision. My work is now nearly finished; but as it will take me two
or three more years to complete it, and as my health is far from strong, I have
been urged to publish this Abstract. I have more especially been induced to do
this, as Mr. Wallace, who is now studying the natural history of the Malay
archipelago, has arrived at almost exactly the same general conclusions that I
have on the origin of species. Last year he sent to me a memoir on this subject,
with a request that I would forward it to Sir Charles Lyell, who sent it to the
Linnean Society, and it is published in the third volume of the Journal of that
Society. Sir C. Lyell and Dr. Hooker, who both knew of my work--the latter
having read my sketch of 1844--honoured me by thinking it advisable to publish,
with Mr. Wallace's excellent memoir, some brief extracts from my manuscripts.
This Abstract, which I now publish, must necessarily be imperfect. I cannot here
give references and authorities for my several statements; and I must trust to
the reader reposing some confidence in my accuracy. No doubt errors will have
crept in, though I hope I have always been cautious in trusting to good
authorities alone. I can here give only the general conclusions at which I have
arrived, with a few facts in illustration, but which, I hope, in most cases will
suffice. No one can feel more sensible than I do of the necessity of hereafter
publishing in detail all the facts, with references, on which my conclusions
have been grounded; and I hope in a future work to do this. For I am well aware
that scarcely a single point is discussed in this volume on which facts cannot
be adduced, often apparently leading to conclusions directly opposite to those
at which I have arrived. A fair result can be obtained only by fully stating and
balancing the facts and arguments on both sides of each question; and this
cannot possibly be here done. I much regret that want of space prevents my
having the satisfaction of acknowledging the generous assistance which I have
received from very many naturalists, some of them personally unknown to me. I
cannot, however, let this opportunity pass without expressing my deep
obligations to Dr. Hooker, who for the last fifteen years has aided me in every
possible way by his large stores of knowledge and his excellent judgment. In
considering the Origin of Species, it is quite conceivable that a naturalist,
reflecting on the mutual affinities of organic beings, on their embryological
relations, their geographical distribution, geological succession, and other
such facts, might come to the conclusion that each species had not been
independently created, but had descended, like varieties, from other species.
Nevertheless, such a conclusion, even if well founded, would be unsatisfactory,
until it could be shown how the innumerable species inhabiting this world have
been modified, so as to acquire that perfection of structure and coadaptation
which most justly excites our admiration. Naturalists continually refer to
external conditions, such as climate, food, etc., as the only possible cause of
variation. In one very limited sense, as we shall hereafter see, this may be
true; but it is preposterous to attribute to mere external conditions, the
structure, for instance, of the woodpecker, with its feet, tail, beak, and
tongue, so admirably adapted to catch insects under the bark of trees. In the
case of the misseltoe, which draws its nourishment from certain trees, which has
seeds that must be transported by certain birds, and which has flowers with
separate sexes absolutely requiring the agency of certain insects to bring
pollen from one flower to the other, it is equally preposterous to account for
the structure of this parasite, with its relations to several distinct organic
beings, by the effects of external conditions, or of habit, or of the volition
of the plant itself. The author of the 'Vestiges of Creation' would, I presume,
say that, after a certain unknown number of generations, some bird had given
birth to a woodpecker, and some plant to the misseltoe, and that these had been
produced perfect as we now see them; but this assumption seems to me to be no
explanation, for it leaves the case of the coadaptations of organic beings to
each other and to their physical conditions of life, untouched and unexplained.
It is, therefore, of the highest importance to gain a clear insight into the
means of modification and coadaptation. At the commencement of my observations
it seemed to me probable that a careful study of domesticated animals and of
cultivated plants would offer the best chance of making out this obscure
problem. Nor have I been disappointed; in this and in all other perplexing cases
I have invariably found that our knowledge, imperfect though it be, of variation
under domestication, afforded the best and safest clue. I may venture to express
my conviction of the high value of such studies, although they have been very
commonly neglected by naturalists. From these considerations, I shall devote the
first chapter of this Abstract to Variation under Domestication. We shall thus
see that a large amount of hereditary modification is at least possible, and,
what is equally or more important, we shall see how great is the power of man in
accumulating by his Selection successive slight variations. I will then pass on
to the variability of species in a state of nature; but I shall, unfortunately,
be compelled to treat this subject far too briefly, as it can be treated
properly only by giving long catalogues of facts. We shall, however, be enabled
to discuss what circumstances are most favourable to variation. In the next
chapter the Struggle for Existence amongst all organic beings throughout the
world, which inevitably follows from their high geometrical powers of increase,
will be treated of. This is the doctrine of Malthus, applied to the whole animal
and vegetable kingdoms. As many more individuals of each species are born than
can possibly survive; and as, consequently, there is a frequently recurring
struggle for existence, it follows that any being, if it vary however slightly
in any manner profitable to itself, under the complex and sometimes varying
conditions of life, will have a better chance of surviving, and thus be
NATURALLY SELECTED. From the strong principle of inheritance, any selected
variety will tend to propagate its new and modified form. This fundamental
subject of Natural Selection will be treated at some length in the fourth
chapter; and we shall then see how Natural Selection almost inevitably causes
much Extinction of the less improved forms of life and induces what I have
called Divergence of Character. In the next chapter I shall discuss the complex
and little known laws of variation and of correlation of growth. In the four
succeeding chapters, the most apparent and gravest difficulties on the theory
will be given: namely, first, the difficulties of transitions, or in
understanding how a simple being or a simple organ can be changed and perfected
into a highly developed being or elaborately constructed organ; secondly the
subject of Instinct, or the mental powers of animals, thirdly, Hybridism, or the
infertility of species and the fertility of varieties when intercrossed; and
fourthly, the imperfection of the Geological Record. In the next chapter I shall
consider the geological succession of organic beings throughout time; in the
eleventh and twelfth, their geographical distribution throughout space; in the
thirteenth, their classification or mutual affinities, both when mature and in
an embryonic condition. In the last chapter I shall give a brief recapitulation
of the whole work, and a few concluding remarks. No one ought to feel surprise
at much remaining as yet unexplained in regard to the origin of species and
varieties, if he makes due allowance for our profound ignorance in regard to the
mutual relations of all the beings which live around us. Who can explain why one
species ranges widely and is very numerous, and why another allied species has a
narrow range and is rare? Yet these relations are of the highest importance, for
they determine the present welfare, and, as I believe, the future success and
modification of every inhabitant of this world. Still less do we know of the
mutual relations of the innumerable inhabitants of the world during the many
past geological epochs in its history. Although much remains obscure, and will
long remain obscure, I can entertain no doubt, after the most deliberate study
and dispassionate judgment of which I am capable, that the view which most
naturalists entertain, and which I formerly entertained--namely, that each
species has been independently created--is erroneous. I am fully convinced that
species are not immutable; but that those belonging to what are called the same
genera are lineal descendants of some other and generally extinct species, in
the same manner as the acknowledged varieties of any one species are the
descendants of that species. Furthermore, I am convinced that Natural Selection
has been the main but not exclusive means of modification. CHAPTER 1. VARIATION
UNDER DOMESTICATION. Causes of Variability. Effects of Habit. Correlation of
Growth. Inheritance. Character of Domestic Varieties. Difficulty of
distinguishing between Varieties and Species. Origin of Domestic Varieties from
one or more Species. Domestic Pigeons, their Differences and Origin. Principle
of Selection anciently followed, its Effects. Methodical and Unconscious
Selection. Unknown Origin of our Domestic Productions. Circumstances favourable
to Man's power of Selection. When we look to the individuals of the same variety
or sub-variety of our older cultivated plants and animals, one of the first
points which strikes us, is, that they generally differ much more from each
other, than do the individuals of any one species or variety in a state of
nature. When we reflect on the vast diversity of the plants and animals which
have been cultivated, and which have varied during all ages under the most
different climates and treatment, I think we are driven to conclude that this
greater variability is simply due to our domestic productions having been raised
under conditions of life not so uniform as, and somewhat different from, those
to which the parent-species have been exposed under nature. There is, also, I
think, some probability in the view propounded by Andrew Knight, that this
variability may be partly connected with excess of food. It seems pretty clear
that organic beings must be exposed during several generations to the new
conditions of life to cause any appreciable amount of variation; and that when
the organisation has once begun to vary, it generally continues to vary for many
generations. No case is on record of a variable being ceasing to be variable
under cultivation. Our oldest cultivated plants, such as wheat, still often
yield new varieties: our oldest domesticated animals are still capable of rapid
improvement or modification. It has been disputed at what period of life the
causes of variability, whatever they may be, generally act; whether during the
early or late period of development of the embryo, or at the instant of
conception. Geoffroy St. Hilaire's experiments show that unnatural treatment of
the embryo causes monstrosities; and monstrosities cannot be separated by any
clear line of distinction from mere variations. But I am strongly inclined to
suspect that the most frequent cause of variability may be attributed to the
male and female reproductive elements having been affected prior to the act of
conception. Several reasons make me believe in this; but the chief one is the
remarkable effect which confinement or cultivation has on the functions of the
reproductive system; this system appearing to be far more susceptible than any
other part of the organisation, to the action of any change in the conditions of
life. Nothing is more easy than to tame an animal, and few things more difficult
than to get it to breed freely under confinement, even in the many cases when
the male and female unite. How many animals there are which will not breed,
though living long under not very close confinement in their native country!
This is generally attributed to vitiated instincts; but how many cultivated
plants display the utmost vigour, and yet rarely or never seed! In some few such
cases it has been found out that very trifling changes, such as a little more or
less water at some particular period of growth, will determine whether or not
the plant sets a seed. I cannot here enter on the copious details which I have
collected on this curious subject; but to show how singular the laws are which
determine the reproduction of animals under confinement, I may just mention that
carnivorous animals, even from the tropics, breed in this country pretty freely
under confinement, with the exception of the plantigrades or bear family;
whereas, carnivorous birds, with the rarest exceptions, hardly ever lay fertile
eggs. Many exotic plants have pollen utterly worthless, in the same exact
condition as in the most sterile hybrids. When, on the one hand, we see
domesticated animals and plants, though often weak and sickly, yet breeding
quite freely under confinement; and when, on the other hand, we see individuals,
though taken young from a state of nature, perfectly tamed, long-lived, and
healthy (of which I could give numerous instances), yet having their
reproductive system so seriously affected by unperceived causes as to fail in
acting, we need not be surprised at this system, when it does act under
confinement, acting not quite regularly, and producing offspring not perfectly
like their parents or variable. Sterility has been said to be the bane of
horticulture; but on this view we owe variability to the same cause which
produces sterility; and variability is the source of all the choicest
productions of the garden. I may add, that as some organisms will breed most
freely under the most unnatural conditions (for instance, the rabbit and ferret
kept in hutches), showing that their reproductive system has not been thus
affected; so will some animals and plants withstand domestication or
cultivation, and vary very slightly--perhaps hardly more than in a state of
nature. A long list could easily be given of "sporting plants;" by this term
gardeners mean a single bud or offset, which suddenly assumes a new and
sometimes very different character from that of the rest of the plant. Such buds
can be propagated by grafting, etc., and sometimes by seed. These "sports" are
extremely rare under nature, but far from rare under cultivation; and in this
case we see that the treatment of the parent has affected a bud or offset, and
not the ovules or pollen. But it is the opinion of most physiologists that there
is no essential difference between a bud and an ovule in their earliest stages
of formation; so that, in fact, "sports" support my view, that variability may
be largely attributed to the ovules or pollen, or to both, having been affected
by the treatment of the parent prior to the act of conception. These cases
anyhow show that variation is not necessarily connected, as some authors have
supposed, with the act of generation. Seedlings from the same fruit, and the
young of the same litter, sometimes differ considerably from each other, though
both the young and the parents, as Muller has remarked, have apparently been
exposed to exactly the same conditions of life; and this shows how unimportant
the direct effects of the conditions of life are in comparison with the laws of
reproduction, and of growth, and of inheritance; for had the action of the
conditions been direct, if any of the young had varied, all would probably have
varied in the same manner. To judge how much, in the case of any variation, we
should attribute to the direct action of heat, moisture, light, food, etc., is
most difficult: my impression is, that with animals such agencies have produced
very little direct effect, though apparently more in the case of plants. Under
this point of view, Mr. Buckman's recent experiments on plants seem extremely
valuable. When all or nearly all the individuals exposed to certain conditions
are affected in the same way, the change at first appears to be directly due to
such conditions; but in some cases it can be shown that quite opposite
conditions produce similar changes of structure. Nevertheless some slight amount
of change may, I think, be attributed to the direct action of the conditions of
life--as, in some cases, increased size from amount of food, colour from
particular kinds of food and from light, and perhaps the thickness of fur from
climate. Habit also has a decided influence, as in the period of flowering with
plants when transported from one climate to another. In animals it has a more
marked effect; for instance, I find in the domestic duck that the bones of the
wing weigh less and the bones of the leg more, in proportion to the whole
skeleton, than do the same bones in the wild-duck; and I presume that this
change may be safely attributed to the domestic duck flying much less, and
walking more, than its wild parent. The great and inherited development of the
udders in cows and goats in countries where they are habitually milked, in
comparison with the state of these organs in other countries, is another
instance of the effect of use. Not a single domestic animal can be named which
has not in some country drooping ears; and the view suggested by some authors,
that the drooping is due to the disuse of the muscles of the ear, from the
animals not being much alarmed by danger, seems probable. There are many laws
regulating variation, some few of which can be dimly seen, and will be hereafter
briefly mentioned. I will here only allude to what may be called correlation of
growth. Any change in the embryo or larva will almost certainly entail changes
in the mature animal. In monstrosities, the correlations between quite distinct
parts are very curious; and many instances are given in Isidore Geoffroy St.
Hilaire's great work on this subject. Breeders believe that long limbs are
almost always accompanied by an elongated head. Some instances of correlation
are quite whimsical; thus cats with blue eyes are invariably deaf; colour and
constitutional peculiarities go together, of which many remarkable cases could
be given amongst animals and plants. From the facts collected by Heusinger, it
appears that white sheep and pigs are differently affected from coloured
individuals by certain vegetable poisons. Hairless dogs have imperfect teeth;
long-haired and coarse-haired animals are apt to have, as is asserted, long or
many horns; pigeons with feathered feet have skin between their outer toes;
pigeons with short beaks have small feet, and those with long beaks large feet.
Hence, if man goes on selecting, and thus augmenting, any peculiarity, he will
almost certainly unconsciously modify other parts of the structure, owing to the
mysterious laws of the correlation of growth. The result of the various, quite
unknown, or dimly seen laws of variation is infinitely complex and diversified.
It is well worth while carefully to study the several treatises published on
some of our old cultivated plants, as on the hyacinth, potato, even the dahlia,
etc.; and it is really surprising to note the endless points in structure and
constitution in which the varieties and sub-varieties differ slightly from each
other. The whole organisation seems to have become plastic, and tends to depart
in some small degree from that of the parental type. Any variation which is not
inherited is unimportant for us. But the number and diversity of inheritable
deviations of structure, both those of slight and those of considerable
physiological importance, is endless. Dr. Prosper Lucas's treatise, in two large
volumes, is the fullest and the best on this subject. No breeder doubts how
strong is the tendency to inheritance: like produces like is his fundamental
belief: doubts have been thrown on this principle by theoretical writers alone.
When a deviation appears not unfrequently, and we see it in the father and
child, we cannot tell whether it may not be due to the same original cause
acting on both; but when amongst individuals, apparently exposed to the same
conditions, any very rare deviation, due to some extraordinary combination of
circumstances, appears in the parent--say, once amongst several million
individuals--and it reappears in the child, the mere doctrine of chances almost
compels us to attribute its reappearance to inheritance. Every one must have
heard of cases of albinism, prickly skin, hairy bodies, etc., appearing in
several members of the same family. If strange and rare deviations of structure
are truly inherited, less strange and commoner deviations may be freely admitted
to be inheritable. Perhaps the correct way of viewing the whole subject, would
be, to look at the inheritance of every character whatever as the rule, and
non-inheritance as the anomaly. The laws governing inheritance are quite
unknown; no one can say why the same peculiarity in different individuals of the
same species, and in individuals of different species, is sometimes inherited
and sometimes not so; why the child often reverts in certain characters to its
grandfather or grandmother or other much more remote ancestor; why a peculiarity
is often transmitted from one sex to both sexes or to one sex alone, more
commonly but not exclusively to the like sex. It is a fact of some little
importance to us, that peculiarities appearing in the males of our domestic
breeds are often transmitted either exclusively, or in a much greater degree, to
males alone. A much more important rule, which I think may be trusted, is that,
at whatever period of life a peculiarity first appears, it tends to appear in
the offspring at a corresponding age, though sometimes earlier. In many cases
this could not be otherwise: thus the inherited peculiarities in the horns of
cattle could appear only in the offspring when nearly mature; peculiarities in
the silkworm are known to appear at the corresponding caterpillar or cocoon
stage. But hereditary diseases and some other facts make me believe that the
rule has a wider extension, and that when there is no apparent reason why a
peculiarity should appear at any particular age, yet that it does tend to appear
in the offspring at the same period at which it first appeared in the parent. I
believe this rule to be of the highest importance in explaining the laws of
embryology. These remarks are of course confined to the first APPEARANCE of the
peculiarity, and not to its primary cause, which may have acted on the ovules or
male element; in nearly the same manner as in the crossed offspring from a
short-horned cow by a long-horned bull, the greater length of horn, though
appearing late in life, is clearly due to the male element. Having alluded to
the subject of reversion, I may here refer to a statement often made by
naturalists--namely, that our domestic varieties, when run wild, gradually but
certainly revert in character to their aboriginal stocks. Hence it has been
argued that no deductions can be drawn from domestic races to species in a state
of nature. I have in vain endeavoured to discover on what decisive facts the
above statement has so often and so boldly been made. There would be great
difficulty in proving its truth: we may safely conclude that very many of the
most strongly-marked domestic varieties could not possibly live in a wild state.
In many cases we do not know what the aboriginal stock was, and so could not
tell whether or not nearly perfect reversion had ensued. It would be quite
necessary, in order to prevent the effects of intercrossing, that only a single
variety should be turned loose in its new home. Nevertheless, as our varieties
certainly do occasionally revert in some of their characters to ancestral forms,
it seems to me not improbable, that if we could succeed in naturalising, or were
to cultivate, during many generations, the several races, for instance, of the
cabbage, in very poor soil (in which case, however, some effect would have to be
attributed to the direct action of the poor soil), that they would to a large
extent, or even wholly, revert to the wild aboriginal stock. Whether or not the
experiment would succeed, is not of great importance for our line of argument;
for by the experiment itself the conditions of life are changed. If it could be
shown that our domestic varieties manifested a strong tendency to
reversion,--that is, to lose their acquired characters, whilst kept under
unchanged conditions, and whilst kept in a considerable body, so that free
intercrossing might check, by blending together, any slight deviations of
structure, in such case, I grant that we could deduce nothing from domestic
varieties in regard to species. But there is not a shadow of evidence in favour
of this view: to assert that we could not breed our cart and race-horses, long
and short-horned cattle, and poultry of various breeds, and esculent vegetables,
for an almost infinite number of generations, would be opposed to all
experience. I may add, that when under nature the conditions of life do change,
variations and reversions of character probably do occur; but natural selection,
as will hereafter be explained, will determine how far the new characters thus
arising shall be preserved. When we look to the hereditary varieties or races of
our domestic animals and plants, and compare them with species closely allied
together, we generally perceive in each domestic race, as already remarked, less
uniformity of character than in true species. Domestic races of the same
species, also, often have a somewhat monstrous character; by which I mean, that,
although differing from each other, and from the other species of the same
genus, in several trifling respects, they often differ in an extreme degree in
some one part, both when compared one with another, and more especially when
compared with all the species in nature to which they are nearest allied. With
these exceptions (and with that of the perfect fertility of varieties when
crossed,--a subject hereafter to be discussed), domestic races of the same
species differ from each other in the same manner as, only in most cases in a
lesser degree than, do closely-allied species of the same genus in a state of
nature. I think this must be admitted, when we find that there are hardly any
domestic races, either amongst animals or plants, which have not been ranked by
some competent judges as mere varieties, and by other competent judges as the
descendants of aboriginally distinct species. If any marked distinction existed
between domestic races and species, this source of doubt could not so
perpetually recur. It has often been stated that domestic races do not differ
from each other in characters of generic value. I think it could be shown that
this statement is hardly correct; but naturalists differ most widely in
determining what characters are of generic value; all such valuations being at
present empirical. Moreover, on the view of the origin of genera which I shall
presently give, we have no right to expect often to meet with generic
differences in our domesticated productions. When we attempt to estimate the
amount of structural difference between the domestic races of the same species,
we are soon involved in doubt, from not knowing whether they have descended from
one or several parent-species. This point, if it could be cleared up, would be
interesting; if, for instance, it could be shown that the greyhound, bloodhound,
terrier, spaniel, and bull-dog, which we all know propagate their kind so truly,
were the offspring of any single species, then such facts would have great
weight in making us doubt about the immutability of the many very closely allied
and natural species--for instance, of the many foxes--inhabiting different
quarters of the world. I do not believe, as we shall presently see, that all our
dogs have descended from any one wild species; but, in the case of some other
domestic races, there is presumptive, or even strong, evidence in favour of this
view. It has often been assumed that man has chosen for domestication animals
and plants having an extraordinary inherent tendency to vary, and likewise to
withstand diverse climates. I do not dispute that these capacities have added
largely to the value of most of our domesticated productions; but how could a
savage possibly know, when he first tamed an animal, whether it would vary in
succeeding generations, and whether it would endure other climates? Has the
little variability of the ass or guinea-fowl, or the small power of endurance of
warmth by the rein-deer, or of cold by the common camel, prevented their
domestication? I cannot doubt that if other animals and plants, equal in number
to our domesticated productions, and belonging to equally diverse classes and
countries, were taken from a state of nature, and could be made to breed for an
equal number of generations under domestication, they would vary on an average
as largely as the parent species of our existing domesticated productions have
varied. In the case of most of our anciently domesticated animals and plants, I
do not think it is possible to come to any definite conclusion, whether they
have descended from one or several species. The argument mainly relied on by
those who believe in the multiple origin of our domestic animals is, that we
find in the most ancient records, more especially on the monuments of Egypt,
much diversity in the breeds; and that some of the breeds closely resemble,
perhaps are identical with, those still existing. Even if this latter fact were
found more strictly and generally true than seems to me to be the case, what
does it show, but that some of our breeds originated there, four or five
thousand years ago? But Mr. Horner's researches have rendered it in some degree
probable that man sufficiently civilized to have manufactured pottery existed in
the valley of the Nile thirteen or fourteen thousand years ago; and who will
pretend to say how long before these ancient periods, savages, like those of
Tierra del Fuego or Australia, who possess a semi-domestic dog, may not have
existed in Egypt? The whole subject must, I think, remain vague; nevertheless, I
may, without here entering on any details, state that, from geographical and
other considerations, I think it highly probable that our domestic dogs have
descended from several wild species. In regard to sheep and goats I can form no
opinion. I should think, from facts communicated to me by Mr. Blyth, on the
habits, voice, and constitution, etc., of the humped Indian cattle, that these
had descended from a different aboriginal stock from our European cattle; and
several competent judges believe that these latter have had more than one wild
parent. With respect to horses, from reasons which I cannot give here, I am
doubtfully inclined to believe, in opposition to several authors, that all the
races have descended from one wild stock. Mr. Blyth, whose opinion, from his
large and varied stores of knowledge, I should value more than that of almost
any one, thinks that all the breeds of poultry have proceeded from the common
wild Indian fowl (Gallus bankiva). In regard to ducks and rabbits, the breeds of
which differ considerably from each other in structure, I do not doubt that they
all have descended from the common wild duck and rabbit. The doctrine of the
origin of our several domestic races from several aboriginal stocks, has been
carried to an absurd extreme by some authors. They believe that every race which
breeds true, let the distinctive characters be ever so slight, has had its wild
prototype. At this rate there must have existed at least a score of species of
wild cattle, as many sheep, and several goats in Europe alone, and several even
within Great Britain. One author believes that there formerly existed in Great
Britain eleven wild species of sheep peculiar to it! When we bear in mind that
Britain has now hardly one peculiar mammal, and France but few distinct from
those of Germany and conversely, and so with Hungary, Spain, etc., but that each
of these kingdoms possesses several peculiar breeds of cattle, sheep, etc., we
must admit that many domestic breeds have originated in Europe; for whence could
they have been derived, as these several countries do not possess a number of
peculiar species as distinct parent-stocks? So it is in India. Even in the case
of the domestic dogs of the whole world, which I fully admit have probably
descended from several wild species, I cannot doubt that there has been an
immense amount of inherited variation. Who can believe that animals closely
resembling the Italian greyhound, the bloodhound, the bull-dog, or Blenheim
spaniel, etc.--so unlike all wild Canidae--ever existed freely in a state of
nature? It has often been loosely said that all our races of dogs have been
produced by the crossing of a few aboriginal species; but by crossing we can get
only forms in some degree intermediate between their parents; and if we account
for our several domestic races by this process, we must admit the former
existence of the most extreme forms, as the Italian greyhound, bloodhound,
bull-dog, etc., in the wild state. Moreover, the possibility of making distinct
races by crossing has been greatly exaggerated. There can be no doubt that a
race may be modified by occasional crosses, if aided by the careful selection of
those individual mongrels, which present any desired character; but that a race
could be obtained nearly intermediate between two extremely different races or
species, I can hardly believe. Sir J. Sebright expressly experimentised for this
object, and failed. The offspring from the first cross between two pure breeds
is tolerably and sometimes (as I have found with pigeons) extremely uniform, and
everything seems simple enough; but when these mongrels are crossed one with
another for several generations, hardly two of them will be alike, and then the
extreme difficulty, or rather utter hopelessness, of the task becomes apparent.
Certainly, a breed intermediate between TWO VERY DISTINCT breeds could not be
got without extreme care and long-continued selection; nor can I find a single
case on record of a permanent race having been thus formed. ON THE BREEDS OF THE
DOMESTIC PIGEON. Believing that it is always best to study some special group, I
have, after deliberation, taken up domestic pigeons. I have kept every breed
which I could purchase or obtain, and have been most kindly favoured with skins
from several quarters of the world, more especially by the Honourable W. Elliot
from India, and by the Honourable C. Murray from Persia. Many treatises in
different languages have been published on pigeons, and some of them are very
important, as being of considerable antiquity. I have associated with several
eminent fanciers, and have been permitted to join two of the London Pigeon
Clubs. The diversity of the breeds is something astonishing. Compare the English
carrier and the short-faced tumbler, and see the wonderful difference in their
beaks, entailing corresponding differences in their skulls. The carrier, more
especially the male bird, is also remarkable from the wonderful development of
the carunculated skin about the head, and this is accompanied by greatly
elongated eyelids, very large external orifices to the nostrils, and a wide gape
of mouth. The short-faced tumbler has a beak in outline almost like that of a
finch; and the common tumbler has the singular and strictly inherited habit of
flying at a great height in a compact flock, and tumbling in the air head over
heels. The runt is a bird of great size, with long, massive beak and large feet;
some of the sub-breeds of runts have very long necks, others very long wings and
tails, others singularly short tails. The barb is allied to the carrier, but,
instead of a very long beak, has a very short and very broad one. The pouter has
a much elongated body, wings, and legs; and its enormously developed crop, which
it glories in inflating, may well excite astonishment and even laughter. The
turbit has a very short and conical beak, with a line of reversed feathers down
the breast; and it has the habit of continually expanding slightly the upper
part of the oesophagus. The Jacobin has the feathers so much reversed along the
back of the neck that they form a hood, and it has, proportionally to its size,
much elongated wing and tail feathers. The trumpeter and laugher, as their names
express, utter a very different coo from the other breeds. The fantail has
thirty or even forty tail-feathers, instead of twelve or fourteen, the normal
number in all members of the great pigeon family; and these feathers are kept
expanded, and are carried so erect that in good birds the head and tail touch;
the oil-gland is quite aborted. Several other less distinct breeds might have
been specified. In the skeletons of the several breeds, the development of the
bones of the face in length and breadth and curvature differs enormously. The
shape, as well as the breadth and length of the ramus of the lower jaw, varies
in a highly remarkable manner. The number of the caudal and sacral vertebrae
vary; as does the number of the ribs, together with their relative breadth and
the presence of processes. The size and shape of the apertures in the sternum
are highly variable; so is the degree of divergence and relative size of the two
arms of the furcula. The proportional width of the gape of mouth, the
proportional length of the eyelids, of the orifice of the nostrils, of the
tongue (not always in strict correlation with the length of beak), the size of
the crop and of the upper part of the oesophagus; the development and abortion
of the oil-gland; the number of the primary wing and caudal feathers; the
relative length of wing and tail to each other and to the body; the relative
length of leg and of the feet; the number of scutellae on the toes, the
development of skin between the toes, are all points of structure which are
variable. The period at which the perfect plumage is acquired varies, as does
the state of the down with which the nestling birds are clothed when hatched.
The shape and size of the eggs vary. The manner of flight differs remarkably; as
does in some breeds the voice and disposition. Lastly, in certain breeds, the
males and females have come to differ to a slight degree from each other.
Altogether at least a score of pigeons might be chosen, which if shown to an
ornithologist, and he were told that they were wild birds, would certainly, I
think, be ranked by him as well-defined species. Moreover, I do not believe that
any ornithologist would place the English carrier, the short-faced tumbler, the
runt, the barb, pouter, and fantail in the same genus; more especially as in
each of these breeds several truly-inherited sub-breeds, or species as he might
have called them, could be shown him. Great as the differences are between the
breeds of pigeons, I am fully convinced that the common opinion of naturalists
is correct, namely, that all have descended from the rock-pigeon (Columba
livia), including under this term several geographical races or sub-species,
which differ from each other in the most trifling respects. As several of the
reasons which have led me to this belief are in some degree applicable in other
cases, I will here briefly give them. If the several breeds are not varieties,
and have not proceeded from the rock-pigeon, they must have descended from at
least seven or eight aboriginal stocks; for it is impossible to make the present
domestic breeds by the crossing of any lesser number: how, for instance, could a
pouter be produced by crossing two breeds unless one of the parent-stocks
possessed the characteristic enormous crop? The supposed aboriginal stocks must
all have been rock-pigeons, that is, not breeding or willingly perching on
trees. But besides C. livia, with its geographical sub-species, only two or
three other species of rock-pigeons are known; and these have not any of the
characters of the domestic breeds. Hence the supposed aboriginal stocks must
either still exist in the countries where they were originally domesticated, and
yet be unknown to ornithologists; and this, considering their size, habits, and
remarkable characters, seems very improbable; or they must have become extinct
in the wild state. But birds breeding on precipices, and good fliers, are
unlikely to be exterminated; and the common rock-pigeon, which has the same
habits with the domestic breeds, has not been exterminated even on several of
the smaller British islets, or on the shores of the Mediterranean. Hence the
supposed extermination of so many species having similar habits with the
rock-pigeon seems to me a very rash assumption. Moreover, the several
above-named domesticated breeds have been transported to all parts of the world,
and, therefore, some of them must have been carried back again into their native
country; but not one has ever become wild or feral, though the dovecot-pigeon,
which is the rock-pigeon in a very slightly altered state, has become feral in
several places. Again, all recent experience shows that it is most difficult to
get any wild animal to breed freely under domestication; yet on the hypothesis
of the multiple origin of our pigeons, it must be assumed that at least seven or
eight species were so thoroughly domesticated in ancient times by half-civilized
man, as to be quite prolific under confinement. An argument, as it seems to me,
of great weight, and applicable in several other cases, is, that the
above-specified breeds, though agreeing generally in constitution, habits,
voice, colouring, and in most parts of their structure, with the wild
rock-pigeon, yet are certainly highly abnormal in other parts of their
structure: we may look in vain throughout the whole great family of Columbidae
for a beak like that of the English carrier, or that of the short-faced tumbler,
or barb; for reversed feathers like those of the jacobin; for a crop like that
of the pouter; for tail-feathers like those of the fantail. Hence it must be
assumed not only that half-civilized man succeeded in thoroughly domesticating
several species, but that he intentionally or by chance picked out
extraordinarily abnormal species; and further, that these very species have
since all become extinct or unknown. So many strange contingencies seem to me
improbable in the highest degree. Some facts in regard to the colouring of
pigeons well deserve consideration. The rock-pigeon is of a slaty-blue, and has
a white rump (the Indian sub-species, C. intermedia of Strickland, having it
bluish); the tail has a terminal dark bar, with the bases of the outer feathers
externally edged with white; the wings have two black bars; some semi-domestic
breeds and some apparently truly wild breeds have, besides the two black bars,
the wings chequered with black. These several marks do not occur together in any
other species of the whole family. Now, in every one of the domestic breeds,
taking thoroughly well-bred birds, all the above marks, even to the white edging
of the outer tail-feathers, sometimes concur perfectly developed. Moreover, when
two birds belonging to two distinct breeds are crossed, neither of which is blue
or has any of the above-specified marks, the mongrel offspring are very apt
suddenly to acquire these characters; for instance, I crossed some uniformly
white fantails with some uniformly black barbs, and they produced mottled brown
and black birds; these I again crossed together, and one grandchild of the pure
white fantail and pure black barb was of as beautiful a blue colour, with the
white rump, double black wing-bar, and barred and white-edged tail-feathers, as
any wild rock-pigeon! We can understand these facts, on the well-known principle
of reversion to ancestral characters, if all the domestic breeds have descended
from the rock-pigeon. But if we deny this, we must make one of the two following
highly improbable suppositions. Either, firstly, that all the several imagined
aboriginal stocks were coloured and marked like the rock-pigeon, although no
other existing species is thus coloured and marked, so that in each separate
breed there might be a tendency to revert to the very same colours and markings.
Or, secondly, that each breed, even the purest, has within a dozen or, at most,
within a score of generations, been crossed by the rock-pigeon: I say within a
dozen or twenty generations, for we know of no fact countenancing the belief
that the child ever reverts to some one ancestor, removed by a greater number of
generations. In a breed which has been crossed only once with some distinct
breed, the tendency to reversion to any character derived from such cross will
naturally become less and less, as in each succeeding generation there will be
less of the foreign blood; but when there has been no cross with a distinct
breed, and there is a tendency in both parents to revert to a character, which
has been lost during some former generation, this tendency, for all that we can
see to the contrary, may be transmitted undiminished for an indefinite number of
generations. These two distinct cases are often confounded in treatises on
inheritance. Lastly, the hybrids or mongrels from between all the domestic
breeds of pigeons are perfectly fertile. I can state this from my own
observations, purposely made on the most distinct breeds. Now, it is difficult,
perhaps impossible, to bring forward one case of the hybrid offspring of two
animals CLEARLY DISTINCT being themselves perfectly fertile. Some authors
believe that long-continued domestication eliminates this strong tendency to
sterility: from the history of the dog I think there is some probability in this
hypothesis, if applied to species closely related together, though it is
unsupported by a single experiment. But to extend the hypothesis so far as to
suppose that species, aboriginally as distinct as carriers, tumblers, pouters,
and fantails now are, should yield offspring perfectly fertile, inter se, seems
to me rash in the extreme. From these several reasons, namely, the improbability
of man having formerly got seven or eight supposed species of pigeons to breed
freely under domestication; these supposed species being quite unknown in a wild
state, and their becoming nowhere feral; these species having very abnormal
characters in certain respects, as compared with all other Columbidae, though so
like in most other respects to the rock-pigeon; the blue colour and various
marks occasionally appearing in all the breeds, both when kept pure and when
crossed; the mongrel offspring being perfectly fertile;--from these several
reasons, taken together, I can feel no doubt that all our domestic breeds have
descended from the Columba livia with its geographical sub-species. In favour of
this view, I may add, firstly, that C. livia, or the rock-pigeon, has been found
capable of domestication in Europe and in India; and that it agrees in habits
and in a great number of points of structure with all the domestic breeds.
Secondly, although an English carrier or short-faced tumbler differs immensely
in certain characters from the rock-pigeon, yet by comparing the several
sub-breeds of these breeds, more especially those brought from distant
countries, we can make an almost perfect series between the extremes of
structure. Thirdly, those characters which are mainly distinctive of each breed,
for instance the wattle and length of beak of the carrier, the shortness of that
of the tumbler, and the number of tail-feathers in the fantail, are in each
breed eminently variable; and the explanation of this fact will be obvious when
we come to treat of selection. Fourthly, pigeons have been watched, and tended
with the utmost care, and loved by many people. They have been domesticated for
thousands of years in several quarters of the world; the earliest known record
of pigeons is in the fifth Aegyptian dynasty, about 3000 B.C., as was pointed
out to me by Professor Lepsius; but Mr. Birch informs me that pigeons are given
in a bill of fare in the previous dynasty. In the time of the Romans, as we hear
from Pliny, immense prices were given for pigeons; "nay, they are come to this
pass, that they can reckon up their pedigree and race." Pigeons were much valued
by Akber Khan in India, about the year 1600; never less than 20,000 pigeons were
taken with the court. "The monarchs of Iran and Turan sent him some very rare
birds;" and, continues the courtly historian, "His Majesty by crossing the
breeds, which method was never practised before, has improved them
astonishingly." About this same period the Dutch were as eager about pigeons as
were the old Romans. The paramount importance of these considerations in
explaining the immense amount of variation which pigeons have undergone, will be
obvious when we treat of Selection. We shall then, also, see how it is that the
breeds so often have a somewhat monstrous character. It is also a most
favourable circumstance for the production of distinct breeds, that male and
female pigeons can be easily mated for life; and thus different breeds can be
kept together in the same aviary. I have discussed the probable origin of
domestic pigeons at some, yet quite insufficient, length; because when I first
kept pigeons and watched the several kinds, knowing well how true they bred, I
felt fully as much difficulty in believing that they could ever have descended
from a common parent, as any naturalist could in coming to a similar conclusion
in regard to the many species of finches, or other large groups of birds, in
nature. One circumstance has struck me much; namely, that all the breeders of
the various domestic animals and the cultivators of plants, with whom I have
ever conversed, or whose treatises I have read, are firmly convinced that the
several breeds to which each has attended, are descended from so many
aboriginally distinct species. Ask, as I have asked, a celebrated raiser of
Hereford cattle, whether his cattle might not have descended from long horns,
and he will laugh you to scorn. I have never met a pigeon, or poultry, or duck,
or rabbit fancier, who was not fully convinced that each main breed was
descended from a distinct species. Van Mons, in his treatise on pears and
apples, shows how utterly he disbelieves that the several sorts, for instance a
Ribston-pippin or Codlin-apple, could ever have proceeded from the seeds of the
same tree. Innumerable other examples could be given. The explanation, I think,
is simple: from long-continued study they are strongly impressed with the
differences between the several races; and though they well know that each race
varies slightly, for they win their prizes by selecting such slight differences,
yet they ignore all general arguments, and refuse to sum up in their minds
slight differences accumulated during many successive generations. May not those
naturalists who, knowing far less of the laws of inheritance than does the
breeder, and knowing no more than he does of the intermediate links in the long
lines of descent, yet admit that many of our domestic races have descended from
the same parents--may they not learn a lesson of caution, when they deride the
idea of species in a state of nature being lineal descendants of other species?
SELECTION. Let us now briefly consider the steps by which domestic races have
been produced, either from one or from several allied species. Some little
effect may, perhaps, be attributed to the direct action of the external
conditions of life, and some little to habit; but he would be a bold man who
would account by such agencies for the differences of a dray and race horse, a
greyhound and bloodhound, a carrier and tumbler pigeon. One of the most
remarkable features in our domesticated races is that we see in them adaptation,
not indeed to the animal's or plant's own good, but to man's use or fancy. Some
variations useful to him have probably arisen suddenly, or by one step; many
botanists, for instance, believe that the fuller's teazle, with its hooks, which
cannot be rivalled by any mechanical contrivance, is only a variety of the wild
Dipsacus; and this amount of change may have suddenly arisen in a seedling. So
it has probably been with the turnspit dog; and this is known to have been the
case with the ancon sheep. But when we compare the dray-horse and race-horse,
the dromedary and camel, the various breeds of sheep fitted either for
cultivated land or mountain pasture, with the wool of one breed good for one
purpose, and that of another breed for another purpose; when we compare the many
breeds of dogs, each good for man in very different ways; when we compare the
game-cock, so pertinacious in battle, with other breeds so little quarrelsome,
with "everlasting layers" which never desire to sit, and with the bantam so
small and elegant; when we compare the host of agricultural, culinary, orchard,
and flower-garden races of plants, most useful to man at different seasons and
for different purposes, or so beautiful in his eyes, we must, I think, look
further than to mere variability. We cannot suppose that all the breeds were
suddenly produced as perfect and as useful as we now see them; indeed, in
several cases, we know that this has not been their history. The key is man's
power of accumulative selection: nature gives successive variations; man adds
them up in certain directions useful to him. In this sense he may be said to
make for himself useful breeds. The great power of this principle of selection
is not hypothetical. It is certain that several of our eminent breeders have,
even within a single lifetime, modified to a large extent some breeds of cattle
and sheep. In order fully to realise what they have done, it is almost necessary
to read several of the many treatises devoted to this subject, and to inspect
the animals. Breeders habitually speak of an animal's organisation as something
quite plastic, which they can model almost as they please. If I had space I
could quote numerous passages to this effect from highly competent authorities.
Youatt, who was probably better acquainted with the works of agriculturalists
than almost any other individual, and who was himself a very good judge of an
animal, speaks of the principle of selection as "that which enables the
agriculturist, not only to modify the character of his flock, but to change it
altogether. It is the magician's wand, by means of which he may summon into life
whatever form and mould he pleases." Lord Somerville, speaking of what breeders
have done for sheep, says:--"It would seem as if they had chalked out upon a
wall a form perfect in itself, and then had given it existence." That most
skilful breeder, Sir John Sebright, used to say, with respect to pigeons, that
"he would produce any given feather in three years, but it would take him six
years to obtain head and beak." In Saxony the importance of the principle of
selection in regard to merino sheep is so fully recognised, that men follow it
as a trade: the sheep are placed on a table and are studied, like a picture by a
connoisseur; this is done three times at intervals of months, and the sheep are
each time marked and classed, so that the very best may ultimately be selected
for breeding. What English breeders have actually effected is proved by the
enormous prices given for animals with a good pedigree; and these have now been
exported to almost every quarter of the world. The improvement is by no means
generally due to crossing different breeds; all the best breeders are strongly
opposed to this practice, except sometimes amongst closely allied sub-breeds.
And when a cross has been made, the closest selection is far more indispensable
even than in ordinary cases. If selection consisted merely in separating some
very distinct variety, and breeding from it, the principle would be so obvious
as hardly to be worth notice; but its importance consists in the great effect
produced by the accumulation in one direction, during successive generations, of
differences absolutely inappreciable by an uneducated eye--differences which I
for one have vainly attempted to appreciate. Not one man in a thousand has
accuracy of eye and judgment sufficient to become an eminent breeder. If gifted
with these qualities, and he studies his subject for years, and devotes his
lifetime to it with indomitable perseverance, he will succeed, and may make
great improvements; if he wants any of these qualities, he will assuredly fail.
Few would readily believe in the natural capacity and years of practice
requisite to become even a skilful pigeon-fancier. The same principles are
followed by horticulturists; but the variations are here often more abrupt. No
one supposes that our choicest productions have been produced by a single
variation from the aboriginal stock. We have proofs that this is not so in some
cases, in which exact records have been kept; thus, to give a very trifling
instance, the steadily-increasing size of the common gooseberry may be quoted.
We see an astonishing improvement in many florists' flowers, when the flowers of
the present day are compared with drawings made only twenty or thirty years ago.
When a race of plants is once pretty well established, the seed-raisers do not
pick out the best plants, but merely go over their seed-beds, and pull up the
"rogues," as they call the plants that deviate from the proper standard. With
animals this kind of selection is, in fact, also followed; for hardly any one is
so careless as to allow his worst animals to breed. In regard to plants, there
is another means of observing the accumulated effects of selection--namely, by
comparing the diversity of flowers in the different varieties of the same
species in the flower-garden; the diversity of leaves, pods, or tubers, or
whatever part is valued, in the kitchen-garden, in comparison with the flowers
of the same varieties; and the diversity of fruit of the same species in the
orchard, in comparison with the leaves and flowers of the same set of varieties.
See how different the leaves of the cabbage are, and how extremely alike the
flowers; how unlike the flowers of the heartsease are, and how alike the leaves;
how much the fruit of the different kinds of gooseberries differ in size,
colour, shape, and hairiness, and yet the flowers present very slight
differences. It is not that the varieties which differ largely in some one point
do not differ at all in other points; this is hardly ever, perhaps never, the
case. The laws of correlation of growth, the importance of which should never be
overlooked, will ensure some differences; but, as a general rule, I cannot doubt
that the continued selection of slight variations, either in the leaves, the
flowers, or the fruit, will produce races differing from each other chiefly in
these characters. It may be objected that the principle of selection has been
reduced to methodical practice for scarcely more than three-quarters of a
century; it has certainly been more attended to of late years, and many
treatises have been published on the subject; and the result, I may add, has
been, in a corresponding degree, rapid and important. But it is very far from
true that the principle is a modern discovery. I could give several references
to the full acknowledgment of the importance of the principle in works of high
antiquity. In rude and barbarous periods of English history choice animals were
often imported, and laws were passed to prevent their exportation: the
destruction of horses under a certain size was ordered, and this may be compared
to the "roguing" of plants by nurserymen. The principle of selection I find
distinctly given in an ancient Chinese encyclopaedia. Explicit rules are laid
down by some of the Roman classical writers. From passages in Genesis, it is
clear that the colour of domestic animals was at that early period attended to.
Savages now sometimes cross their dogs with wild canine animals, to improve the
breed, and they formerly did so, as is attested by passages in Pliny. The
savages in South Africa match their draught cattle by colour, as do some of the
Esquimaux their teams of dogs. Livingstone shows how much good domestic breeds
are valued by the negroes of the interior of Africa who have not associated with
Europeans. Some of these facts do not show actual selection, but they show that
the breeding of domestic animals was carefully attended to in ancient times, and
is now attended to by the lowest savages. It would, indeed, have been a strange
fact, had attention not been paid to breeding, for the inheritance of good and
bad qualities is so obvious. At the present time, eminent breeders try by
methodical selection, with a distinct object in view, to make a new strain or
sub-breed, superior to anything existing in the country. But, for our purpose, a
kind of Selection, which may be called Unconscious, and which results from every
one trying to possess and breed from the best individual animals, is more
important. Thus, a man who intends keeping pointers naturally tries to get as
good dogs as he can, and afterwards breeds from his own best dogs, but he has no
wish or expectation of permanently altering the breed. Nevertheless I cannot
doubt that this process, continued during centuries, would improve and modify
any breed, in the same way as Bakewell, Collins, etc., by this very same
process, only carried on more methodically, did greatly modify, even during
their own lifetimes, the forms and qualities of their cattle. Slow and
insensible changes of this kind could never be recognised unless actual
measurements or careful drawings of the breeds in question had been made long
ago, which might serve for comparison. In some cases, however, unchanged or but
little changed individuals of the same breed may be found in less civilised
districts, where the breed has been less improved. There is reason to believe
that King Charles's spaniel has been unconsciously modified to a large extent
since the time of that monarch. Some highly competent authorities are convinced
that the setter is directly derived from the spaniel, and has probably been
slowly altered from it. It is known that the English pointer has been greatly
changed within the last century, and in this case the change has, it is
believed, been chiefly effected by crosses with the fox-hound; but what concerns
us is, that the change has been effected unconsciously and gradually, and yet so
effectually, that, though the old Spanish pointer certainly came from Spain, Mr.
Borrow has not seen, as I am informed by him, any native dog in Spain like our
pointer. By a similar process of selection, and by careful training, the whole
body of English racehorses have come to surpass in fleetness and size the parent
Arab stock, so that the latter, by the regulations for the Goodwood Races, are
favoured in the weights they carry. Lord Spencer and others have shown how the
cattle of England have increased in weight and in early maturity, compared with
the stock formerly kept in this country. By comparing the accounts given in old
pigeon treatises of carriers and tumblers with these breeds as now existing in
Britain, India, and Persia, we can, I think, clearly trace the stages through
which they have insensibly passed, and come to differ so greatly from the
rock-pigeon. Youatt gives an excellent illustration of the effects of a course
of selection, which may be considered as unconsciously followed, in so far that
the breeders could never have expected or even have wished to have produced the
result which ensued--namely, the production of two distinct strains. The two
flocks of Leicester sheep kept by Mr. Buckley and Mr. Burgess, as Mr. Youatt
remarks, "have been purely bred from the original stock of Mr. Bakewell for
upwards of fifty years. There is not a suspicion existing in the mind of any one
at all acquainted with the subject that the owner of either of them has deviated
in any one instance from the pure blood of Mr. Bakewell's flock, and yet the
difference between the sheep possessed by these two gentlemen is so great that
they have the appearance of being quite different varieties." If there exist
savages so barbarous as never to think of the inherited character of the
offspring of their domestic animals, yet any one animal particularly useful to
them, for any special purpose, would be carefully preserved during famines and
other accidents, to which savages are so liable, and such choice animals would
thus generally leave more offspring than the inferior ones; so that in this case
there would be a kind of unconscious selection going on. We see the value set on
animals even by the barbarians of Tierra del Fuego, by their killing and
devouring their old women, in times of dearth, as of less value than their dogs.
In plants the same gradual process of improvement, through the occasional
preservation of the best individuals, whether or not sufficiently distinct to be
ranked at their first appearance as distinct varieties, and whether or not two
or more species or races have become blended together by crossing, may plainly
be recognised in the increased size and beauty which we now see in the varieties
of the heartsease, rose, pelargonium, dahlia, and other plants, when compared
with the older varieties or with their parent-stocks. No one would ever expect
to get a first-rate heartsease or dahlia from the seed of a wild plant. No one
would expect to raise a first-rate melting pear from the seed of a wild pear,
though he might succeed from a poor seedling growing wild, if it had come from a
garden-stock. The pear, though cultivated in classical times, appears, from
Pliny's description, to have been a fruit of very inferior quality. I have seen
great surprise expressed in horticultural works at the wonderful skill of
gardeners, in having produced such splendid results from such poor materials;
but the art, I cannot doubt, has been simple, and, as far as the final result is
concerned, has been followed almost unconsciously. It has consisted in always
cultivating the best known variety, sowing its seeds, and, when a slightly
better variety has chanced to appear, selecting it, and so onwards. But the
gardeners of the classical period, who cultivated the best pear they could
procure, never thought what splendid fruit we should eat; though we owe our
excellent fruit, in some small degree, to their having naturally chosen and
preserved the best varieties they could anywhere find. A large amount of change
in our cultivated plants, thus slowly and unconsciously accumulated, explains,
as I believe, the well-known fact, that in a vast number of cases we cannot
recognise, and therefore do not know, the wild parent-stocks of the plants which
have been longest cultivated in our flower and kitchen gardens. If it has taken
centuries or thousands of years to improve or modify most of our plants up to
their present standard of usefulness to man, we can understand how it is that
neither Australia, the Cape of Good Hope, nor any other region inhabited by
quite uncivilised man, has afforded us a single plant worth culture. It is not
that these countries, so rich in species, do not by a strange chance possess the
aboriginal stocks of any useful plants, but that the native plants have not been
improved by continued selection up to a standard of perfection comparable with
that given to the plants in countries anciently civilised. In regard to the
domestic animals kept by uncivilised man, it should not be overlooked that they
almost always have to struggle for their own food, at least during certain
seasons. And in two countries very differently circumstanced, individuals of the
same species, having slightly different constitutions or structure, would often
succeed better in the one country than in the other, and thus by a process of
"natural selection," as will hereafter be more fully explained, two sub-breeds
might be formed. This, perhaps, partly explains what has been remarked by some
authors, namely, that the varieties kept by savages have more of the character
of species than the varieties kept in civilised countries. On the view here
given of the all-important part which selection by man has played, it becomes at
once obvious, how it is that our domestic races show adaptation in their
structure or in their habits to man's wants or fancies. We can, I think, further
understand the frequently abnormal character of our domestic races, and likewise
their differences being so great in external characters and relatively so slight
in internal parts or organs. Man can hardly select, or only with much
difficulty, any deviation of structure excepting such as is externally visible;
and indeed he rarely cares for what is internal. He can never act by selection,
excepting on variations which are first given to him in some slight degree by
nature. No man would ever try to make a fantail, till he saw a pigeon with a
tail developed in some slight degree in an unusual manner, or a pouter till he
saw a pigeon with a crop of somewhat unusual size; and the more abnormal or
unusual any character was when it first appeared, the more likely it would be to
catch his attention. But to use such an expression as trying to make a fantail,
is, I have no doubt, in most cases, utterly incorrect. The man who first
selected a pigeon with a slightly larger tail, never dreamed what the
descendants of that pigeon would become through long-continued, partly
unconscious and partly methodical selection. Perhaps the parent bird of all
fantails had only fourteen tail-feathers somewhat expanded, like the present
Java fantail, or like individuals of other and distinct breeds, in which as many
as seventeen tail-feathers have been counted. Perhaps the first pouter-pigeon
did not inflate its crop much more than the turbit now does the upper part of
its oesophagus,--a habit which is disregarded by all fanciers, as it is not one
of the points of the breed. Nor let it be thought that some great deviation of
structure would be necessary to catch the fancier's eye: he perceives extremely
small differences, and it is in human nature to value any novelty, however
slight, in one's own possession. Nor must the value which would formerly be set
on any slight differences in the individuals of the same species, be judged of
by the value which would now be set on them, after several breeds have once
fairly been established. Many slight differences might, and indeed do now, arise
amongst pigeons, which are rejected as faults or deviations from the standard of
perfection of each breed. The common goose has not given rise to any marked
varieties; hence the Thoulouse and the common breed, which differ only in
colour, that most fleeting of characters, have lately been exhibited as distinct
at our poultry-shows. I think these views further explain what has sometimes
been noticed--namely that we know nothing about the origin or history of any of
our domestic breeds. But, in fact, a breed, like a dialect of a language, can
hardly be said to have had a definite origin. A man preserves and breeds from an
individual with some slight deviation of structure, or takes more care than
usual in matching his best animals and thus improves them, and the improved
individuals slowly spread in the immediate neighbourhood. But as yet they will
hardly have a distinct name, and from being only slightly valued, their history
will be disregarded. When further improved by the same slow and gradual process,
they will spread more widely, and will get recognised as something distinct and
valuable, and will then probably first receive a provincial name. In
semi-civilised countries, with little free communication, the spreading and
knowledge of any new sub-breed will be a slow process. As soon as the points of
value of the new sub-breed are once fully acknowledged, the principle, as I have
called it, of unconscious selection will always tend,--perhaps more at one
period than at another, as the breed rises or falls in fashion,--perhaps more in
one district than in another, according to the state of civilisation of the
inhabitants--slowly to add to the characteristic features of the breed, whatever
they may be. But the chance will be infinitely small of any record having been
preserved of such slow, varying, and insensible changes. I must now say a few
words on the circumstances, favourable, or the reverse, to man's power of
selection. A high degree of variability is obviously favourable, as freely
giving the materials for selection to work on; not that mere individual
differences are not amply sufficient, with extreme care, to allow of the
accumulation of a large amount of modification in almost any desired direction.
But as variations manifestly useful or pleasing to man appear only occasionally,
the chance of their appearance will be much increased by a large number of
individuals being kept; and hence this comes to be of the highest importance to
success. On this principle Marshall has remarked, with respect to the sheep of
parts of Yorkshire, that "as they generally belong to poor people, and are
mostly IN SMALL LOTS, they never can be improved." On the other hand,
nurserymen, from raising large stocks of the same plants, are generally far more
successful than amateurs in getting new and valuable varieties. The keeping of a
large number of individuals of a species in any country requires that the
species should be placed under favourable conditions of life, so as to breed
freely in that country. When the individuals of any species are scanty, all the
individuals, whatever their quality may be, will generally be allowed to breed,
and this will effectually prevent selection. But probably the most important
point of all, is, that the animal or plant should be so highly useful to man, or
so much valued by him, that the closest attention should be paid to even the
slightest deviation in the qualities or structure of each individual. Unless
such attention be paid nothing can be effected. I have seen it gravely remarked,
that it was most fortunate that the strawberry began to vary just when gardeners
began to attend closely to this plant. No doubt the strawberry had always varied
since it was cultivated, but the slight varieties had been neglected. As soon,
however, as gardeners picked out individual plants with slightly larger,
earlier, or better fruit, and raised seedlings from them, and again picked out
the best seedlings and bred from them, then, there appeared (aided by some
crossing with distinct species) those many admirable varieties of the strawberry
which have been raised during the last thirty or forty years. In the case of
animals with separate sexes, facility in preventing crosses is an important
element of success in the formation of new races,--at least, in a country which
is already stocked with other races. In this respect enclosure of the land plays
a part. Wandering savages or the inhabitants of open plains rarely possess more
than one breed of the same species. Pigeons can be mated for life, and this is a
great convenience to the fancier, for thus many races may be kept true, though
mingled in the same aviary; and this circumstance must have largely favoured the
improvement and formation of new breeds. Pigeons, I may add, can be propagated
in great numbers and at a very quick rate, and inferior birds may be freely
rejected, as when killed they serve for food. On the other hand, cats, from
their nocturnal rambling habits, cannot be matched, and, although so much valued
by women and children, we hardly ever see a distinct breed kept up; such breeds
as we do sometimes see are almost always imported from some other country, often
from islands. Although I do not doubt that some domestic animals vary less than
others, yet the rarity or absence of distinct breeds of the cat, the donkey,
peacock, goose, etc., may be attributed in main part to selection not having
been brought into play: in cats, from the difficulty in pairing them; in
donkeys, from only a few being kept by poor people, and little attention paid to
their breeding; in peacocks, from not being very easily reared and a large stock
not kept; in geese, from being valuable only for two purposes, food and
feathers, and more especially from no pleasure having been felt in the display
of distinct breeds. To sum up on the origin of our Domestic Races of animals and
plants. I believe that the conditions of life, from their action on the
reproductive system, are so far of the highest importance as causing
variability. I do not believe that variability is an inherent and necessary
contingency, under all circumstances, with all organic beings, as some authors
have thought. The effects of variability are modified by various degrees of
inheritance and of reversion. Variability is governed by many unknown laws, more
especially by that of correlation of growth. Something may be attributed to the
direct action of the conditions of life. Something must be attributed to use and
disuse. The final result is thus rendered infinitely complex. In some cases, I
do not doubt that the intercrossing of species, aboriginally distinct, has
played an important part in the origin of our domestic productions. When in any
country several domestic breeds have once been established, their occasional
intercrossing, with the aid of selection, has, no doubt, largely aided in the
formation of new sub-breeds; but the importance of the crossing of varieties
has, I believe, been greatly exaggerated, both in regard to animals and to those
plants which are propagated by seed. In plants which are temporarily propagated
by cuttings, buds, etc., the importance of the crossing both of distinct species
and of varieties is immense; for the cultivator here quite disregards the
extreme variability both of hybrids and mongrels, and the frequent sterility of
hybrids; but the cases of plants not propagated by seed are of little importance
to us, for their endurance is only temporary. Over all these causes of Change I
am convinced that the accumulative action of Selection, whether applied
methodically and more quickly, or unconsciously and more slowly, but more
efficiently, is by far the predominant Power. CHAPTER 2. VARIATION UNDER NATURE.
Variability. Individual differences. Doubtful species. Wide ranging, much
diffused, and common species vary most. Species of the larger genera in any
country vary more than the species of the smaller genera. Many of the species of
the larger genera resemble varieties in being very closely, but unequally,
related to each other, and in having restricted ranges. Before applying the
principles arrived at in the last chapter to organic beings in a state of
nature, we must briefly discuss whether these latter are subject to any
variation. To treat this subject at all properly, a long catalogue of dry facts
should be given; but these I shall reserve for my future work. Nor shall I here
discuss the various definitions which have been given of the term species. No
one definition has as yet satisfied all naturalists; yet every naturalist knows
vaguely what he means when he speaks of a species. Generally the term includes
the unknown element of a distinct act of creation. The term "variety" is almost
equally difficult to define; but here community of descent is almost universally
implied, though it can rarely be proved. We have also what are called
monstrosities; but they graduate into varieties. By a monstrosity I presume is
meant some considerable deviation of structure in one part, either injurious to
or not useful to the species, and not generally propagated. Some authors use the
term "variation" in a technical sense, as implying a modification directly due
to the physical conditions of life; and "variations" in this sense are supposed
not to be inherited: but who can say that the dwarfed condition of shells in the
brackish waters of the Baltic, or dwarfed plants on Alpine summits, or the
thicker fur of an animal from far northwards, would not in some cases be
inherited for at least some few generations? and in this case I presume that the
form would be called a variety. Again, we have many slight differences which may
be called individual differences, such as are known frequently to appear in the
offspring from the same parents, or which may be presumed to have thus arisen,
from being frequently observed in the individuals of the same species inhabiting
the same confined locality. No one supposes that all the individuals of the same
species are cast in the very same mould. These individual differences are highly
important for us, as they afford materials for natural selection to accumulate,
in the same manner as man can accumulate in any given direction individual
differences in his domesticated productions. These individual differences
generally affect what naturalists consider unimportant parts; but I could show
by a long catalogue of facts, that parts which must be called important, whether
viewed under a physiological or classificatory point of view, sometimes vary in
the individuals of the same species. I am convinced that the most experienced
naturalist would be surprised at the number of the cases of variability, even in
important parts of structure, which he could collect on good authority, as I
have collected, during a course of years. It should be remembered that
systematists are far from pleased at finding variability in important
characters, and that there are not many men who will laboriously examine
internal and important organs, and compare them in many specimens of the same
species. I should never have expected that the branching of the main nerves
close to the great central ganglion of an insect would have been variable in the
same species; I should have expected that changes of this nature could have been
effected only by slow degrees: yet quite recently Mr. Lubbock has shown a degree
of variability in these main nerves in Coccus, which may almost be compared to
the irregular branching of the stem of a tree. This philosophical naturalist, I
may add, has also quite recently shown that the muscles in the larvae of certain
insects are very far from uniform. Authors sometimes argue in a circle when they
state that important organs never vary; for these same authors practically rank
that character as important (as some few naturalists have honestly confessed)
which does not vary; and, under this point of view, no instance of an important
part varying will ever be found: but under any other point of view many
instances assuredly can be given. There is one point connected with individual
differences, which seems to me extremely perplexing: I refer to those genera
which have sometimes been called "protean" or "polymorphic," in which the
species present an inordinate amount of variation; and hardly two naturalists
can agree which forms to rank as species and which as varieties. We may instance
Rubus, Rosa, and Hieracium amongst plants, several genera of insects, and
several genera of Brachiopod shells. In most polymorphic genera some of the
species have fixed and definite characters. Genera which are polymorphic in one
country seem to be, with some few exceptions, polymorphic in other countries,
and likewise, judging from Brachiopod shells, at former periods of time. These
facts seem to be very perplexing, for they seem to show that this kind of
variability is independent of the conditions of life. I am inclined to suspect
that we see in these polymorphic genera variations in points of structure which
are of no service or disservice to the species, and which consequently have not
been seized on and rendered definite by natural selection, as hereafter will be
explained. Those forms which possess in some considerable degree the character
of species, but which are so closely similar to some other forms, or are so
closely linked to them by intermediate gradations, that naturalists do not like
to rank them as distinct species, are in several respects the most important for
us. We have every reason to believe that many of these doubtful and
closely-allied forms have permanently retained their characters in their own
country for a long time; for as long, as far as we know, as have good and true
species. Practically, when a naturalist can unite two forms together by others
having intermediate characters, he treats the one as a variety of the other,
ranking the most common, but sometimes the one first described, as the species,
and the other as the variety. But cases of great difficulty, which I will not
here enumerate, sometimes occur in deciding whether or not to rank one form as a
variety of another, even when they are closely connected by intermediate links;
nor will the commonly-assumed hybrid nature of the intermediate links always
remove the difficulty. In very many cases, however, one form is ranked as a
variety of another, not because the intermediate links have actually been found,
but because analogy leads the observer to suppose either that they do now
somewhere exist, or may formerly have existed; and here a wide door for the
entry of doubt and conjecture is opened. Hence, in determining whether a form
should be ranked as a species or a variety, the opinion of naturalists having
sound judgment and wide experience seems the only guide to follow. We must,
however, in many cases, decide by a majority of naturalists, for few well-marked
and well-known varieties can be named which have not been ranked as species by
at least some competent judges. That varieties of this doubtful nature are far
from uncommon cannot be disputed. Compare the several floras of Great Britain,
of France or of the United States, drawn up by different botanists, and see what
a surprising number of forms have been ranked by one botanist as good species,
and by another as mere varieties. Mr. H. C. Watson, to whom I lie under deep
obligation for assistance of all kinds, has marked for me 182 British plants,
which are generally considered as varieties, but which have all been ranked by
botanists as species; and in making this list he has omitted many trifling
varieties, but which nevertheless have been ranked by some botanists as species,
and he has entirely omitted several highly polymorphic genera. Under genera,
including the most polymorphic forms, Mr. Babington gives 251 species, whereas
Mr. Bentham gives only 112,--a difference of 139 doubtful forms! Amongst animals
which unite for each birth, and which are highly locomotive, doubtful forms,
ranked by one zoologist as a species and by another as a variety, can rarely be
found within the same country, but are common in separated areas. How many of
those birds and insects in North America and Europe, which differ very slightly
from each other, have been ranked by one eminent naturalist as undoubted
species, and by another as varieties, or, as they are often called, as
geographical races! Many years ago, when comparing, and seeing others compare,
the birds from the separate islands of the Galapagos Archipelago, both one with
another, and with those from the American mainland, I was much struck how
entirely vague and arbitrary is the distinction between species and varieties.
On the islets of the little Madeira group there are many insects which are
characterized as varieties in Mr. Wollaston's admirable work, but which it
cannot be doubted would be ranked as distinct species by many entomologists.
Even Ireland has a few animals, now generally regarded as varieties, but which
have been ranked as species by some zoologists. Several most experienced
ornithologists consider our British red grouse as only a strongly-marked race of
a Norwegian species, whereas the greater number rank it as an undoubted species
peculiar to Great Britain. A wide distance between the homes of two doubtful
forms leads many naturalists to rank both as distinct species; but what
distance, it has been well asked, will suffice? if that between America and
Europe is ample, will that between the Continent and the Azores, or Madeira, or
the Canaries, or Ireland, be sufficient? It must be admitted that many forms,
considered by highly-competent judges as varieties, have so perfectly the
character of species that they are ranked by other highly-competent judges as
good and true species. But to discuss whether they are rightly called species or
varieties, before any definition of these terms has been generally accepted, is
vainly to beat the air. Many of the cases of strongly-marked varieties or
doubtful species well deserve consideration; for several interesting lines of
argument, from geographical distribution, analogical variation, hybridism, etc.,
have been brought to bear on the attempt to determine their rank. I will here
give only a single instance,--the well-known one of the primrose and cowslip, or
Primula veris and elatior. These plants differ considerably in appearance; they
have a different flavour and emit a different odour; they flower at slightly
different periods; they grow in somewhat different stations; they ascend
mountains to different heights; they have different geographical ranges; and
lastly, according to very numerous experiments made during several years by that
most careful observer Gartner, they can be crossed only with much difficulty. We
could hardly wish for better evidence of the two forms being specifically
distinct. On the other hand, they are united by many intermediate links, and it
is very doubtful whether these links are hybrids; and there is, as it seems to
me, an overwhelming amount of experimental evidence, showing that they descend
from common parents, and consequently must be ranked as varieties. Close
investigation, in most cases, will bring naturalists to an agreement how to rank
doubtful forms. Yet it must be confessed, that it is in the best-known countries
that we find the greatest number of forms of doubtful value. I have been struck
with the fact, that if any animal or plant in a state of nature be highly useful
to man, or from any cause closely attract his attention, varieties of it will
almost universally be found recorded. These varieties, moreover, will be often
ranked by some authors as species. Look at the common oak, how closely it has
been studied; yet a German author makes more than a dozen species out of forms,
which are very generally considered as varieties; and in this country the
highest botanical authorities and practical men can be quoted to show that the
sessile and pedunculated oaks are either good and distinct species or mere
varieties. When a young naturalist commences the study of a group of organisms
quite unknown to him, he is at first much perplexed to determine what
differences to consider as specific, and what as varieties; for he knows nothing
of the amount and kind of variation to which the group is subject; and this
shows, at least, how very generally there is some variation. But if he confine
his attention to one class within one country, he will soon make up his mind how
to rank most of the doubtful forms. His general tendency will be to make many
species, for he will become impressed, just like the pigeon or poultry-fancier
before alluded to, with the amount of difference in the forms which he is
continually studying; and he has little general knowledge of analogical
variation in other groups and in other countries, by which to correct his first
impressions. As he extends the range of his observations, he will meet with more
cases of difficulty; for he will encounter a greater number of closely-allied
forms. But if his observations be widely extended, he will in the end generally
be enabled to make up his own mind which to call varieties and which species;
but he will succeed in this at the expense of admitting much variation,--and the
truth of this admission will often be disputed by other naturalists. When,
moreover, he comes to study allied forms brought from countries not now
continuous, in which case he can hardly hope to find the intermediate links
between his doubtful forms, he will have to trust almost entirely to analogy,
and his difficulties will rise to a climax. Certainly no clear line of
demarcation has as yet been drawn between species and sub-species--that is, the
forms which in the opinion of some naturalists come very near to, but do not
quite arrive at the rank of species; or, again, between sub-species and
well-marked varieties, or between lesser varieties and individual differences.
These differences blend into each other in an insensible series; and a series
impresses the mind with the idea of an actual passage. Hence I look at
individual differences, though of small interest to the systematist, as of high
importance for us, as being the first step towards such slight varieties as are
barely thought worth recording in works on natural history. And I look at
varieties which are in any degree more distinct and permanent, as steps leading
to more strongly marked and more permanent varieties; and at these latter, as
leading to sub-species, and to species. The passage from one stage of difference
to another and higher stage may be, in some cases, due merely to the
long-continued action of different physical conditions in two different regions;
but I have not much faith in this view; and I attribute the passage of a
variety, from a state in which it differs very slightly from its parent to one
in which it differs more, to the action of natural selection in accumulating (as
will hereafter be more fully explained) differences of structure in certain
definite directions. Hence I believe a well-marked variety may be justly called
an incipient species; but whether this belief be justifiable must be judged of
by the general weight of the several facts and views given throughout this work.
It need not be supposed that all varieties or incipient species necessarily
attain the rank of species. They may whilst in this incipient state become
extinct, or they may endure as varieties for very long periods, as has been
shown to be the case by Mr. Wollaston with the varieties of certain fossil
land-shells in Madeira. If a variety were to flourish so as to exceed in numbers
the parent species, it would then rank as the species, and the species as the
variety; or it might come to supplant and exterminate the parent species; or
both might co-exist, and both rank as independent species. But we shall
hereafter have to return to this subject. From these remarks it will be seen
that I look at the term species, as one arbitrarily given for the sake of
convenience to a set of individuals closely resembling each other, and that it
does not essentially differ from the term variety, which is given to less
distinct and more fluctuating forms. The term variety, again, in comparison with
mere individual differences, is also applied arbitrarily, and for mere
convenience sake. Guided by theoretical considerations, I thought that some
interesting results might be obtained in regard to the nature and relations of
the species which vary most, by tabulating all the varieties in several
well-worked floras. At first this seemed a simple task; but Mr. H. C. Watson, to
whom I am much indebted for valuable advice and assistance on this subject, soon
convinced me that there were many difficulties, as did subsequently Dr. Hooker,
even in stronger terms. I shall reserve for my future work the discussion of
these difficulties, and the tables themselves of the proportional numbers of the
varying species. Dr. Hooker permits me to add, that after having carefully read
my manuscript, and examined the tables, he thinks that the following statements
are fairly well established. The whole subject, however, treated as it
necessarily here is with much brevity, is rather perplexing, and allusions
cannot be avoided to the "struggle for existence," "divergence of character,"
and other questions, hereafter to be discussed. Alph. De Candolle and others
have shown that plants which have very wide ranges generally present varieties;
and this might have been expected, as they become exposed to diverse physical
conditions, and as they come into competition (which, as we shall hereafter see,
is a far more important circumstance) with different sets of organic beings. But
my tables further show that, in any limited country, the species which are most
common, that is abound most in individuals, and the species which are most
widely diffused within their own country (and this is a different consideration
from wide range, and to a certain extent from commonness), often give rise to
varieties sufficiently well-marked to have been recorded in botanical works.
Hence it is the most flourishing, or, as they may be called, the dominant
species,--those which range widely over the world, are the most diffused in
their own country, and are the most numerous in individuals,--which oftenest
produce well-marked varieties, or, as I consider them, incipient species. And
this, perhaps, might have been anticipated; for, as varieties, in order to
become in any degree permanent, necessarily have to struggle with the other
inhabitants of the country, the species which are already dominant will be the
most likely to yield offspring which, though in some slight degree modified,
will still inherit those advantages that enabled their parents to become
dominant over their compatriots. If the plants inhabiting a country and
described in any Flora be divided into two equal masses, all those in the larger
genera being placed on one side, and all those in the smaller genera on the
other side, a somewhat larger number of the very common and much diffused or
dominant species will be found on the side of the larger genera. This, again,
might have been anticipated; for the mere fact of many species of the same genus
inhabiting any country, shows that there is something in the organic or
inorganic conditions of that country favourable to the genus; and, consequently,
we might have expected to have found in the larger genera, or those including
many species, a large proportional number of dominant species. But so many
causes tend to obscure this result, that I am surprised that my tables show even
a small majority on the side of the larger genera. I will here allude to only
two causes of obscurity. Fresh-water and salt-loving plants have generally very
wide ranges and are much diffused, but this seems to be connected with the
nature of the stations inhabited by them, and has little or no relation to the
size of the genera to which the species belong. Again, plants low in the scale
of organisation are generally much more widely diffused than plants higher in
the scale; and here again there is no close relation to the size of the genera.
The cause of lowly-organised plants ranging widely will be discussed in our
chapter on geographical distribution. From looking at species as only
strongly-marked and well-defined varieties, I was led to anticipate that the
species of the larger genera in each country would oftener present varieties,
than the species of the smaller genera; for wherever many closely related
species (i.e. species of the same genus) have been formed, many varieties or
incipient species ought, as a general rule, to be now forming. Where many large
trees grow, we expect to find saplings. Where many species of a genus have been
formed through variation, circumstances have been favourable for variation; and
hence we might expect that the circumstances would generally be still favourable
to variation. On the other hand, if we look at each species as a special act of
creation, there is no apparent reason why more varieties should occur in a group
having many species, than in one having few. To test the truth of this
anticipation I have arranged the plants of twelve countries, and the
coleopterous insects of two districts, into two nearly equal masses, the species
of the larger genera on one side, and those of the smaller genera on the other
side, and it has invariably proved to be the case that a larger proportion of
the species on the side of the larger genera present varieties, than on the side
of the smaller genera. Moreover, the species of the large genera which present
any varieties, invariably present a larger average number of varieties than do
the species of the small genera. Both these results follow when another division
is made, and when all the smallest genera, with from only one to four species,
are absolutely excluded from the tables. These facts are of plain signification
on the view that species are only strongly marked and permanent varieties; for
wherever many species of the same genus have been formed, or where, if we may
use the expression, the manufactory of species has been active, we ought
generally to find the manufactory still in action, more especially as we have
every reason to believe the process of manufacturing new species to be a slow
one. And this certainly is the case, if varieties be looked at as incipient
species; for my tables clearly show as a general rule that, wherever many
species of a genus have been formed, the species of that genus present a number
of varieties, that is of incipient species, beyond the average. It is not that
all large genera are now varying much, and are thus increasing in the number of
their species, or that no small genera are now varying and increasing; for if
this had been so, it would have been fatal to my theory; inasmuch as geology
plainly tells us that small genera have in the lapse of time often increased
greatly in size; and that large genera have often come to their maxima,
declined, and disappeared. All that we want to show is, that where many species
of a genus have been formed, on an average many are still forming; and this
holds good. There are other relations between the species of large genera and
their recorded varieties which deserve notice. We have seen that there is no
infallible criterion by which to distinguish species and well-marked varieties;
and in those cases in which intermediate links have not been found between
doubtful forms, naturalists are compelled to come to a determination by the
amount of difference between them, judging by analogy whether or not the amount
suffices to raise one or both to the rank of species. Hence the amount of
difference is one very important criterion in settling whether two forms should
be ranked as species or varieties. Now Fries has remarked in regard to plants,
and Westwood in regard to insects, that in large genera the amount of difference
between the species is often exceedingly small. I have endeavoured to test this
numerically by averages, and, as far as my imperfect results go, they always
confirm the view. I have also consulted some sagacious and most experienced
observers, and, after deliberation, they concur in this view. In this respect,
therefore, the species of the larger genera resemble varieties, more than do the
species of the smaller genera. Or the case may be put in another way, and it may
be said, that in the larger genera, in which a number of varieties or incipient
species greater than the average are now manufacturing, many of the species
already manufactured still to a certain extent resemble varieties, for they
differ from each other by a less than usual amount of difference. Moreover, the
species of the large genera are related to each other, in the same manner as the
varieties of any one species are related to each other. No naturalist pretends
that all the species of a genus are equally distinct from each other; they may
generally be divided into sub-genera, or sections, or lesser groups. As Fries
has well remarked, little groups of species are generally clustered like
satellites around certain other species. And what are varieties but groups of
forms, unequally related to each other, and clustered round certain forms--that
is, round their parent-species? Undoubtedly there is one most important point of
difference between varieties and species; namely, that the amount of difference
between varieties, when compared with each other or with their parent-species,
is much less than that between the species of the same genus. But when we come
to discuss the principle, as I call it, of Divergence of Character, we shall see
how this may be explained, and how the lesser differences between varieties will
tend to increase into the greater differences between species. There is one
other point which seems to me worth notice. Varieties generally have much
restricted ranges: this statement is indeed scarcely more than a truism, for if
a variety were found to have a wider range than that of its supposed
parent-species, their denominations ought to be reversed. But there is also
reason to believe, that those species which are very closely allied to other
species, and in so far resemble varieties, often have much restricted ranges.
For instance, Mr. H. C. Watson has marked for me in the well-sifted London
Catalogue of plants (4th edition) 63 plants which are therein ranked as species,
but which he considers as so closely allied to other species as to be of
doubtful value: these 63 reputed species range on an average over 6.9 of the
provinces into which Mr. Watson has divided Great Britain. Now, in this same
catalogue, 53 acknowledged varieties are recorded, and these range over 7.7
provinces; whereas, the species to which these varieties belong range over 14.3
provinces. So that the acknowledged varieties have very nearly the same
restricted average range, as have those very closely allied forms, marked for me
by Mr. Watson as doubtful species, but which are almost universally ranked by
British botanists as good and true species. Finally, then, varieties have the
same general characters as species, for they cannot be distinguished from
species,--except, firstly, by the discovery of intermediate linking forms, and
the occurrence of such links cannot affect the actual characters of the forms
which they connect; and except, secondly, by a certain amount of difference, for
two forms, if differing very little, are generally ranked as varieties,
notwithstanding that intermediate linking forms have not been discovered; but
the amount of difference considered necessary to give to two forms the rank of
species is quite indefinite. In genera having more than the average number of
species in any country, the species of these genera have more than the average
number of varieties. In large genera the species are apt to be closely, but
unequally, allied together, forming little clusters round certain species.
Species very closely allied to other species apparently have restricted ranges.
In all these several respects the species of large genera present a strong
analogy with varieties. And we can clearly understand these analogies, if
species have once existed as varieties, and have thus originated: whereas, these
analogies are utterly inexplicable if each species has been independently
created. We have, also, seen that it is the most flourishing and dominant
species of the larger genera which on an average vary most; and varieties, as we
shall hereafter see, tend to become converted into new and distinct species. The
larger genera thus tend to become larger; and throughout nature the forms of
life which are now dominant tend to become still more dominant by leaving many
modified and dominant descendants. But by steps hereafter to be explained, the
larger genera also tend to break up into smaller genera. And thus, the forms of
life throughout the universe become divided into groups subordinate to groups.
CHAPTER 3. STRUGGLE FOR EXISTENCE. Bears on natural selection. The term used in
a wide sense. Geometrical powers of increase. Rapid increase of naturalised
animals and plants. Nature of the checks to increase. Competition universal.
Effects of climate. Protection from the number of individuals. Complex relations
of all animals and plants throughout nature. Struggle for life most severe
between individuals and varieties of the same species; often severe between
species of the same genus. The relation of organism to organism the most
important of all relations. Before entering on the subject of this chapter, I
must make a few preliminary remarks, to show how the struggle for existence
bears on Natural Selection. It has been seen in the last chapter that amongst
organic beings in a state of nature there is some individual variability; indeed
I am not aware that this has ever been disputed. It is immaterial for us whether
a multitude of doubtful forms be called species or sub-species or varieties;
what rank, for instance, the two or three hundred doubtful forms of British
plants are entitled to hold, if the existence of any well-marked varieties be
admitted. But the mere existence of individual variability and of some few
well-marked varieties, though necessary as the foundation for the work, helps us
but little in understanding how species arise in nature. How have all those
exquisite adaptations of one part of the organisation to another part, and to
the conditions of life, and of one distinct organic being to another being, been
perfected? We see these beautiful co-adaptations most plainly in the woodpecker
and missletoe; and only a little less plainly in the humblest parasite which
clings to the hairs of a quadruped or feathers of a bird; in the structure of
the beetle which dives through the water; in the plumed seed which is wafted by
the gentlest breeze; in short, we see beautiful adaptations everywhere and in
every part of the organic world. Again, it may be asked, how is it that
varieties, which I have called incipient species, become ultimately converted
into good and distinct species, which in most cases obviously differ from each
other far more than do the varieties of the same species? How do those groups of
species, which constitute what are called distinct genera, and which differ from
each other more than do the species of the same genus, arise? All these results,
as we shall more fully see in the next chapter, follow inevitably from the
struggle for life. Owing to this struggle for life, any variation, however
slight and from whatever cause proceeding, if it be in any degree profitable to
an individual of any species, in its infinitely complex relations to other
organic beings and to external nature, will tend to the preservation of that
individual, and will generally be inherited by its offspring. The offspring,
also, will thus have a better chance of surviving, for, of the many individuals
of any species which are periodically born, but a small number can survive. I
have called this principle, by which each slight variation, if useful, is
preserved, by the term of Natural Selection, in order to mark its relation to
man's power of selection. We have seen that man by selection can certainly
produce great results, and can adapt organic beings to his own uses, through the
accumulation of slight but useful variations, given to him by the hand of
Nature. But Natural Selection, as we shall hereafter see, is a power incessantly
ready for action, and is as immeasurably superior to man's feeble efforts, as
the works of Nature are to those of Art. We will now discuss in a little more
detail the struggle for existence. In my future work this subject shall be
treated, as it well deserves, at much greater length. The elder De Candolle and
Lyell have largely and philosophically shown that all organic beings are exposed
to severe competition. In regard to plants, no one has treated this subject with
more spirit and ability than W. Herbert, Dean of Manchester, evidently the
result of his great horticultural knowledge. Nothing is easier than to admit in
words the truth of the universal struggle for life, or more difficult--at least
I have found it so--than constantly to bear this conclusion in mind. Yet unless
it be thoroughly engrained in the mind, I am convinced that the whole economy of
nature, with every fact on distribution, rarity, abundance, extinction, and
variation, will be dimly seen or quite misunderstood. We behold the face of
nature bright with gladness, we often see superabundance of food; we do not see,
or we forget, that the birds which are idly singing round us mostly live on
insects or seeds, and are thus constantly destroying life; or we forget how
largely these songsters, or their eggs, or their nestlings, are destroyed by
birds and beasts of prey; we do not always bear in mind, that though food may be
now superabundant, it is not so at all seasons of each recurring year. I should
premise that I use the term Struggle for Existence in a large and metaphorical
sense, including dependence of one being on another, and including (which is
more important) not only the life of the individual, but success in leaving
progeny. Two canine animals in a time of dearth, may be truly said to struggle
with each other which shall get food and live. But a plant on the edge of a
desert is said to struggle for life against the drought, though more properly it
should be said to be dependent on the moisture. A plant which annually produces
a thousand seeds, of which on an average only one comes to maturity, may be more
truly said to struggle with the plants of the same and other kinds which already
clothe the ground. The missletoe is dependent on the apple and a few other
trees, but can only in a far-fetched sense be said to struggle with these trees,
for if too many of these parasites grow on the same tree, it will languish and
die. But several seedling missletoes, growing close together on the same branch,
may more truly be said to struggle with each other. As the missletoe is
disseminated by birds, its existence depends on birds; and it may metaphorically
be said to struggle with other fruit-bearing plants, in order to tempt birds to
devour and thus disseminate its seeds rather than those of other plants. In
these several senses, which pass into each other, I use for convenience sake the
general term of struggle for existence. A struggle for existence inevitably
follows from the high rate at which all organic beings tend to increase. Every
being, which during its natural lifetime produces several eggs or seeds, must
suffer destruction during some period of its life, and during some season or
occasional year, otherwise, on the principle of geometrical increase, its
numbers would quickly become so inordinately great that no country could support
the product. Hence, as more individuals are produced than can possibly survive,
there must in every case be a struggle for existence, either one individual with
another of the same species, or with the individuals of distinct species, or
with the physical conditions of life. It is the doctrine of Malthus applied with
manifold force to the whole animal and vegetable kingdoms; for in this case
there can be no artificial increase of food, and no prudential restraint from
marriage. Although some species may be now increasing, more or less rapidly, in
numbers, all cannot do so, for the world would not hold them. There is no
exception to the rule that every organic being naturally increases at so high a
rate, that if not destroyed, the earth would soon be covered by the progeny of a
single pair. Even slow-breeding man has doubled in twenty-five years, and at
this rate, in a few thousand years, there would literally not be standing room
for his progeny. Linnaeus has calculated that if an annual plant produced only
two seeds--and there is no plant so unproductive as this--and their seedlings
next year produced two, and so on, then in twenty years there would be a million
plants. The elephant is reckoned to be the slowest breeder of all known animals,
and I have taken some pains to estimate its probable minimum rate of natural
increase: it will be under the mark to assume that it breeds when thirty years
old, and goes on breeding till ninety years old, bringing forth three pair of
young in this interval; if this be so, at the end of the fifth century there
would be alive fifteen million elephants, descended from the first pair. But we
have better evidence on this subject than mere theoretical calculations, namely,
the numerous recorded cases of the astonishingly rapid increase of various
animals in a state of nature, when circumstances have been favourable to them
during two or three following seasons. Still more striking is the evidence from
our domestic animals of many kinds which have run wild in several parts of the
world: if the statements of the rate of increase of slow-breeding cattle and
horses in South America, and latterly in Australia, had not been well
authenticated, they would have been quite incredible. So it is with plants:
cases could be given of introduced plants which have become common throughout
whole islands in a period of less than ten years. Several of the plants now most
numerous over the wide plains of La Plata, clothing square leagues of surface
almost to the exclusion of all other plants, have been introduced from Europe;
and there are plants which now range in India, as I hear from Dr. Falconer, from
Cape Comorin to the Himalaya, which have been imported from America since its
discovery. In such cases, and endless instances could be given, no one supposes
that the fertility of these animals or plants has been suddenly and temporarily
increased in any sensible degree. The obvious explanation is that the conditions
of life have been very favourable, and that there has consequently been less
destruction of the old and young, and that nearly all the young have been
enabled to breed. In such cases the geometrical ratio of increase, the result of
which never fails to be surprising, simply explains the extraordinarily rapid
increase and wide diffusion of naturalised productions in their new homes. In a
state of nature almost every plant produces seed, and amongst animals there are
very few which do not annually pair. Hence we may confidently assert, that all
plants and animals are tending to increase at a geometrical ratio, that all
would most rapidly stock every station in which they could any how exist, and
that the geometrical tendency to increase must be checked by destruction at some
period of life. Our familiarity with the larger domestic animals tends, I think,
to mislead us: we see no great destruction falling on them, and we forget that
thousands are annually slaughtered for food, and that in a state of nature an
equal number would have somehow to be disposed of. The only difference between
organisms which annually produce eggs or seeds by the thousand, and those which
produce extremely few, is, that the slow-breeders would require a few more years
to people, under favourable conditions, a whole district, let it be ever so
large. The condor lays a couple of eggs and the ostrich a score, and yet in the
same country the condor may be the more numerous of the two: the Fulmar petrel
lays but one egg, yet it is believed to be the most numerous bird in the world.
One fly deposits hundreds of eggs, and another, like the hippobosca, a single
one; but this difference does not determine how many individuals of the two
species can be supported in a district. A large number of eggs is of some
importance to those species, which depend on a rapidly fluctuating amount of
food, for it allows them rapidly to increase in number. But the real importance
of a large number of eggs or seeds is to make up for much destruction at some
period of life; and this period in the great majority of cases is an early one.
If an animal can in any way protect its own eggs or young, a small number may be
produced, and yet the average stock be fully kept up; but if many eggs or young
are destroyed, many must be produced, or the species will become extinct. It
would suffice to keep up the full number of a tree, which lived on an average
for a thousand years, if a single seed were produced once in a thousand years,
supposing that this seed were never destroyed, and could be ensured to germinate
in a fitting place. So that in all cases, the average number of any animal or
plant depends only indirectly on the number of its eggs or seeds. In looking at
Nature, it is most necessary to keep the foregoing considerations always in
mind--never to forget that every single organic being around us may be said to
be striving to the utmost to increase in numbers; that each lives by a struggle
at some period of its life; that heavy destruction inevitably falls either on
the young or old, during each generation or at recurrent intervals. Lighten any
check, mitigate the destruction ever so little, and the number of the species
will almost instantaneously increase to any amount. The face of Nature may be
compared to a yielding surface, with ten thousand sharp wedges packed close
together and driven inwards by incessant blows, sometimes one wedge being
struck, and then another with greater force. What checks the natural tendency of
each species to increase in number is most obscure. Look at the most vigorous
species; by as much as it swarms in numbers, by so much will its tendency to
increase be still further increased. We know not exactly what the checks are in
even one single instance. Nor will this surprise any one who reflects how
ignorant we are on this head, even in regard to mankind, so incomparably better
known than any other animal. This subject has been ably treated by several
authors, and I shall, in my future work, discuss some of the checks at
considerable length, more especially in regard to the feral animals of South
America. Here I will make only a few remarks, just to recall to the reader's
mind some of the chief points. Eggs or very young animals seem generally to
suffer most, but this is not invariably the case. With plants there is a vast
destruction of seeds, but, from some observations which I have made, I believe
that it is the seedlings which suffer most from germinating in ground already
thickly stocked with other plants. Seedlings, also, are destroyed in vast
numbers by various enemies; for instance, on a piece of ground three feet long
and two wide, dug and cleared, and where there could be no choking from other
plants, I marked all the seedlings of our native weeds as they came up, and out
of the 357 no less than 295 were destroyed, chiefly by slugs and insects. If
turf which has long been mown, and the case would be the same with turf closely
browsed by quadrupeds, be let to grow, the more vigorous plants gradually kill
the less vigorous, though fully grown, plants: thus out of twenty species
growing on a little plot of turf (three feet by four) nine species perished from
the other species being allowed to grow up freely. The amount of food for each
species of course gives the extreme limit to which each can increase; but very
frequently it is not the obtaining food, but the serving as prey to other
animals, which determines the average numbers of a species. Thus, there seems to
be little doubt that the stock of partridges, grouse, and hares on any large
estate depends chiefly on the destruction of vermin. If not one head of game
were shot during the next twenty years in England, and, at the same time, if no
vermin were destroyed, there would, in all probability, be less game than at
present, although hundreds of thousands of game animals are now annually killed.
On the other hand, in some cases, as with the elephant and rhinoceros, none are
destroyed by beasts of prey: even the tiger in India most rarely dares to attack
a young elephant protected by its dam. Climate plays an important part in
determining the average numbers of a species, and periodical seasons of extreme
cold or drought, I believe to be the most effective of all checks. I estimated
that the winter of 1854-55 destroyed four-fifths of the birds in my own grounds;
and this is a tremendous destruction, when we remember that ten per cent. is an
extraordinarily severe mortality from epidemics with man. The action of climate
seems at first sight to be quite independent of the struggle for existence; but
in so far as climate chiefly acts in reducing food, it brings on the most severe
struggle between the individuals, whether of the same or of distinct species,
which subsist on the same kind of food. Even when climate, for instance extreme
cold, acts directly, it will be the least vigorous, or those which have got
least food through the advancing winter, which will suffer most. When we travel
from south to north, or from a damp region to a dry, we invariably see some
species gradually getting rarer and rarer, and finally disappearing; and the
change of climate being conspicuous, we are tempted to attribute the whole
effect to its direct action. But this is a very false view: we forget that each
species, even where it most abounds, is constantly suffering enormous
destruction at some period of its life, from enemies or from competitors for the
same place and food; and if these enemies or competitors be in the least degree
favoured by any slight change of climate, they will increase in numbers, and, as
each area is already fully stocked with inhabitants, the other species will
decrease. When we travel southward and see a species decreasing in numbers, we
may feel sure that the cause lies quite as much in other species being favoured,
as in this one being hurt. So it is when we travel northward, but in a somewhat
lesser degree, for the number of species of all kinds, and therefore of
competitors, decreases northwards; hence in going northward, or in ascending a
mountain, we far oftener meet with stunted forms, due to the DIRECTLY injurious
action of climate, than we do in proceeding southwards or in descending a
mountain. When we reach the Arctic regions, or snow-capped summits, or absolute
deserts, the struggle for life is almost exclusively with the elements. That
climate acts in main part indirectly by favouring other species, we may clearly
see in the prodigious number of plants in our gardens which can perfectly well
endure our climate, but which never become naturalised, for they cannot compete
with our native plants, nor resist destruction by our native animals. When a
species, owing to highly favourable circumstances, increases inordinately in
numbers in a small tract, epidemics--at least, this seems generally to occur
with our game animals--often ensue: and here we have a limiting check
independent of the struggle for life. But even some of these so-called epidemics
appear to be due to parasitic worms, which have from some cause, possibly in
part through facility of diffusion amongst the crowded animals, been
disproportionably favoured: and here comes in a sort of struggle between the
parasite and its prey. On the other hand, in many cases, a large stock of
individuals of the same species, relatively to the numbers of its enemies, is
absolutely necessary for its preservation. Thus we can easily raise plenty of
corn and rape-seed, etc., in our fields, because the seeds are in great excess
compared with the number of birds which feed on them; nor can the birds, though
having a superabundance of food at this one season, increase in number
proportionally to the supply of seed, as their numbers are checked during
winter: but any one who has tried, knows how troublesome it is to get seed from
a few wheat or other such plants in a garden; I have in this case lost every
single seed. This view of the necessity of a large stock of the same species for
its preservation, explains, I believe, some singular facts in nature, such as
that of very rare plants being sometimes extremely abundant in the few spots
where they do occur; and that of some social plants being social, that is,
abounding in individuals, even on the extreme confines of their range. For in
such cases, we may believe, that a plant could exist only where the conditions
of its life were so favourable that many could exist together, and thus save
each other from utter destruction. I should add that the good effects of
frequent intercrossing, and the ill effects of close interbreeding, probably
come into play in some of these cases; but on this intricate subject I will not
here enlarge. Many cases are on record showing how complex and unexpected are
the checks and relations between organic beings, which have to struggle together
in the same country. I will give only a single instance, which, though a simple
one, has interested me. In Staffordshire, on the estate of a relation where I
had ample means of investigation, there was a large and extremely barren heath,
which had never been touched by the hand of man; but several hundred acres of
exactly the same nature had been enclosed twenty-five years previously and
planted with Scotch fir. The change in the native vegetation of the planted part
of the heath was most remarkable, more than is generally seen in passing from
one quite different soil to another: not only the proportional numbers of the
heath-plants were wholly changed, but twelve species of plants (not counting
grasses and carices) flourished in the plantations, which could not be found on
the heath. The effect on the insects must have been still greater, for six
insectivorous birds were very common in the plantations, which were not to be
seen on the heath; and the heath was frequented by two or three distinct
insectivorous birds. Here we see how potent has been the effect of the
introduction of a single tree, nothing whatever else having been done, with the
exception that the land had been enclosed, so that cattle could not enter. But
how important an element enclosure is, I plainly saw near Farnham, in Surrey.
Here there are extensive heaths, with a few clumps of old Scotch firs on the
distant hill-tops: within the last ten years large spaces have been enclosed,
and self-sown firs are now springing up in multitudes, so close together that
all cannot live. When I ascertained that these young trees had not been sown or
planted, I was so much surprised at their numbers that I went to several points
of view, whence I could examine hundreds of acres of the unenclosed heath, and
literally I could not see a single Scotch fir, except the old planted clumps.
But on looking closely between the stems of the heath, I found a multitude of
seedlings and little trees, which had been perpetually browsed down by the
cattle. In one square yard, at a point some hundred yards distant from one of
the old clumps, I counted thirty-two little trees; and one of them, judging from
the rings of growth, had during twenty-six years tried to raise its head above
the stems of the heath, and had failed. No wonder that, as soon as the land was
enclosed, it became thickly clothed with vigorously growing young firs. Yet the
heath was so extremely barren and so extensive that no one would ever have
imagined that cattle would have so closely and effectually searched it for food.
Here we see that cattle absolutely determine the existence of the Scotch fir;
but in several parts of the world insects determine the existence of cattle.
Perhaps Paraguay offers the most curious instance of this; for here neither
cattle nor horses nor dogs have ever run wild, though they swarm southward and
northward in a feral state; and Azara and Rengger have shown that this is caused
by the greater number in Paraguay of a certain fly, which lays its eggs in the
navels of these animals when first born. The increase of these flies, numerous
as they are, must be habitually checked by some means, probably by birds. Hence,
if certain insectivorous birds (whose numbers are probably regulated by hawks or
beasts of prey) were to increase in Paraguay, the flies would decrease--then
cattle and horses would become feral, and this would certainly greatly alter (as
indeed I have observed in parts of South America) the vegetation: this again
would largely affect the insects; and this, as we just have seen in
Staffordshire, the insectivorous birds, and so onwards in ever-increasing
circles of complexity. We began this series by insectivorous birds, and we have
ended with them. Not that in nature the relations can ever be as simple as this.
Battle within battle must ever be recurring with varying success; and yet in the
long-run the forces are so nicely balanced, that the face of nature remains
uniform for long periods of time, though assuredly the merest trifle would often
give the victory to one organic being over another. Nevertheless so profound is
our ignorance, and so high our presumption, that we marvel when we hear of the
extinction of an organic being; and as we do not see the cause, we invoke
cataclysms to desolate the world, or invent laws on the duration of the forms of
life! I am tempted to give one more instance showing how plants and animals,
most remote in the scale of nature, are bound together by a web of complex
relations. I shall hereafter have occasion to show that the exotic Lobelia
fulgens, in this part of England, is never visited by insects, and consequently,
from its peculiar structure, never can set a seed. Many of our orchidaceous
plants absolutely require the visits of moths to remove their pollen-masses and
thus to fertilise them. I have, also, reason to believe that humble-bees are
indispensable to the fertilisation of the heartsease (Viola tricolor), for other
bees do not visit this flower. From experiments which I have tried, I have found
that the visits of bees, if not indispensable, are at least highly beneficial to
the fertilisation of our clovers; but humble-bees alone visit the common red
clover (Trifolium pratense), as other bees cannot reach the nectar. Hence I have
very little doubt, that if the whole genus of humble-bees became extinct or very
rare in England, the heartsease and red clover would become very rare, or wholly
disappear. The number of humble-bees in any district depends in a great degree
on the number of field-mice, which destroy their combs and nests; and Mr. H.
Newman, who has long attended to the habits of humble-bees, believes that "more
than two thirds of them are thus destroyed all over England." Now the number of
mice is largely dependent, as every one knows, on the number of cats; and Mr.
Newman says, "Near villages and small towns I have found the nests of
humble-bees more numerous than elsewhere, which I attribute to the number of
cats that destroy the mice." Hence it is quite credible that the presence of a
feline animal in large numbers in a district might determine, through the
intervention first of mice and then of bees, the frequency of certain flowers in
that district! In the case of every species, many different checks, acting at
different periods of life, and during different seasons or years, probably come
into play; some one check or some few being generally the most potent, but all
concurring in determining the average number or even the existence of the
species. In some cases it can be shown that widely-different checks act on the
same species in different districts. When we look at the plants and bushes
clothing an entangled bank, we are tempted to attribute their proportional
numbers and kinds to what we call chance. But how false a view is this! Every
one has heard that when an American forest is cut down, a very different
vegetation springs up; but it has been observed that the trees now growing on
the ancient Indian mounds, in the Southern United States, display the same
beautiful diversity and proportion of kinds as in the surrounding virgin
forests. What a struggle between the several kinds of trees must here have gone
on during long centuries, each annually scattering its seeds by the thousand;
what war between insect and insect--between insects, snails, and other animals
with birds and beasts of prey--all striving to increase, and all feeding on each
other or on the trees or their seeds and seedlings, or on the other plants which
first clothed the ground and thus checked the growth of the trees! Throw up a
handful of feathers, and all must fall to the ground according to definite laws;
but how simple is this problem compared to the action and reaction of the
innumerable plants and animals which have determined, in the course of
centuries, the proportional numbers and kinds of trees now growing on the old
Indian ruins! The dependency of one organic being on another, as of a parasite
on its prey, lies generally between beings remote in the scale of nature. This
is often the case with those which may strictly be said to struggle with each
other for existence, as in the case of locusts and grass-feeding quadrupeds. But
the struggle almost invariably will be most severe between the individuals of
the same species, for they frequent the same districts, require the same food,
and are exposed to the same dangers. In the case of varieties of the same
species, the struggle will generally be almost equally severe, and we sometimes
see the contest soon decided: for instance, if several varieties of wheat be
sown together, and the mixed seed be resown, some of the varieties which best
suit the soil or climate, or are naturally the most fertile, will beat the
others and so yield more seed, and will consequently in a few years quite
supplant the other varieties. To keep up a mixed stock of even such extremely
close varieties as the variously coloured sweet-peas, they must be each year
harvested separately, and the seed then mixed in due proportion, otherwise the
weaker kinds will steadily decrease in numbers and disappear. So again with the
varieties of sheep: it has been asserted that certain mountain-varieties will
starve out other mountain-varieties, so that they cannot be kept together. The
same result has followed from keeping together different varieties of the
medicinal leech. It may even be doubted whether the varieties of any one of our
domestic plants or animals have so exactly the same strength, habits, and
constitution, that the original proportions of a mixed stock could be kept up
for half a dozen generations, if they were allowed to struggle together, like
beings in a state of nature, and if the seed or young were not annually sorted.
As species of the same genus have usually, though by no means invariably, some
similarity in habits and constitution, and always in structure, the struggle
will generally be more severe between species of the same genus, when they come
into competition with each other, than between species of distinct genera. We
see this in the recent extension over parts of the United States of one species
of swallow having caused the decrease of another species. The recent increase of
the missel-thrush in parts of Scotland has caused the decrease of the
song-thrush. How frequently we hear of one species of rat taking the place of
another species under the most different climates! In Russia the small Asiatic
cockroach has everywhere driven before it its great congener. One species of
charlock will supplant another, and so in other cases. We can dimly see why the
competition should be most severe between allied forms, which fill nearly the
same place in the economy of nature; but probably in no one case could we
precisely say why one species has been victorious over another in the great
battle of life. A corollary of the highest importance may be deduced from the
foregoing remarks, namely, that the structure of every organic being is related,
in the most essential yet often hidden manner, to that of all other organic
beings, with which it comes into competition for food or residence, or from
which it has to escape, or on which it preys. This is obvious in the structure
of the teeth and talons of the tiger; and in that of the legs and claws of the
parasite which clings to the hair on the tiger's body. But in the beautifully
plumed seed of the dandelion, and in the flattened and fringed legs of the
water-beetle, the relation seems at first confined to the elements of air and
water. Yet the advantage of plumed seeds no doubt stands in the closest relation
to the land being already thickly clothed by other plants; so that the seeds may
be widely distributed and fall on unoccupied ground. In the water-beetle, the
structure of its legs, so well adapted for diving, allows it to compete with
other aquatic insects, to hunt for its own prey, and to escape serving as prey
to other animals. The store of nutriment laid up within the seeds of many plants
seems at first sight to have no sort of relation to other plants. But from the
strong growth of young plants produced from such seeds (as peas and beans), when
sown in the midst of long grass, I suspect that the chief use of the nutriment
in the seed is to favour the growth of the young seedling, whilst struggling
with other plants growing vigorously all around. Look at a plant in the midst of
its range, why does it not double or quadruple its numbers? We know that it can
perfectly well withstand a little more heat or cold, dampness or dryness, for
elsewhere it ranges into slightly hotter or colder, damper or drier districts.
In this case we can clearly see that if we wished in imagination to give the
plant the power of increasing in number, we should have to give it some
advantage over its competitors, or over the animals which preyed on it. On the
confines of its geographical range, a change of constitution with respect to
climate would clearly be an advantage to our plant; but we have reason to
believe that only a few plants or animals range so far, that they are destroyed
by the rigour of the climate alone. Not until we reach the extreme confines of
life, in the arctic regions or on the borders of an utter desert, will
competition cease. The land may be extremely cold or dry, yet there will be
competition between some few species, or between the individuals of the same
species, for the warmest or dampest spots. Hence, also, we can see that when a
plant or animal is placed in a new country amongst new competitors, though the
climate may be exactly the same as in its former home, yet the conditions of its
life will generally be changed in an essential manner. If we wished to increase
its average numbers in its new home, we should have to modify it in a different
way to what we should have done in its native country; for we should have to
give it some advantage over a different set of competitors or enemies. It is
good thus to try in our imagination to give any form some advantage over
another. Probably in no single instance should we know what to do, so as to
succeed. It will convince us of our ignorance on the mutual relations of all
organic beings; a conviction as necessary, as it seems to be difficult to
acquire. All that we can do, is to keep steadily in mind that each organic being
is striving to increase at a geometrical ratio; that each at some period of its
life, during some season of the year, during each generation or at intervals,
has to struggle for life, and to suffer great destruction. When we reflect on
this struggle, we may console ourselves with the full belief, that the war of
nature is not incessant, that no fear is felt, that death is generally prompt,
and that the vigorous, the healthy, and the happy survive and multiply. CHAPTER
4. NATURAL SELECTION. Natural Selection: its power compared with man's
selection, its power on characters of trifling importance, its power at all ages
and on both sexes. Sexual Selection. On the generality of intercrosses between
individuals of the same species. Circumstances favourable and unfavourable to
Natural Selection, namely, intercrossing, isolation, number of individuals. Slow
action. Extinction caused by Natural Selection. Divergence of Character, related
to the diversity of inhabitants of any small area, and to naturalisation. Action
of Natural Selection, through Divergence of Character and Extinction, on the
descendants from a common parent. Explains the Grouping of all organic beings.
How will the struggle for existence, discussed too briefly in the last chapter,
act in regard to variation? Can the principle of selection, which we have seen
is so potent in the hands of man, apply in nature? I think we shall see that it
can act most effectually. Let it be borne in mind in what an endless number of
strange peculiarities our domestic productions, and, in a lesser degree, those
under nature, vary; and how strong the hereditary tendency is. Under
domestication, it may be truly said that the whole organisation becomes in some
degree plastic. Let it be borne in mind how infinitely complex and close-fitting
are the mutual relations of all organic beings to each other and to their
physical conditions of life. Can it, then, be thought improbable, seeing that
variations useful to man have undoubtedly occurred, that other variations useful
in some way to each being in the great and complex battle of life, should
sometimes occur in the course of thousands of generations? If such do occur, can
we doubt (remembering that many more individuals are born than can possibly
survive) that individuals having any advantage, however slight, over others,
would have the best chance of surviving and of procreating their kind? On the
other hand, we may feel sure that any variation in the least degree injurious
would be rigidly destroyed. This preservation of favourable variations and the
rejection of injurious variations, I call Natural Selection. Variations neither
useful nor injurious would not be affected by natural selection, and would be
left a fluctuating element, as perhaps we see in the species called polymorphic.
We shall best understand the probable course of natural selection by taking the
case of a country undergoing some physical change, for instance, of climate. The
proportional numbers of its inhabitants would almost immediately undergo a
change, and some species might become extinct. We may conclude, from what we
have seen of the intimate and complex manner in which the inhabitants of each
country are bound together, that any change in the numerical proportions of some
of the inhabitants, independently of the change of climate itself, would most
seriously affect many of the others. If the country were open on its borders,
new forms would certainly immigrate, and this also would seriously disturb the
relations of some of the former inhabitants. Let it be remembered how powerful
the influence of a single introduced tree or mammal has been shown to be. But in
the case of an island, or of a country partly surrounded by barriers, into which
new and better adapted forms could not freely enter, we should then have places
in the economy of nature which would assuredly be better filled up, if some of
the original inhabitants were in some manner modified; for, had the area been
open to immigration, these same places would have been seized on by intruders.
In such case, every slight modification, which in the course of ages chanced to
arise, and which in any way favoured the individuals of any of the species, by
better adapting them to their altered conditions, would tend to be preserved;
and natural selection would thus have free scope for the work of improvement. We
have reason to believe, as stated in the first chapter, that a change in the
conditions of life, by specially acting on the reproductive system, causes or
increases variability; and in the foregoing case the conditions of life are
supposed to have undergone a change, and this would manifestly be favourable to
natural selection, by giving a better chance of profitable variations occurring;
and unless profitable variations do occur, natural selection can do nothing. Not
that, as I believe, any extreme amount of variability is necessary; as man can
certainly produce great results by adding up in any given direction mere
individual differences, so could Nature, but far more easily, from having
incomparably longer time at her disposal. Nor do I believe that any great
physical change, as of climate, or any unusual degree of isolation to check
immigration, is actually necessary to produce new and unoccupied places for
natural selection to fill up by modifying and improving some of the varying
inhabitants. For as all the inhabitants of each country are struggling together
with nicely balanced forces, extremely slight modifications in the structure or
habits of one inhabitant would often give it an advantage over others; and still
further modifications of the same kind would often still further increase the
advantage. No country can be named in which all the native inhabitants are now
so perfectly adapted to each other and to the physical conditions under which
they live, that none of them could anyhow be improved; for in all countries, the
natives have been so far conquered by naturalised productions, that they have
allowed foreigners to take firm possession of the land. And as foreigners have
thus everywhere beaten some of the natives, we may safely conclude that the
natives might have been modified with advantage, so as to have better resisted
such intruders. As man can produce and certainly has produced a great result by
his methodical and unconscious means of selection, what may not nature effect?
Man can act only on external and visible characters: nature cares nothing for
appearances, except in so far as they may be useful to any being. She can act on
every internal organ, on every shade of constitutional difference, on the whole
machinery of life. Man selects only for his own good; Nature only for that of
the being which she tends. Every selected character is fully exercised by her;
and the being is placed under well-suited conditions of life. Man keeps the
natives of many climates in the same country; he seldom exercises each selected
character in some peculiar and fitting manner; he feeds a long and a short
beaked pigeon on the same food; he does not exercise a long-backed or
long-legged quadruped in any peculiar manner; he exposes sheep with long and
short wool to the same climate. He does not allow the most vigorous males to
struggle for the females. He does not rigidly destroy all inferior animals, but
protects during each varying season, as far as lies in his power, all his
productions. He often begins his selection by some half-monstrous form; or at
least by some modification prominent enough to catch his eye, or to be plainly
useful to him. Under nature, the slightest difference of structure or
constitution may well turn the nicely-balanced scale in the struggle for life,
and so be preserved. How fleeting are the wishes and efforts of man! how short
his time! and consequently how poor will his products be, compared with those
accumulated by nature during whole geological periods. Can we wonder, then, that
nature's productions should be far "truer" in character than man's productions;
that they should be infinitely better adapted to the most complex conditions of
life, and should plainly bear the stamp of far higher workmanship? It may be
said that natural selection is daily and hourly scrutinising, throughout the
world, every variation, even the slightest; rejecting that which is bad,
preserving and adding up all that is good; silently and insensibly working,
whenever and wherever opportunity offers, at the improvement of each organic
being in relation to its organic and inorganic conditions of life. We see
nothing of these slow changes in progress, until the hand of time has marked the
long lapse of ages, and then so imperfect is our view into long past geological
ages, that we only see that the forms of life are now different from what they
formerly were. Although natural selection can act only through and for the good
of each being, yet characters and structures, which we are apt to consider as of
very trifling importance, may thus be acted on. When we see leaf-eating insects
green, and bark-feeders mottled-grey; the alpine ptarmigan white in winter, the
red-grouse the colour of heather, and the black-grouse that of peaty earth, we
must believe that these tints are of service to these birds and insects in
preserving them from danger. Grouse, if not destroyed at some period of their
lives, would increase in countless numbers; they are known to suffer largely
from birds of prey; and hawks are guided by eyesight to their prey,--so much so,
that on parts of the Continent persons are warned not to keep white pigeons, as
being the most liable to destruction. Hence I can see no reason to doubt that
natural selection might be most effective in giving the proper colour to each
kind of grouse, and in keeping that colour, when once acquired, true and
constant. Nor ought we to think that the occasional destruction of an animal of
any particular colour would produce little effect: we should remember how
essential it is in a flock of white sheep to destroy every lamb with the
faintest trace of black. In plants the down on the fruit and the colour of the
flesh are considered by botanists as characters of the most trifling importance:
yet we hear from an excellent horticulturist, Downing, that in the United States
smooth-skinned fruits suffer far more from a beetle, a curculio, than those with
down; that purple plums suffer far more from a certain disease than yellow
plums; whereas another disease attacks yellow-fleshed peaches far more than
those with other coloured flesh. If, with all the aids of art, these slight
differences make a great difference in cultivating the several varieties,
assuredly, in a state of nature, where the trees would have to struggle with
other trees and with a host of enemies, such differences would effectually
settle which variety, whether a smooth or downy, a yellow or purple fleshed
fruit, should succeed. In looking at many small points of difference between
species, which, as far as our ignorance permits us to judge, seem to be quite
unimportant, we must not forget that climate, food, etc., probably produce some
slight and direct effect. It is, however, far more necessary to bear in mind
that there are many unknown laws of correlation of growth, which, when one part
of the organisation is modified through variation, and the modifications are
accumulated by natural selection for the good of the being, will cause other
modifications, often of the most unexpected nature. As we see that those
variations which under domestication appear at any particular period of life,
tend to reappear in the offspring at the same period;--for instance, in the
seeds of the many varieties of our culinary and agricultural plants; in the
caterpillar and cocoon stages of the varieties of the silkworm; in the eggs of
poultry, and in the colour of the down of their chickens; in the horns of our
sheep and cattle when nearly adult;--so in a state of nature, natural selection
will be enabled to act on and modify organic beings at any age, by the
accumulation of profitable variations at that age, and by their inheritance at a
corresponding age. If it profit a plant to have its seeds more and more widely
disseminated by the wind, I can see no greater difficulty in this being effected
through natural selection, than in the cotton-planter increasing and improving
by selection the down in the pods on his cotton-trees. Natural selection may
modify and adapt the larva of an insect to a score of contingencies, wholly
different from those which concern the mature insect. These modifications will
no doubt affect, through the laws of correlation, the structure of the adult;
and probably in the case of those insects which live only for a few hours, and
which never feed, a large part of their structure is merely the correlated
result of successive changes in the structure of their larvae. So, conversely,
modifications in the adult will probably often affect the structure of the
larva; but in all cases natural selection will ensure that modifications
consequent on other modifications at a different period of life, shall not be in
the least degree injurious: for if they became so, they would cause the
extinction of the species. Natural selection will modify the structure of the
young in relation to the parent, and of the parent in relation to the young. In
social animals it will adapt the structure of each individual for the benefit of
the community; if each in consequence profits by the selected change. What
natural selection cannot do, is to modify the structure of one species, without
giving it any advantage, for the good of another species; and though statements
to this effect may be found in works of natural history, I cannot find one case
which will bear investigation. A structure used only once in an animal's whole
life, if of high importance to it, might be modified to any extent by natural
selection; for instance, the great jaws possessed by certain insects, and used
exclusively for opening the cocoon--or the hard tip to the beak of nestling
birds, used for breaking the egg. It has been asserted, that of the best
short-beaked tumbler-pigeons more perish in the egg than are able to get out of
it; so that fanciers assist in the act of hatching. Now, if nature had to make
the beak of a full-grown pigeon very short for the bird's own advantage, the
process of modification would be very slow, and there would be simultaneously
the most rigorous selection of the young birds within the egg, which had the
most powerful and hardest beaks, for all with weak beaks would inevitably
perish: or, more delicate and more easily broken shells might be selected, the
thickness of the shell being known to vary like every other structure. SEXUAL
SELECTION. Inasmuch as peculiarities often appear under domestication in one sex
and become hereditarily attached to that sex, the same fact probably occurs
under nature, and if so, natural selection will be able to modify one sex in its
functional relations to the other sex, or in relation to wholly different habits
of life in the two sexes, as is sometimes the case with insects. And this leads
me to say a few words on what I call Sexual Selection. This depends, not on a
struggle for existence, but on a struggle between the males for possession of
the females; the result is not death to the unsuccessful competitor, but few or
no offspring. Sexual selection is, therefore, less rigorous than natural
selection. Generally, the most vigorous males, those which are best fitted for
their places in nature, will leave most progeny. But in many cases, victory will
depend not on general vigour, but on having special weapons, confined to the
male sex. A hornless stag or spurless cock would have a poor chance of leaving
offspring. Sexual selection by always allowing the victor to breed might surely
give indomitable courage, length to the spur, and strength to the wing to strike
in the spurred leg, as well as the brutal cock-fighter, who knows well that he
can improve his breed by careful selection of the best cocks. How low in the
scale of nature this law of battle descends, I know not; male alligators have
been described as fighting, bellowing, and whirling round, like Indians in a
war-dance, for the possession of the females; male salmons have been seen
fighting all day long; male stag-beetles often bear wounds from the huge
mandibles of other males. The war is, perhaps, severest between the males of
polygamous animals, and these seem oftenest provided with special weapons. The
males of carnivorous animals are already well armed; though to them and to
others, special means of defence may be given through means of sexual selection,
as the mane to the lion, the shoulder-pad to the boar, and the hooked jaw to the
male salmon; for the shield may be as important for victory, as the sword or
spear. Amongst birds, the contest is often of a more peaceful character. All
those who have attended to the subject, believe that there is the severest
rivalry between the males of many species to attract by singing the females. The
rock-thrush of Guiana, birds of Paradise, and some others, congregate; and
successive males display their gorgeous plumage and perform strange antics
before the females, which standing by as spectators, at last choose the most
attractive partner. Those who have closely attended to birds in confinement well
know that they often take individual preferences and dislikes: thus Sir R. Heron
has described how one pied peacock was eminently attractive to all his hen
birds. It may appear childish to attribute any effect to such apparently weak
means: I cannot here enter on the details necessary to support this view; but if
man can in a short time give elegant carriage and beauty to his bantams,
according to his standard of beauty, I can see no good reason to doubt that
female birds, by selecting, during thousands of generations, the most melodious
or beautiful males, according to their standard of beauty, might produce a
marked effect. I strongly suspect that some well-known laws with respect to the
plumage of male and female birds, in comparison with the plumage of the young,
can be explained on the view of plumage having been chiefly modified by sexual
selection, acting when the birds have come to the breeding age or during the
breeding season; the modifications thus produced being inherited at
corresponding ages or seasons, either by the males alone, or by the males and
females; but I have not space here to enter on this subject. Thus it is, as I
believe, that when the males and females of any animal have the same general
habits of life, but differ in structure, colour, or ornament, such differences
have been mainly caused by sexual selection; that is, individual males have had,
in successive generations, some slight advantage over other males, in their
weapons, means of defence, or charms; and have transmitted these advantages to
their male offspring. Yet, I would not wish to attribute all such sexual
differences to this agency: for we see peculiarities arising and becoming
attached to the male sex in our domestic animals (as the wattle in male
carriers, horn-like protuberances in the cocks of certain fowls, etc.), which we
cannot believe to be either useful to the males in battle, or attractive to the
females. We see analogous cases under nature, for instance, the tuft of hair on
the breast of the turkey-cock, which can hardly be either useful or ornamental
to this bird;--indeed, had the tuft appeared under domestication, it would have
been called a monstrosity. ILLUSTRATIONS OF THE ACTION OF NATURAL SELECTION. In
order to make it clear how, as I believe, natural selection acts, I must beg
permission to give one or two imaginary illustrations. Let us take the case of a
wolf, which preys on various animals, securing some by craft, some by strength,
and some by fleetness; and let us suppose that the fleetest prey, a deer for
instance, had from any change in the country increased in numbers, or that other
prey had decreased in numbers, during that season of the year when the wolf is
hardest pressed for food. I can under such circumstances see no reason to doubt
that the swiftest and slimmest wolves would have the best chance of surviving,
and so be preserved or selected,--provided always that they retained strength to
master their prey at this or at some other period of the year, when they might
be compelled to prey on other animals. I can see no more reason to doubt this,
than that man can improve the fleetness of his greyhounds by careful and
methodical selection, or by that unconscious selection which results from each
man trying to keep the best dogs without any thought of modifying the breed.
Even without any change in the proportional numbers of the animals on which our
wolf preyed, a cub might be born with an innate tendency to pursue certain kinds
of prey. Nor can this be thought very improbable; for we often observe great
differences in the natural tendencies of our domestic animals; one cat, for
instance, taking to catch rats, another mice; one cat, according to Mr. St.
John, bringing home winged game, another hares or rabbits, and another hunting
on marshy ground and almost nightly catching woodcocks or snipes. The tendency
to catch rats rather than mice is known to be inherited. Now, if any slight
innate change of habit or of structure benefited an individual wolf, it would
have the best chance of surviving and of leaving offspring. Some of its young
would probably inherit the same habits or structure, and by the repetition of
this process, a new variety might be formed which would either supplant or
coexist with the parent-form of wolf. Or, again, the wolves inhabiting a
mountainous district, and those frequenting the lowlands, would naturally be
forced to hunt different prey; and from the continued preservation of the
individuals best fitted for the two sites, two varieties might slowly be formed.
These varieties would cross and blend where they met; but to this subject of
intercrossing we shall soon have to return. I may add, that, according to Mr.
Pierce, there are two varieties of the wolf inhabiting the Catskill Mountains in
the United States, one with a light greyhound-like form, which pursues deer, and
the other more bulky, with shorter legs, which more frequently attacks the
shepherd's flocks. Let us now take a more complex case. Certain plants excrete a
sweet juice, apparently for the sake of eliminating something injurious from
their sap: this is effected by glands at the base of the stipules in some
Leguminosae, and at the back of the leaf of the common laurel. This juice,
though small in quantity, is greedily sought by insects. Let us now suppose a
little sweet juice or nectar to be excreted by the inner bases of the petals of
a flower. In this case insects in seeking the nectar would get dusted with
pollen, and would certainly often transport the pollen from one flower to the
stigma of another flower. The flowers of two distinct individuals of the same
species would thus get crossed; and the act of crossing, we have good reason to
believe (as will hereafter be more fully alluded to), would produce very
vigorous seedlings, which consequently would have the best chance of flourishing
and surviving. Some of these seedlings would probably inherit the
nectar-excreting power. Those individual flowers which had the largest glands or
nectaries, and which excreted most nectar, would be oftenest visited by insects,
and would be oftenest crossed; and so in the long-run would gain the upper hand.
Those flowers, also, which had their stamens and pistils placed, in relation to
the size and habits of the particular insects which visited them, so as to
favour in any degree the transportal of their pollen from flower to flower,
would likewise be favoured or selected. We might have taken the case of insects
visiting flowers for the sake of collecting pollen instead of nectar; and as
pollen is formed for the sole object of fertilisation, its destruction appears a
simple loss to the plant; yet if a little pollen were carried, at first
occasionally and then habitually, by the pollen-devouring insects from flower to
flower, and a cross thus effected, although nine-tenths of the pollen were
destroyed, it might still be a great gain to the plant; and those individuals
which produced more and more pollen, and had larger and larger anthers, would be
selected. When our plant, by this process of the continued preservation or
natural selection of more and more attractive flowers, had been rendered highly
attractive to insects, they would, unintentionally on their part, regularly
carry pollen from flower to flower; and that they can most effectually do this,
I could easily show by many striking instances. I will give only one--not as a
very striking case, but as likewise illustrating one step in the separation of
the sexes of plants, presently to be alluded to. Some holly-trees bear only male
flowers, which have four stamens producing rather a small quantity of pollen,
and a rudimentary pistil; other holly-trees bear only female flowers; these have
a full-sized pistil, and four stamens with shrivelled anthers, in which not a
grain of pollen can be detected. Having found a female tree exactly sixty yards
from a male tree, I put the stigmas of twenty flowers, taken from different
branches, under the microscope, and on all, without exception, there were
pollen-grains, and on some a profusion of pollen. As the wind had set for
several days from the female to the male tree, the pollen could not thus have
been carried. The weather had been cold and boisterous, and therefore not
favourable to bees, nevertheless every female flower which I examined had been
effectually fertilised by the bees, accidentally dusted with pollen, having
flown from tree to tree in search of nectar. But to return to our imaginary
case: as soon as the plant had been rendered so highly attractive to insects
that pollen was regularly carried from flower to flower, another process might
commence. No naturalist doubts the advantage of what has been called the
"physiological division of labour;" hence we may believe that it would be
advantageous to a plant to produce stamens alone in one flower or on one whole
plant, and pistils alone in another flower or on another plant. In plants under
culture and placed under new conditions of life, sometimes the male organs and
sometimes the female organs become more or less impotent; now if we suppose this
to occur in ever so slight a degree under nature, then as pollen is already
carried regularly from flower to flower, and as a more complete separation of
the sexes of our plant would be advantageous on the principle of the division of
labour, individuals with this tendency more and more increased, would be
continually favoured or selected, until at last a complete separation of the
sexes would be effected. Let us now turn to the nectar-feeding insects in our
imaginary case: we may suppose the plant of which we have been slowly increasing
the nectar by continued selection, to be a common plant; and that certain
insects depended in main part on its nectar for food. I could give many facts,
showing how anxious bees are to save time; for instance, their habit of cutting
holes and sucking the nectar at the bases of certain flowers, which they can,
with a very little more trouble, enter by the mouth. Bearing such facts in mind,
I can see no reason to doubt that an accidental deviation in the size and form
of the body, or in the curvature and length of the proboscis, etc., far too
slight to be appreciated by us, might profit a bee or other insect, so that an
individual so characterised would be able to obtain its food more quickly, and
so have a better chance of living and leaving descendants. Its descendants would
probably inherit a tendency to a similar slight deviation of structure. The
tubes of the corollas of the common red and incarnate clovers (Trifolium
pratense and incarnatum) do not on a hasty glance appear to differ in length;
yet the hive-bee can easily suck the nectar out of the incarnate clover, but not
out of the common red clover, which is visited by humble-bees alone; so that
whole fields of the red clover offer in vain an abundant supply of precious
nectar to the hive-bee. Thus it might be a great advantage to the hive-bee to
have a slightly longer or differently constructed proboscis. On the other hand,
I have found by experiment that the fertility of clover greatly depends on bees
visiting and moving parts of the corolla, so as to push the pollen on to the
stigmatic surface. Hence, again, if humble-bees were to become rare in any
country, it might be a great advantage to the red clover to have a shorter or
more deeply divided tube to its corolla, so that the hive-bee could visit its
flowers. Thus I can understand how a flower and a bee might slowly become,
either simultaneously or one after the other, modified and adapted in the most
perfect manner to each other, by the continued preservation of individuals
presenting mutual and slightly favourable deviations of structure. I am well
aware that this doctrine of natural selection, exemplified in the above
imaginary instances, is open to the same objections which were at first urged
against Sir Charles Lyell's noble views on "the modern changes of the earth, as
illustrative of geology;" but we now very seldom hear the action, for instance,
of the coast-waves, called a trifling and insignificant cause, when applied to
the excavation of gigantic valleys or to the formation of the longest lines of
inland cliffs. Natural selection can act only by the preservation and
accumulation of infinitesimally small inherited modifications, each profitable
to the preserved being; and as modern geology has almost banished such views as
the excavation of a great valley by a single diluvial wave, so will natural
selection, if it be a true principle, banish the belief of the continued
creation of new organic beings, or of any great and sudden modification in their
structure. ON THE INTERCROSSING OF INDIVIDUALS. I must here introduce a short
digression. In the case of animals and plants with separated sexes, it is of
course obvious that two individuals must always unite for each birth; but in the
case of hermaphrodites this is far from obvious. Nevertheless I am strongly
inclined to believe that with all hermaphrodites two individuals, either
occasionally or habitually, concur for the reproduction of their kind. This
view, I may add, was first suggested by Andrew Knight. We shall presently see
its importance; but I must here treat the subject with extreme brevity, though I
have the materials prepared for an ample discussion. All vertebrate animals, all
insects, and some other large groups of animals, pair for each birth. Modern
research has much diminished the number of supposed hermaphrodites, and of real
hermaphrodites a large number pair; that is, two individuals regularly unite for
reproduction, which is all that concerns us. But still there are many
hermaphrodite animals which certainly do not habitually pair, and a vast
majority of plants are hermaphrodites. What reason, it may be asked, is there
for supposing in these cases that two individuals ever concur in reproduction?
As it is impossible here to enter on details, I must trust to some general
considerations alone. In the first place, I have collected so large a body of
facts, showing, in accordance with the almost universal belief of breeders, that
with animals and plants a cross between different varieties, or between
individuals of the same variety but of another strain, gives vigour and
fertility to the offspring; and on the other hand, that CLOSE interbreeding
diminishes vigour and fertility; that these facts alone incline me to believe
that it is a general law of nature (utterly ignorant though we be of the meaning
of the law) that no organic being self-fertilises itself for an eternity of
generations; but that a cross with another individual is occasionally--perhaps
at very long intervals--indispensable. On the belief that this is a law of
nature, we can, I think, understand several large classes of facts, such as the
following, which on any other view are inexplicable. Every hybridizer knows how
unfavourable exposure to wet is to the fertilisation of a flower, yet what a
multitude of flowers have their anthers and stigmas fully exposed to the
weather! but if an occasional cross be indispensable, the fullest freedom for
the entrance of pollen from another individual will explain this state of
exposure, more especially as the plant's own anthers and pistil generally stand
so close together that self-fertilisation seems almost inevitable. Many flowers,
on the other hand, have their organs of fructification closely enclosed, as in
the great papilionaceous or pea-family; but in several, perhaps in all, such
flowers, there is a very curious adaptation between the structure of the flower
and the manner in which bees suck the nectar; for, in doing this, they either
push the flower's own pollen on the stigma, or bring pollen from another flower.
So necessary are the visits of bees to papilionaceous flowers, that I have
found, by experiments published elsewhere, that their fertility is greatly
diminished if these visits be prevented. Now, it is scarcely possible that bees
should fly from flower to flower, and not carry pollen from one to the other, to
the great good, as I believe, of the plant. Bees will act like a camel-hair
pencil, and it is quite sufficient just to touch the anthers of one flower and
then the stigma of another with the same brush to ensure fertilisation; but it
must not be supposed that bees would thus produce a multitude of hybrids between
distinct species; for if you bring on the same brush a plant's own pollen and
pollen from another species, the former will have such a prepotent effect, that
it will invariably and completely destroy, as has been shown by Gartner, any
influence from the foreign pollen. When the stamens of a flower suddenly spring
towards the pistil, or slowly move one after the other towards it, the
contrivance seems adapted solely to ensure self-fertilisation; and no doubt it
is useful for this end: but, the agency of insects is often required to cause
the stamens to spring forward, as Kolreuter has shown to be the case with the
barberry; and curiously in this very genus, which seems to have a special
contrivance for self-fertilisation, it is well known that if very closely-allied
forms or varieties are planted near each other, it is hardly possible to raise
pure seedlings, so largely do they naturally cross. In many other cases, far
from there being any aids for self-fertilisation, there are special
contrivances, as I could show from the writings of C. C. Sprengel and from my
own observations, which effectually prevent the stigma receiving pollen from its
own flower: for instance, in Lobelia fulgens, there is a really beautiful and
elaborate contrivance by which every one of the infinitely numerous
pollen-granules are swept out of the conjoined anthers of each flower, before
the stigma of that individual flower is ready to receive them; and as this
flower is never visited, at least in my garden, by insects, it never sets a
seed, though by placing pollen from one flower on the stigma of another, I
raised plenty of seedlings; and whilst another species of Lobelia growing close
by, which is visited by bees, seeds freely. In very many other cases, though
there be no special mechanical contrivance to prevent the stigma of a flower
receiving its own pollen, yet, as C. C. Sprengel has shown, and as I can
confirm, either the anthers burst before the stigma is ready for fertilisation,
or the stigma is ready before the pollen of that flower is ready, so that these
plants have in fact separated sexes, and must habitually be crossed. How strange
are these facts! How strange that the pollen and stigmatic surface of the same
flower, though placed so close together, as if for the very purpose of
self-fertilisation, should in so many cases be mutually useless to each other!
How simply are these facts explained on the view of an occasional cross with a
distinct individual being advantageous or indispensable! If several varieties of
the cabbage, radish, onion, and of some other plants, be allowed to seed near
each other, a large majority, as I have found, of the seedlings thus raised will
turn out mongrels: for instance, I raised 233 seedling cabbages from some plants
of different varieties growing near each other, and of these only 78 were true
to their kind, and some even of these were not perfectly true. Yet the pistil of
each cabbage-flower is surrounded not only by its own six stamens, but by those
of the many other flowers on the same plant. How, then, comes it that such a
vast number of the seedlings are mongrelized? I suspect that it must arise from
the pollen of a distinct VARIETY having a prepotent effect over a flower's own
pollen; and that this is part of the general law of good being derived from the
intercrossing of distinct individuals of the same species. When distinct SPECIES
are crossed the case is directly the reverse, for a plant's own pollen is always
prepotent over foreign pollen; but to this subject we shall return in a future
chapter. In the case of a gigantic tree covered with innumerable flowers, it may
be objected that pollen could seldom be carried from tree to tree, and at most
only from flower to flower on the same tree, and that flowers on the same tree
can be considered as distinct individuals only in a limited sense. I believe
this objection to be valid, but that nature has largely provided against it by
giving to trees a strong tendency to bear flowers with separated sexes. When the
sexes are separated, although the male and female flowers may be produced on the
same tree, we can see that pollen must be regularly carried from flower to
flower; and this will give a better chance of pollen being occasionally carried
from tree to tree. That trees belonging to all Orders have their sexes more
often separated than other plants, I find to be the case in this country; and at
my request Dr. Hooker tabulated the trees of New Zealand, and Dr. Asa Gray those
of the United States, and the result was as I anticipated. On the other hand,
Dr. Hooker has recently informed me that he finds that the rule does not hold in
Australia; and I have made these few remarks on the sexes of trees simply to
call attention to the subject. Turning for a very brief space to animals: on the
land there are some hermaphrodites, as land-mollusca and earth-worms; but these
all pair. As yet I have not found a single case of a terrestrial animal which
fertilises itself. We can understand this remarkable fact, which offers so
strong a contrast with terrestrial plants, on the view of an occasional cross
being indispensable, by considering the medium in which terrestrial animals
live, and the nature of the fertilising element; for we know of no means,
analogous to the action of insects and of the wind in the case of plants, by
which an occasional cross could be effected with terrestrial animals without the
concurrence of two individuals. Of aquatic animals, there are many
self-fertilising hermaphrodites; but here currents in the water offer an obvious
means for an occasional cross. And, as in the case of flowers, I have as yet
failed, after consultation with one of the highest authorities, namely,
Professor Huxley, to discover a single case of an hermaphrodite animal with the
organs of reproduction so perfectly enclosed within the body, that access from
without and the occasional influence of a distinct individual can be shown to be
physically impossible. Cirripedes long appeared to me to present a case of very
great difficulty under this point of view; but I have been enabled, by a
fortunate chance, elsewhere to prove that two individuals, though both are
self-fertilising hermaphrodites, do sometimes cross. It must have struck most
naturalists as a strange anomaly that, in the case of both animals and plants,
species of the same family and even of the same genus, though agreeing closely
with each other in almost their whole organisation, yet are not rarely, some of
them hermaphrodites, and some of them unisexual. But if, in fact, all
hermaphrodites do occasionally intercross with other individuals, the difference
between hermaphrodites and unisexual species, as far as function is concerned,
becomes very small. From these several considerations and from the many special
facts which I have collected, but which I am not here able to give, I am
strongly inclined to suspect that, both in the vegetable and animal kingdoms, an
occasional intercross with a distinct individual is a law of nature. I am well
aware that there are, on this view, many cases of difficulty, some of which I am
trying to investigate. Finally then, we may conclude that in many organic
beings, a cross between two individuals is an obvious necessity for each birth;
in many others it occurs perhaps only at long intervals; but in none, as I
suspect, can self-fertilisation go on for perpetuity. CIRCUMSTANCES FAVOURABLE
TO NATURAL SELECTION. This is an extremely intricate subject. A large amount of
inheritable and diversified variability is favourable, but I believe mere
individual differences suffice for the work. A large number of individuals, by
giving a better chance for the appearance within any given period of profitable
variations, will compensate for a lesser amount of variability in each
individual, and is, I believe, an extremely important element of success. Though
nature grants vast periods of time for the work of natural selection, she does
not grant an indefinite period; for as all organic beings are striving, it may
be said, to seize on each place in the economy of nature, if any one species
does not become modified and improved in a corresponding degree with its
competitors, it will soon be exterminated. In man's methodical selection, a
breeder selects for some definite object, and free intercrossing will wholly
stop his work. But when many men, without intending to alter the breed, have a
nearly common standard of perfection, and all try to get and breed from the best
animals, much improvement and modification surely but slowly follow from this
unconscious process of selection, notwithstanding a large amount of crossing
with inferior animals. Thus it will be in nature; for within a confined area,
with some place in its polity not so perfectly occupied as might be, natural
selection will always tend to preserve all the individuals varying in the right
direction, though in different degrees, so as better to fill up the unoccupied
place. But if the area be large, its several districts will almost certainly
present different conditions of life; and then if natural selection be modifying
and improving a species in the several districts, there will be intercrossing
with the other individuals of the same species on the confines of each. And in
this case the effects of intercrossing can hardly be counterbalanced by natural
selection always tending to modify all the individuals in each district in
exactly the same manner to the conditions of each; for in a continuous area, the
conditions will generally graduate away insensibly from one district to another.
The intercrossing will most affect those animals which unite for each birth,
which wander much, and which do not breed at a very quick rate. Hence in animals
of this nature, for instance in birds, varieties will generally be confined to
separated countries; and this I believe to be the case. In hermaphrodite
organisms which cross only occasionally, and likewise in animals which unite for
each birth, but which wander little and which can increase at a very rapid rate,
a new and improved variety might be quickly formed on any one spot, and might
there maintain itself in a body, so that whatever intercrossing took place would
be chiefly between the individuals of the same new variety. A local variety when
once thus formed might subsequently slowly spread to other districts. On the
above principle, nurserymen always prefer getting seed from a large body of
plants of the same variety, as the chance of intercrossing with other varieties
is thus lessened. Even in the case of slow-breeding animals, which unite for
each birth, we must not overrate the effects of intercrosses in retarding
natural selection; for I can bring a considerable catalogue of facts, showing
that within the same area, varieties of the same animal can long remain
distinct, from haunting different stations, from breeding at slightly different
seasons, or from varieties of the same kind preferring to pair together.
Intercrossing plays a very important part in nature in keeping the individuals
of the same species, or of the same variety, true and uniform in character. It
will obviously thus act far more efficiently with those animals which unite for
each birth; but I have already attempted to show that we have reason to believe
that occasional intercrosses take place with all animals and with all plants.
Even if these take place only at long intervals, I am convinced that the young
thus produced will gain so much in vigour and fertility over the offspring from
long-continued self-fertilisation, that they will have a better chance of
surviving and propagating their kind; and thus, in the long run, the influence
of intercrosses, even at rare intervals, will be great. If there exist organic
beings which never intercross, uniformity of character can be retained amongst
them, as long as their conditions of life remain the same, only through the
principle of inheritance, and through natural selection destroying any which
depart from the proper type; but if their conditions of life change and they
undergo modification, uniformity of character can be given to their modified
offspring, solely by natural selection preserving the same favourable
variations. Isolation, also, is an important element in the process of natural
selection. In a confined or isolated area, if not very large, the organic and
inorganic conditions of life will generally be in a great degree uniform; so
that natural selection will tend to modify all the individuals of a varying
species throughout the area in the same manner in relation to the same
conditions. Intercrosses, also, with the individuals of the same species, which
otherwise would have inhabited the surrounding and differently circumstanced
districts, will be prevented. But isolation probably acts more efficiently in
checking the immigration of better adapted organisms, after any physical change,
such as of climate or elevation of the land, etc.; and thus new places in the
natural economy of the country are left open for the old inhabitants to struggle
for, and become adapted to, through modifications in their structure and
constitution. Lastly, isolation, by checking immigration and consequently
competition, will give time for any new variety to be slowly improved; and this
may sometimes be of importance in the production of new species. If, however, an
isolated area be very small, either from being surrounded by barriers, or from
having very peculiar physical conditions, the total number of the individuals
supported on it will necessarily be very small; and fewness of individuals will
greatly retard the production of new species through natural selection, by
decreasing the chance of the appearance of favourable variations. If we turn to
nature to test the truth of these remarks, and look at any small isolated area,
such as an oceanic island, although the total number of the species inhabiting
it, will be found to be small, as we shall see in our chapter on geographical
distribution; yet of these species a very large proportion are endemic,--that
is, have been produced there, and nowhere else. Hence an oceanic island at first
sight seems to have been highly favourable for the production of new species.
But we may thus greatly deceive ourselves, for to ascertain whether a small
isolated area, or a large open area like a continent, has been most favourable
for the production of new organic forms, we ought to make the comparison within
equal times; and this we are incapable of doing. Although I do not doubt that
isolation is of considerable importance in the production of new species, on the
whole I am inclined to believe that largeness of area is of more importance,
more especially in the production of species, which will prove capable of
enduring for a long period, and of spreading widely. Throughout a great and open
area, not only will there be a better chance of favourable variations arising
from the large number of individuals of the same species there supported, but
the conditions of life are infinitely complex from the large number of already
existing species; and if some of these many species become modified and
improved, others will have to be improved in a corresponding degree or they will
be exterminated. Each new form, also, as soon as it has been much improved, will
be able to spread over the open and continuous area, and will thus come into
competition with many others. Hence more new places will be formed, and the
competition to fill them will be more severe, on a large than on a small and
isolated area. Moreover, great areas, though now continuous, owing to
oscillations of level, will often have recently existed in a broken condition,
so that the good effects of isolation will generally, to a certain extent, have
concurred. Finally, I conclude that, although small isolated areas probably have
been in some respects highly favourable for the production of new species, yet
that the course of modification will generally have been more rapid on large
areas; and what is more important, that the new forms produced on large areas,
which already have been victorious over many competitors, will be those that
will spread most widely, will give rise to most new varieties and species, and
will thus play an important part in the changing history of the organic world.
We can, perhaps, on these views, understand some facts which will be again
alluded to in our chapter on geographical distribution; for instance, that the
productions of the smaller continent of Australia have formerly yielded, and
apparently are now yielding, before those of the larger Europaeo-Asiatic area.
Thus, also, it is that continental productions have everywhere become so largely
naturalised on islands. On a small island, the race for life will have been less
severe, and there will have been less modification and less extermination.
Hence, perhaps, it comes that the flora of Madeira, according to Oswald Heer,
resembles the extinct tertiary flora of Europe. All fresh-water basins, taken
together, make a small area compared with that of the sea or of the land; and,
consequently, the competition between fresh-water productions will have been
less severe than elsewhere; new forms will have been more slowly formed, and old
forms more slowly exterminated. And it is in fresh water that we find seven
genera of Ganoid fishes, remnants of a once preponderant order: and in fresh
water we find some of the most anomalous forms now known in the world, as the
Ornithorhynchus and Lepidosiren, which, like fossils, connect to a certain
extent orders now widely separated in the natural scale. These anomalous forms
may almost be called living fossils; they have endured to the present day, from
having inhabited a confined area, and from having thus been exposed to less
severe competition. To sum up the circumstances favourable and unfavourable to
natural selection, as far as the extreme intricacy of the subject permits. I
conclude, looking to the future, that for terrestrial productions a large
continental area, which will probably undergo many oscillations of level, and
which consequently will exist for long periods in a broken condition, will be
the most favourable for the production of many new forms of life, likely to
endure long and to spread widely. For the area will first have existed as a
continent, and the inhabitants, at this period numerous in individuals and
kinds, will have been subjected to very severe competition. When converted by
subsidence into large separate islands, there will still exist many individuals
of the same species on each island: intercrossing on the confines of the range
of each species will thus be checked: after physical changes of any kind,
immigration will be prevented, so that new places in the polity of each island
will have to be filled up by modifications of the old inhabitants; and time will
be allowed for the varieties in each to become well modified and perfected.
When, by renewed elevation, the islands shall be re-converted into a continental
area, there will again be severe competition: the most favoured or improved
varieties will be enabled to spread: there will be much extinction of the less
improved forms, and the relative proportional numbers of the various inhabitants
of the renewed continent will again be changed; and again there will be a fair
field for natural selection to improve still further the inhabitants, and thus
produce new species. That natural selection will always act with extreme
slowness, I fully admit. Its action depends on there being places in the polity
of nature, which can be better occupied by some of the inhabitants of the
country undergoing modification of some kind. The existence of such places will
often depend on physical changes, which are generally very slow, and on the
immigration of better adapted forms having been checked. But the action of
natural selection will probably still oftener depend on some of the inhabitants
becoming slowly modified; the mutual relations of many of the other inhabitants
being thus disturbed. Nothing can be effected, unless favourable variations
occur, and variation itself is apparently always a very slow process. The
process will often be greatly retarded by free intercrossing. Many will exclaim
that these several causes are amply sufficient wholly to stop the action of
natural selection. I do not believe so. On the other hand, I do believe that
natural selection will always act very slowly, often only at long intervals of
time, and generally on only a very few of the inhabitants of the same region at
the same time. I further believe, that this very slow, intermittent action of
natural selection accords perfectly well with what geology tells us of the rate
and manner at which the inhabitants of this world have changed. Slow though the
process of selection may be, if feeble man can do much by his powers of
artificial selection, I can see no limit to the amount of change, to the beauty
and infinite complexity of the coadaptations between all organic beings, one
with another and with their physical conditions of life, which may be effected
in the long course of time by nature's power of selection. EXTINCTION. This
subject will be more fully discussed in our chapter on Geology; but it must be
here alluded to from being intimately connected with natural selection. Natural
selection acts solely through the preservation of variations in some way
advantageous, which consequently endure. But as from the high geometrical powers
of increase of all organic beings, each area is already fully stocked with
inhabitants, it follows that as each selected and favoured form increases in
number, so will the less favoured forms decrease and become rare. Rarity, as
geology tells us, is the precursor to extinction. We can, also, see that any
form represented by few individuals will, during fluctuations in the seasons or
in the number of its enemies, run a good chance of utter extinction. But we may
go further than this; for as new forms are continually and slowly being
produced, unless we believe that the number of specific forms goes on
perpetually and almost indefinitely increasing, numbers inevitably must become
extinct. That the number of specific forms has not indefinitely increased,
geology shows us plainly; and indeed we can see reason why they should not have
thus increased, for the number of places in the polity of nature is not
indefinitely great,--not that we have any means of knowing that any one region
has as yet got its maximum of species. Probably no region is as yet fully
stocked, for at the Cape of Good Hope, where more species of plants are crowded
together than in any other quarter of the world, some foreign plants have become
naturalised, without causing, as far as we know, the extinction of any natives.
Furthermore, the species which are most numerous in individuals will have the
best chance of producing within any given period favourable variations. We have
evidence of this, in the facts given in the second chapter, showing that it is
the common species which afford the greatest number of recorded varieties, or
incipient species. Hence, rare species will be less quickly modified or improved
within any given period, and they will consequently be beaten in the race for
life by the modified descendants of the commoner species. From these several
considerations I think it inevitably follows, that as new species in the course
of time are formed through natural selection, others will become rarer and
rarer, and finally extinct. The forms which stand in closest competition with
those undergoing modification and improvement, will naturally suffer most. And
we have seen in the chapter on the Struggle for Existence that it is the most
closely-allied forms,--varieties of the same species, and species of the same
genus or of related genera,--which, from having nearly the same structure,
constitution, and habits, generally come into the severest competition with each
other. Consequently, each new variety or species, during the progress of its
formation, will generally press hardest on its nearest kindred, and tend to
exterminate them. We see the same process of extermination amongst our
domesticated productions, through the selection of improved forms by man. Many
curious instances could be given showing how quickly new breeds of cattle,
sheep, and other animals, and varieties of flowers, take the place of older and
inferior kinds. In Yorkshire, it is historically known that the ancient black
cattle were displaced by the long-horns, and that these "were swept away by the
short-horns" (I quote the words of an agricultural writer) "as if by some
murderous pestilence." DIVERGENCE OF CHARACTER. The principle, which I have
designated by this term, is of high importance on my theory, and explains, as I
believe, several important facts. In the first place, varieties, even
strongly-marked ones, though having somewhat of the character of species--as is
shown by the hopeless doubts in many cases how to rank them--yet certainly
differ from each other far less than do good and distinct species. Nevertheless,
according to my view, varieties are species in the process of formation, or are,
as I have called them, incipient species. How, then, does the lesser difference
between varieties become augmented into the greater difference between species?
That this does habitually happen, we must infer from most of the innumerable
species throughout nature presenting well-marked differences; whereas varieties,
the supposed prototypes and parents of future well-marked species, present
slight and ill-defined differences. Mere chance, as we may call it, might cause
one variety to differ in some character from its parents, and the offspring of
this variety again to differ from its parent in the very same character and in a
greater degree; but this alone would never account for so habitual and large an
amount of difference as that between varieties of the same species and species
of the same genus. As has always been my practice, let us seek light on this
head from our domestic productions. We shall here find something analogous. A
fancier is struck by a pigeon having a slightly shorter beak; another fancier is
struck by a pigeon having a rather longer beak; and on the acknowledged
principle that "fanciers do not and will not admire a medium standard, but like
extremes," they both go on (as has actually occurred with tumbler-pigeons)
choosing and breeding from birds with longer and longer beaks, or with shorter
and shorter beaks. Again, we may suppose that at an early period one man
preferred swifter horses; another stronger and more bulky horses. The early
differences would be very slight; in the course of time, from the continued
selection of swifter horses by some breeders, and of stronger ones by others,
the differences would become greater, and would be noted as forming two
sub-breeds; finally, after the lapse of centuries, the sub-breeds would become
converted into two well-established and distinct breeds. As the differences
slowly become greater, the inferior animals with intermediate characters, being
neither very swift nor very strong, will have been neglected, and will have
tended to disappear. Here, then, we see in man's productions the action of what
may be called the principle of divergence, causing differences, at first barely
appreciable, steadily to increase, and the breeds to diverge in character both
from each other and from their common parent. But how, it may be asked, can any
analogous principle apply in nature? I believe it can and does apply most
efficiently, from the simple circumstance that the more diversified the
descendants from any one species become in structure, constitution, and habits,
by so much will they be better enabled to seize on many and widely diversified
places in the polity of nature, and so be enabled to increase in numbers. We can
clearly see this in the case of animals with simple habits. Take the case of a
carnivorous quadruped, of which the number that can be supported in any country
has long ago arrived at its full average. If its natural powers of increase be
allowed to act, it can succeed in increasing (the country not undergoing any
change in its conditions) only by its varying descendants seizing on places at
present occupied by other animals: some of them, for instance, being enabled to
feed on new kinds of prey, either dead or alive; some inhabiting new stations,
climbing trees, frequenting water, and some perhaps becoming less carnivorous.
The more diversified in habits and structure the descendants of our carnivorous
animal became, the more places they would be enabled to occupy. What applies to
one animal will apply throughout all time to all animals--that is, if they
vary--for otherwise natural selection can do nothing. So it will be with plants.
It has been experimentally proved, that if a plot of ground be sown with one
species of grass, and a similar plot be sown with several distinct genera of
grasses, a greater number of plants and a greater weight of dry herbage can thus
be raised. The same has been found to hold good when first one variety and then
several mixed varieties of wheat have been sown on equal spaces of ground.
Hence, if any one species of grass were to go on varying, and those varieties
were continually selected which differed from each other in at all the same
manner as distinct species and genera of grasses differ from each other, a
greater number of individual plants of this species of grass, including its
modified descendants, would succeed in living on the same piece of ground. And
we well know that each species and each variety of grass is annually sowing
almost countless seeds; and thus, as it may be said, is striving its utmost to
increase its numbers. Consequently, I cannot doubt that in the course of many
thousands of generations, the most distinct varieties of any one species of
grass would always have the best chance of succeeding and of increasing in
numbers, and thus of supplanting the less distinct varieties; and varieties,
when rendered very distinct from each other, take the rank of species. The truth
of the principle, that the greatest amount of life can be supported by great
diversification of structure, is seen under many natural circumstances. In an
extremely small area, especially if freely open to immigration, and where the
contest between individual and individual must be severe, we always find great
diversity in its inhabitants. For instance, I found that a piece of turf, three
feet by four in size, which had been exposed for many years to exactly the same
conditions, supported twenty species of plants, and these belonged to eighteen
genera and to eight orders, which shows how much these plants differed from each
other. So it is with the plants and insects on small and uniform islets; and so
in small ponds of fresh water. Farmers find that they can raise most food by a
rotation of plants belonging to the most different orders: nature follows what
may be called a simultaneous rotation. Most of the animals and plants which live
close round any small piece of ground, could live on it (supposing it not to be
in any way peculiar in its nature), and may be said to be striving to the utmost
to live there; but, it is seen, that where they come into the closest
competition with each other, the advantages of diversification of structure,
with the accompanying differences of habit and constitution, determine that the
inhabitants, which thus jostle each other most closely, shall, as a general
rule, belong to what we call different genera and orders. The same principle is
seen in the naturalisation of plants through man's agency in foreign lands. It
might have been expected that the plants which have succeeded in becoming
naturalised in any land would generally have been closely allied to the
indigenes; for these are commonly looked at as specially created and adapted for
their own country. It might, also, perhaps have been expected that naturalised
plants would have belonged to a few groups more especially adapted to certain
stations in their new homes. But the case is very different; and Alph. De
Candolle has well remarked in his great and admirable work, that floras gain by
naturalisation, proportionally with the number of the native genera and species,
far more in new genera than in new species. To give a single instance: in the
last edition of Dr. Asa Gray's 'Manual of the Flora of the Northern United
States,' 260 naturalised plants are enumerated, and these belong to 162 genera.
We thus see that these naturalised plants are of a highly diversified nature.
They differ, moreover, to a large extent from the indigenes, for out of the 162
genera, no less than 100 genera are not there indigenous, and thus a large
proportional addition is made to the genera of these States. By considering the
nature of the plants or animals which have struggled successfully with the
indigenes of any country, and have there become naturalised, we can gain some
crude idea in what manner some of the natives would have had to be modified, in
order to have gained an advantage over the other natives; and we may, I think,
at least safely infer that diversification of structure, amounting to new
generic differences, would have been profitable to them. The advantage of
diversification in the inhabitants of the same region is, in fact, the same as
that of the physiological division of labour in the organs of the same
individual body--a subject so well elucidated by Milne Edwards. No physiologist
doubts that a stomach by being adapted to digest vegetable matter alone, or
flesh alone, draws most nutriment from these substances. So in the general
economy of any land, the more widely and perfectly the animals and plants are
diversified for different habits of life, so will a greater number of
individuals be capable of there supporting themselves. A set of animals, with
their organisation but little diversified, could hardly compete with a set more
perfectly diversified in structure. It may be doubted, for instance, whether the
Australian marsupials, which are divided into groups differing but little from
each other, and feebly representing, as Mr. Waterhouse and others have remarked,
our carnivorous, ruminant, and rodent mammals, could successfully compete with
these well-pronounced orders. In the Australian mammals, we see the process of
diversification in an early and incomplete stage of development. After the
foregoing discussion, which ought to have been much amplified, we may, I think,
assume that the modified descendants of any one species will succeed by so much
the better as they become more diversified in structure, and are thus enabled to
encroach on places occupied by other beings. Now let us see how this principle
of great benefit being derived from divergence of character, combined with the
principles of natural selection and of extinction, will tend to act. The
accompanying diagram will aid us in understanding this rather perplexing
subject. Let A to L represent the species of a genus large in its own country;
these species are supposed to resemble each other in unequal degrees, as is so
generally the case in nature, and as is represented in the diagram by the
letters standing at unequal distances. I have said a large genus, because we
have seen in the second chapter, that on an average more of the species of large
genera vary than of small genera; and the varying species of the large genera
present a greater number of varieties. We have, also, seen that the species,
which are the commonest and the most widely-diffused, vary more than rare
species with restricted ranges. Let (A) be a common, widely-diffused, and
varying species, belonging to a genus large in its own country. The little fan
of diverging dotted lines of unequal lengths proceeding from (A), may represent
its varying offspring. The variations are supposed to be extremely slight, but
of the most diversified nature; they are not supposed all to appear
simultaneously, but often after long intervals of time; nor are they all
supposed to endure for equal periods. Only those variations which are in some
way profitable will be preserved or naturally selected. And here the importance
of the principle of benefit being derived from divergence of character comes in;
for this will generally lead to the most different or divergent variations
(represented by the outer dotted lines) being preserved and accumulated by
natural selection. When a dotted line reaches one of the horizontal lines, and
is there marked by a small numbered letter, a sufficient amount of variation is
supposed to have been accumulated to have formed a fairly well-marked variety,
such as would be thought worthy of record in a systematic work. The intervals
between the horizontal lines in the diagram, may represent each a thousand
generations; but it would have been better if each had represented ten thousand
generations. After a thousand generations, species (A) is supposed to have
produced two fairly well-marked varieties, namely a1 and m1. These two varieties
will generally continue to be exposed to the same conditions which made their
parents variable, and the tendency to variability is in itself hereditary,
consequently they will tend to vary, and generally to vary in nearly the same
manner as their parents varied. Moreover, these two varieties, being only
slightly modified forms, will tend to inherit those advantages which made their
common parent (A) more numerous than most of the other inhabitants of the same
country; they will likewise partake of those more general advantages which made
the genus to which the parent-species belonged, a large genus in its own
country. And these circumstances we know to be favourable to the production of
new varieties. If, then, these two varieties be variable, the most divergent of
their variations will generally be preserved during the next thousand
generations. And after this interval, variety a1 is supposed in the diagram to
have produced variety a2, which will, owing to the principle of divergence,
differ more from (A) than did variety a1. Variety m1 is supposed to have
produced two varieties, namely m2 and s2, differing from each other, and more
considerably from their common parent (A). We may continue the process by
similar steps for any length of time; some of the varieties, after each thousand
generations, producing only a single variety, but in a more and more modified
condition, some producing two or three varieties, and some failing to produce
any. Thus the varieties or modified descendants, proceeding from the common
parent (A), will generally go on increasing in number and diverging in
character. In the diagram the process is represented up to the ten-thousandth
generation, and under a condensed and simplified form up to the
fourteen-thousandth generation. But I must here remark that I do not suppose
that the process ever goes on so regularly as is represented in the diagram,
though in itself made somewhat irregular. I am far from thinking that the most
divergent varieties will invariably prevail and multiply: a medium form may
often long endure, and may or may not produce more than one modified descendant;
for natural selection will always act according to the nature of the places
which are either unoccupied or not perfectly occupied by other beings; and this
will depend on infinitely complex relations. But as a general rule, the more
diversified in structure the descendants from any one species can be rendered,
the more places they will be enabled to seize on, and the more their modified
progeny will be increased. In our diagram the line of succession is broken at
regular intervals by small numbered letters marking the successive forms which
have become sufficiently distinct to be recorded as varieties. But these breaks
are imaginary, and might have been inserted anywhere, after intervals long
enough to have allowed the accumulation of a considerable amount of divergent
variation. As all the modified descendants from a common and widely-diffused
species, belonging to a large genus, will tend to partake of the same advantages
which made their parent successful in life, they will generally go on
multiplying in number as well as diverging in character: this is represented in
the diagram by the several divergent branches proceeding from (A). The modified
offspring from the later and more highly improved branches in the lines of
descent, will, it is probable, often take the place of, and so destroy, the
earlier and less improved branches: this is represented in the diagram by some
of the lower branches not reaching to the upper horizontal lines. In some cases
I do not doubt that the process of modification will be confined to a single
line of descent, and the number of the descendants will not be increased;
although the amount of divergent modification may have been increased in the
successive generations. This case would be represented in the diagram, if all
the lines proceeding from (A) were removed, excepting that from a1 to a10. In
the same way, for instance, the English race-horse and English pointer have
apparently both gone on slowly diverging in character from their original
stocks, without either having given off any fresh branches or races. After ten
thousand generations, species (A) is supposed to have produced three forms, a10,
f10, and m10, which, from having diverged in character during the successive
generations, will have come to differ largely, but perhaps unequally, from each
other and from their common parent. If we suppose the amount of change between
each horizontal line in our diagram to be excessively small, these three forms
may still be only well-marked varieties; or they may have arrived at the
doubtful category of sub-species; but we have only to suppose the steps in the
process of modification to be more numerous or greater in amount, to convert
these three forms into well-defined species: thus the diagram illustrates the
steps by which the small differences distinguishing varieties are increased into
the larger differences distinguishing species. By continuing the same process
for a greater number of generations (as shown in the diagram in a condensed and
simplified manner), we get eight species, marked by the letters between a14 and
m14, all descended from (A). Thus, as I believe, species are multiplied and
genera are formed. In a large genus it is probable that more than one species
would vary. In the diagram I have assumed that a second species (I) has
produced, by analogous steps, after ten thousand generations, either two
well-marked varieties (w10 and z10) or two species, according to the amount of
change supposed to be represented between the horizontal lines. After fourteen
thousand generations, six new species, marked by the letters n14 to z14, are
supposed to have been produced. In each genus, the species, which are already
extremely different in character, will generally tend to produce the greatest
number of modified descendants; for these will have the best chance of filling
new and widely different places in the polity of nature: hence in the diagram I
have chosen the extreme species (A), and the nearly extreme species (I), as
those which have largely varied, and have given rise to new varieties and
species. The other nine species (marked by capital letters) of our original
genus, may for a long period continue transmitting unaltered descendants; and
this is shown in the diagram by the dotted lines not prolonged far upwards from
want of space. But during the process of modification, represented in the
diagram, another of our principles, namely that of extinction, will have played
an important part. As in each fully stocked country natural selection
necessarily acts by the selected form having some advantage in the struggle for
life over other forms, there will be a constant tendency in the improved
descendants of any one species to supplant and exterminate in each stage of
descent their predecessors and their original parent. For it should be
remembered that the competition will generally be most severe between those
forms which are most nearly related to each other in habits, constitution, and
structure. Hence all the intermediate forms between the earlier and later
states, that is between the less and more improved state of a species, as well
as the original parent-species itself, will generally tend to become extinct. So
it probably will be with many whole collateral lines of descent, which will be
conquered by later and improved lines of descent. If, however, the modified
offspring of a species get into some distinct country, or become quickly adapted
to some quite new station, in which child and parent do not come into
competition, both may continue to exist. If then our diagram be assumed to
represent a considerable amount of modification, species (A) and all the earlier
varieties will have become extinct, having been replaced by eight new species
(a14 to m14); and (I) will have been replaced by six (n14 to z14) new species.
But we may go further than this. The original species of our genus were supposed
to resemble each other in unequal degrees, as is so generally the case in
nature; species (A) being more nearly related to B, C, and D, than to the other
species; and species (I) more to G, H, K, L, than to the others. These two
species (A) and (I), were also supposed to be very common and widely diffused
species, so that they must originally have had some advantage over most of the
other species of the genus. Their modified descendants, fourteen in number at
the fourteen-thousandth generation, will probably have inherited some of the
same advantages: they have also been modified and improved in a diversified
manner at each stage of descent, so as to have become adapted to many related
places in the natural economy of their country. It seems, therefore, to me
extremely probable that they will have taken the places of, and thus
exterminated, not only their parents (A) and (I), but likewise some of the
original species which were most nearly related to their parents. Hence very few
of the original species will have transmitted offspring to the
fourteen-thousandth generation. We may suppose that only one (F), of the two
species which were least closely related to the other nine original species, has
transmitted descendants to this late stage of descent. The new species in our
diagram descended from the original eleven species, will now be fifteen in
number. Owing to the divergent tendency of natural selection, the extreme amount
of difference in character between species a14 and z14 will be much greater than
that between the most different of the original eleven species. The new species,
moreover, will be allied to each other in a widely different manner. Of the
eight descendants from (A) the three marked a14, q14, p14, will be nearly
related from having recently branched off from a10; b14 and f14, from having
diverged at an earlier period from a5, will be in some degree distinct from the
three first-named species; and lastly, o14, e14, and m14, will be nearly related
one to the other, but from having diverged at the first commencement of the
process of modification, will be widely different from the other five species,
and may constitute a sub-genus or even a distinct genus. The six descendants
from (I) will form two sub-genera or even genera. But as the original species
(I) differed largely from (A), standing nearly at the extreme points of the
original genus, the six descendants from (I) will, owing to inheritance, differ
considerably from the eight descendants from (A); the two groups, moreover, are
supposed to have gone on diverging in different directions. The intermediate
species, also (and this is a very important consideration), which connected the
original species (A) and (I), have all become, excepting (F), extinct, and have
left no descendants. Hence the six new species descended from (I), and the eight
descended from (A), will have to be ranked as very distinct genera, or even as
distinct sub-families. Thus it is, as I believe, that two or more genera are
produced by descent, with modification, from two or more species of the same
genus. And the two or more parent-species are supposed to have descended from
some one species of an earlier genus. In our diagram, this is indicated by the
broken lines, beneath the capital letters, converging in sub-branches downwards
towards a single point; this point representing a single species, the supposed
single parent of our several new sub-genera and genera. It is worth while to
reflect for a moment on the character of the new species F14, which is supposed
not to have diverged much in character, but to have retained the form of (F),
either unaltered or altered only in a slight degree. In this case, its
affinities to the other fourteen new species will be of a curious and circuitous
nature. Having descended from a form which stood between the two parent-species
(A) and (I), now supposed to be extinct and unknown, it will be in some degree
intermediate in character between the two groups descended from these species.
But as these two groups have gone on diverging in character from the type of
their parents, the new species (F14) will not be directly intermediate between
them, but rather between types of the two groups; and every naturalist will be
able to bring some such case before his mind. In the diagram, each horizontal
line has hitherto been supposed to represent a thousand generations, but each
may represent a million or hundred million generations, and likewise a section
of the successive strata of the earth's crust including extinct remains. We
shall, when we come to our chapter on Geology, have to refer again to this
subject, and I think we shall then see that the diagram throws light on the
affinities of extinct beings, which, though generally belonging to the same
orders, or families, or genera, with those now living, yet are often, in some
degree, intermediate in character between existing groups; and we can understand
this fact, for the extinct species lived at very ancient epochs when the
branching lines of descent had diverged less. I see no reason to limit the
process of modification, as now explained, to the formation of genera alone. If,
in our diagram, we suppose the amount of change represented by each successive
group of diverging dotted lines to be very great, the forms marked a14 to p14,
those marked b14 and f14, and those marked o14 to m14, will form three very
distinct genera. We shall also have two very distinct genera descended from (I)
and as these latter two genera, both from continued divergence of character and
from inheritance from a different parent, will differ widely from the three
genera descended from (A), the two little groups of genera will form two
distinct families, or even orders, according to the amount of divergent
modification supposed to be represented in the diagram. And the two new
families, or orders, will have descended from two species of the original genus;
and these two species are supposed to have descended from one species of a still
more ancient and unknown genus. We have seen that in each country it is the
species of the larger genera which oftenest present varieties or incipient
species. This, indeed, might have been expected; for as natural selection acts
through one form having some advantage over other forms in the struggle for
existence, it will chiefly act on those which already have some advantage; and
the largeness of any group shows that its species have inherited from a common
ancestor some advantage in common. Hence, the struggle for the production of new
and modified descendants, will mainly lie between the larger groups, which are
all trying to increase in number. One large group will slowly conquer another
large group, reduce its numbers, and thus lessen its chance of further variation
and improvement. Within the same large group, the later and more highly
perfected sub-groups, from branching out and seizing on many new places in the
polity of Nature, will constantly tend to supplant and destroy the earlier and
less improved sub-groups. Small and broken groups and sub-groups will finally
tend to disappear. Looking to the future, we can predict that the groups of
organic beings which are now large and triumphant, and which are least broken
up, that is, which as yet have suffered least extinction, will for a long period
continue to increase. But which groups will ultimately prevail, no man can
predict; for we well know that many groups, formerly most extensively developed,
have now become extinct. Looking still more remotely to the future, we may
predict that, owing to the continued and steady increase of the larger groups, a
multitude of smaller groups will become utterly extinct, and leave no modified
descendants; and consequently that of the species living at any one period,
extremely few will transmit descendants to a remote futurity. I shall have to
return to this subject in the chapter on Classification, but I may add that on
this view of extremely few of the more ancient species having transmitted
descendants, and on the view of all the descendants of the same species making a
class, we can understand how it is that there exist but very few classes in each
main division of the animal and vegetable kingdoms. Although extremely few of
the most ancient species may now have living and modified descendants, yet at
the most remote geological period, the earth may have been as well peopled with
many species of many genera, families, orders, and classes, as at the present
day. SUMMARY OF CHAPTER. If during the long course of ages and under varying
conditions of life, organic beings vary at all in the several parts of their
organisation, and I think this cannot be disputed; if there be, owing to the
high geometrical powers of increase of each species, at some age, season, or
year, a severe struggle for life, and this certainly cannot be disputed; then,
considering the infinite complexity of the relations of all organic beings to
each other and to their conditions of existence, causing an infinite diversity
in structure, constitution, and habits, to be advantageous to them, I think it
would be a most extraordinary fact if no variation ever had occurred useful to
each being's own welfare, in the same way as so many variations have occurred
useful to man. But if variations useful to any organic being do occur, assuredly
individuals thus characterised will have the best chance of being preserved in
the struggle for life; and from the strong principle of inheritance they will
tend to produce offspring similarly characterised. This principle of
preservation, I have called, for the sake of brevity, Natural Selection. Natural
selection, on the principle of qualities being inherited at corresponding ages,
can modify the egg, seed, or young, as easily as the adult. Amongst many
animals, sexual selection will give its aid to ordinary selection, by assuring
to the most vigorous and best adapted males the greatest number of offspring.
Sexual selection will also give characters useful to the males alone, in their
struggles with other males. Whether natural selection has really thus acted in
nature, in modifying and adapting the various forms of life to their several
conditions and stations, must be judged of by the general tenour and balance of
evidence given in the following chapters. But we already see how it entails
extinction; and how largely extinction has acted in the world's history, geology
plainly declares. Natural selection, also, leads to divergence of character; for
more living beings can be supported on the same area the more they diverge in
structure, habits, and constitution, of which we see proof by looking at the
inhabitants of any small spot or at naturalised productions. Therefore during
the modification of the descendants of any one species, and during the incessant
struggle of all species to increase in numbers, the more diversified these
descendants become, the better will be their chance of succeeding in the battle
of life. Thus the small differences distinguishing varieties of the same
species, will steadily tend to increase till they come to equal the greater
differences between species of the same genus, or even of distinct genera. We
have seen that it is the common, the widely-diffused, and widely-ranging
species, belonging to the larger genera, which vary most; and these will tend to
transmit to their modified offspring that superiority which now makes them
dominant in their own countries. Natural selection, as has just been remarked,
leads to divergence of character and to much extinction of the less improved and
intermediate forms of life. On these principles, I believe, the nature of the
affinities of all organic beings may be explained. It is a truly wonderful
fact--the wonder of which we are apt to overlook from familiarity--that all
animals and all plants throughout all time and space should be related to each
other in group subordinate to group, in the manner which we everywhere
behold--namely, varieties of the same species most closely related together,
species of the same genus less closely and unequally related together, forming
sections and sub-genera, species of distinct genera much less closely related,
and genera related in different degrees, forming sub-families, families, orders,
sub-classes, and classes. The several subordinate groups in any class cannot be
ranked in a single file, but seem rather to be clustered round points, and these
round other points, and so on in almost endless cycles. On the view that each
species has been independently created, I can see no explanation of this great
fact in the classification of all organic beings; but, to the best of my
judgment, it is explained through inheritance and the complex action of natural
selection, entailing extinction and divergence of character, as we have seen
illustrated in the diagram. The affinities of all the beings of the same class
have sometimes been represented by a great tree. I believe this simile largely
speaks the truth. The green and budding twigs may represent existing species;
and those produced during each former year may represent the long succession of
extinct species. At each period of growth all the growing twigs have tried to
branch out on all sides, and to overtop and kill the surrounding twigs and
branches, in the same manner as species and groups of species have tried to
overmaster other species in the great battle for life. The limbs divided into
great branches, and these into lesser and lesser branches, were themselves once,
when the tree was small, budding twigs; and this connexion of the former and
present buds by ramifying branches may well represent the classification of all
extinct and living species in groups subordinate to groups. Of the many twigs
which flourished when the tree was a mere bush, only two or three, now grown
into great branches, yet survive and bear all the other branches; so with the
species which lived during long-past geological periods, very few now have
living and modified descendants. From the first growth of the tree, many a limb
and branch has decayed and dropped off; and these lost branches of various sizes
may represent those whole orders, families, and genera which have now no living
representatives, and which are known to us only from having been found in a
fossil state. As we here and there see a thin straggling branch springing from a
fork low down in a tree, and which by some chance has been favoured and is still
alive on its summit, so we occasionally see an animal like the Ornithorhynchus
or Lepidosiren, which in some small degree connects by its affinities two large
branches of life, and which has apparently been saved from fatal competition by
having inhabited a protected station. As buds give rise by growth to fresh buds,
and these, if vigorous, branch out and overtop on all sides many a feebler
branch, so by generation I believe it has been with the great Tree of Life,
which fills with its dead and broken branches the crust of the earth, and covers
the surface with its ever branching and beautiful ramifications. CHAPTER 5. LAWS
OF VARIATION. Effects of external conditions. Use and disuse, combined with
natural selection; organs of flight and of vision. Acclimatisation. Correlation
of growth. Compensation and economy of growth. False correlations. Multiple,
rudimentary, and lowly organised structures variable. Parts developed in an
unusual manner are highly variable: specific characters more variable than
generic: secondary sexual characters variable. Species of the same genus vary in
an analogous manner. Reversions to long lost characters. Summary. I have
hitherto sometimes spoken as if the variations--so common and multiform in
organic beings under domestication, and in a lesser degree in those in a state
of nature--had been due to chance. This, of course, is a wholly incorrect
expression, but it serves to acknowledge plainly our ignorance of the cause of
each particular variation. Some authors believe it to be as much the function of
the reproductive system to produce individual differences, or very slight
deviations of structure, as to make the child like its parents. But the much
greater variability, as well as the greater frequency of monstrosities, under
domestication or cultivation, than under nature, leads me to believe that
deviations of structure are in some way due to the nature of the conditions of
life, to which the parents and their more remote ancestors have been exposed
during several generations. I have remarked in the first chapter--but a long
catalogue of facts which cannot be here given would be necessary to show the
truth of the remark--that the reproductive system is eminently susceptible to
changes in the conditions of life; and to this system being functionally
disturbed in the parents, I chiefly attribute the varying or plastic condition
of the offspring. The male and female sexual elements seem to be affected before
that union takes place which is to form a new being. In the case of "sporting"
plants, the bud, which in its earliest condition does not apparently differ
essentially from an ovule, is alone affected. But why, because the reproductive
system is disturbed, this or that part should vary more or less, we are
profoundly ignorant. Nevertheless, we can here and there dimly catch a faint ray
of light, and we may feel sure that there must be some cause for each deviation
of structure, however slight. How much direct effect difference of climate,
food, etc., produces on any being is extremely doubtful. My impression is, that
the effect is extremely small in the case of animals, but perhaps rather more in
that of plants. We may, at least, safely conclude that such influences cannot
have produced the many striking and complex co-adaptations of structure between
one organic being and another, which we see everywhere throughout nature. Some
little influence may be attributed to climate, food, etc.: thus, E. Forbes
speaks confidently that shells at their southern limit, and when living in
shallow water, are more brightly coloured than those of the same species further
north or from greater depths. Gould believes that birds of the same species are
more brightly coloured under a clear atmosphere, than when living on islands or
near the coast. So with insects, Wollaston is convinced that residence near the
sea affects their colours. Moquin-Tandon gives a list of plants which when
growing near the sea-shore have their leaves in some degree fleshy, though not
elsewhere fleshy. Several other such cases could be given. The fact of varieties
of one species, when they range into the zone of habitation of other species,
often acquiring in a very slight degree some of the characters of such species,
accords with our view that species of all kinds are only well-marked and
permanent varieties. Thus the species of shells which are confined to tropical
and shallow seas are generally brighter-coloured than those confined to cold and
deeper seas. The birds which are confined to continents are, according to Mr.
Gould, brighter-coloured than those of islands. The insect-species confined to
sea-coasts, as every collector knows, are often brassy or lurid. Plants which
live exclusively on the sea-side are very apt to have fleshy leaves. He who
believes in the creation of each species, will have to say that this shell, for
instance, was created with bright colours for a warm sea; but that this other
shell became bright-coloured by variation when it ranged into warmer or
shallower waters. When a variation is of the slightest use to a being, we cannot
tell how much of it to attribute to the accumulative action of natural
selection, and how much to the conditions of life. Thus, it is well known to
furriers that animals of the same species have thicker and better fur the more
severe the climate is under which they have lived; but who can tell how much of
this difference may be due to the warmest-clad individuals having been favoured
and preserved during many generations, and how much to the direct action of the
severe climate? for it would appear that climate has some direct action on the
hair of our domestic quadrupeds. Instances could be given of the same variety
being produced under conditions of life as different as can well be conceived;
and, on the other hand, of different varieties being produced from the same
species under the same conditions. Such facts show how indirectly the conditions
of life must act. Again, innumerable instances are known to every naturalist of
species keeping true, or not varying at all, although living under the most
opposite climates. Such considerations as these incline me to lay very little
weight on the direct action of the conditions of life. Indirectly, as already
remarked, they seem to play an important part in affecting the reproductive
system, and in thus inducing variability; and natural selection will then
accumulate all profitable variations, however slight, until they become plainly
developed and appreciable by us. EFFECTS OF USE AND DISUSE. From the facts
alluded to in the first chapter, I think there can be little doubt that use in
our domestic animals strengthens and enlarges certain parts, and disuse
diminishes them; and that such modifications are inherited. Under free nature,
we can have no standard of comparison, by which to judge of the effects of
long-continued use or disuse, for we know not the parent-forms; but many animals
have structures which can be explained by the effects of disuse. As Professor
Owen has remarked, there is no greater anomaly in nature than a bird that cannot
fly; yet there are several in this state. The logger-headed duck of South
America can only flap along the surface of the water, and has its wings in
nearly the same condition as the domestic Aylesbury duck. As the larger
ground-feeding birds seldom take flight except to escape danger, I believe that
the nearly wingless condition of several birds, which now inhabit or have lately
inhabited several oceanic islands, tenanted by no beast of prey, has been caused
by disuse. The ostrich indeed inhabits continents and is exposed to danger from
which it cannot escape by flight, but by kicking it can defend itself from
enemies, as well as any of the smaller quadrupeds. We may imagine that the early
progenitor of the ostrich had habits like those of a bustard, and that as
natural selection increased in successive generations the size and weight of its
body, its legs were used more, and its wings less, until they became incapable
of flight. Kirby has remarked (and I have observed the same fact) that the
anterior tarsi, or feet, of many male dung-feeding beetles are very often broken
off; he examined seventeen specimens in his own collection, and not one had even
a relic left. In the Onites apelles the tarsi are so habitually lost, that the
insect has been described as not having them. In some other genera they are
present, but in a rudimentary condition. In the Ateuchus or sacred beetle of the
Egyptians, they are totally deficient. There is not sufficient evidence to
induce us to believe that mutilations are ever inherited; and I should prefer
explaining the entire absence of the anterior tarsi in Ateuchus, and their
rudimentary condition in some other genera, by the long-continued effects of
disuse in their progenitors; for as the tarsi are almost always lost in many
dung-feeding beetles, they must be lost early in life, and therefore cannot be
much used by these insects. In some cases we might easily put down to disuse
modifications of structure which are wholly, or mainly, due to natural
selection. Mr. Wollaston has discovered the remarkable fact that 200 beetles,
out of the 550 species inhabiting Madeira, are so far deficient in wings that
they cannot fly; and that of the twenty-nine endemic genera, no less than
twenty-three genera have all their species in this condition! Several facts,
namely, that beetles in many parts of the world are very frequently blown to sea
and perish; that the beetles in Madeira, as observed by Mr. Wollaston, lie much
concealed, until the wind lulls and the sun shines; that the proportion of
wingless beetles is larger on the exposed Dezertas than in Madeira itself; and
especially the extraordinary fact, so strongly insisted on by Mr. Wollaston, of
the almost entire absence of certain large groups of beetles, elsewhere
excessively numerous, and which groups have habits of life almost necessitating
frequent flight;--these several considerations have made me believe that the
wingless condition of so many Madeira beetles is mainly due to the action of
natural selection, but combined probably with disuse. For during thousands of
successive generations each individual beetle which flew least, either from its
wings having been ever so little less perfectly developed or from indolent
habit, will have had the best chance of surviving from not being blown out to
sea; and, on the other hand, those beetles which most readily took to flight
will oftenest have been blown to sea and thus have been destroyed. The insects
in Madeira which are not ground-feeders, and which, as the flower-feeding
coleoptera and lepidoptera, must habitually use their wings to gain their
subsistence, have, as Mr. Wollaston suspects, their wings not at all reduced,
but even enlarged. This is quite compatible with the action of natural
selection. For when a new insect first arrived on the island, the tendency of
natural selection to enlarge or to reduce the wings, would depend on whether a
greater number of individuals were saved by successfully battling with the
winds, or by giving up the attempt and rarely or never flying. As with mariners
shipwrecked near a coast, it would have been better for the good swimmers if
they had been able to swim still further, whereas it would have been better for
the bad swimmers if they had not been able to swim at all and had stuck to the
wreck. The eyes of moles and of some burrowing rodents are rudimentary in size,
and in some cases are quite covered up by skin and fur. This state of the eyes
is probably due to gradual reduction from disuse, but aided perhaps by natural
selection. In South America, a burrowing rodent, the tuco-tuco, or Ctenomys, is
even more subterranean in its habits than the mole; and I was assured by a
Spaniard, who had often caught them, that they were frequently blind; one which
I kept alive was certainly in this condition, the cause, as appeared on
dissection, having been inflammation of the nictitating membrane. As frequent
inflammation of the eyes must be injurious to any animal, and as eyes are
certainly not indispensable to animals with subterranean habits, a reduction in
their size with the adhesion of the eyelids and growth of fur over them, might
in such case be an advantage; and if so, natural selection would constantly aid
the effects of disuse. It is well known that several animals, belonging to the
most different classes, which inhabit the caves of Styria and of Kentucky, are
blind. In some of the crabs the foot-stalk for the eye remains, though the eye
is gone; the stand for the telescope is there, though the telescope with its
glasses has been lost. As it is difficult to imagine that eyes, though useless,
could be in any way injurious to animals living in darkness, I attribute their
loss wholly to disuse. In one of the blind animals, namely, the cave-rat, the
eyes are of immense size; and Professor Silliman thought that it regained, after
living some days in the light, some slight power of vision. In the same manner
as in Madeira the wings of some of the insects have been enlarged, and the wings
of others have been reduced by natural selection aided by use and disuse, so in
the case of the cave-rat natural selection seems to have struggled with the loss
of light and to have increased the size of the eyes; whereas with all the other
inhabitants of the caves, disuse by itself seems to have done its work. It is
difficult to imagine conditions of life more similar than deep limestone caverns
under a nearly similar climate; so that on the common view of the blind animals
having been separately created for the American and European caverns, close
similarity in their organisation and affinities might have been expected; but,
as Schiodte and others have remarked, this is not the case, and the cave-insects
of the two continents are not more closely allied than might have been
anticipated from the general resemblance of the other inhabitants of North
America and Europe. On my view we must suppose that American animals, having
ordinary powers of vision, slowly migrated by successive generations from the
outer world into the deeper and deeper recesses of the Kentucky caves, as did
European animals into the caves of Europe. We have some evidence of this
gradation of habit; for, as Schiodte remarks, "animals not far remote from
ordinary forms, prepare the transition from light to darkness. Next follow those
that are constructed for twilight; and, last of all, those destined for total
darkness." By the time that an animal had reached, after numberless generations,
the deepest recesses, disuse will on this view have more or less perfectly
obliterated its eyes, and natural selection will often have effected other
changes, such as an increase in the length of the antennae or palpi, as a
compensation for blindness. Notwithstanding such modifications, we might expect
still to see in the cave-animals of America, affinities to the other inhabitants
of that continent, and in those of Europe, to the inhabitants of the European
continent. And this is the case with some of the American cave-animals, as I
hear from Professor Dana; and some of the European cave-insects are very closely
allied to those of the surrounding country. It would be most difficult to give
any rational explanation of the affinities of the blind cave-animals to the
other inhabitants of the two continents on the ordinary view of their
independent creation. That several of the inhabitants of the caves of the Old
and New Worlds should be closely related, we might expect from the well-known
relationship of most of their other productions. Far from feeling any surprise
that some of the cave-animals should be very anomalous, as Agassiz has remarked
in regard to the blind fish, the Amblyopsis, and as is the case with the blind
Proteus with reference to the reptiles of Europe, I am only surprised that more
wrecks of ancient life have not been preserved, owing to the less severe
competition to which the inhabitants of these dark abodes will probably have
been exposed. ACCLIMATISATION. Habit is hereditary with plants, as in the period
of flowering, in the amount of rain requisite for seeds to germinate, in the
time of sleep, etc., and this leads me to say a few words on acclimatisation. As
it is extremely common for species of the same genus to inhabit very hot and
very cold countries, and as I believe that all the species of the same genus
have descended from a single parent, if this view be correct, acclimatisation
must be readily effected during long-continued descent. It is notorious that
each species is adapted to the climate of its own home: species from an arctic
or even from a temperate region cannot endure a tropical climate, or conversely.
So again, many succulent plants cannot endure a damp climate. But the degree of
adaptation of species to the climates under which they live is often overrated.
We may infer this from our frequent inability to predict whether or not an
imported plant will endure our climate, and from the number of plants and
animals brought from warmer countries which here enjoy good health. We have
reason to believe that species in a state of nature are limited in their ranges
by the competition of other organic beings quite as much as, or more than, by
adaptation to particular climates. But whether or not the adaptation be
generally very close, we have evidence, in the case of some few plants, of their
becoming, to a certain extent, naturally habituated to different temperatures,
or becoming acclimatised: thus the pines and rhododendrons, raised from seed
collected by Dr. Hooker from trees growing at different heights on the Himalaya,
were found in this country to possess different constitutional powers of
resisting cold. Mr. Thwaites informs me that he has observed similar facts in
Ceylon, and analogous observations have been made by Mr. H. C. Watson on
European species of plants brought from the Azores to England. In regard to
animals, several authentic cases could be given of species within historical
times having largely extended their range from warmer to cooler latitudes, and
conversely; but we do not positively know that these animals were strictly
adapted to their native climate, but in all ordinary cases we assume such to be
the case; nor do we know that they have subsequently become acclimatised to
their new homes. As I believe that our domestic animals were originally chosen
by uncivilised man because they were useful and bred readily under confinement,
and not because they were subsequently found capable of far-extended
transportation, I think the common and extraordinary capacity in our domestic
animals of not only withstanding the most different climates but of being
perfectly fertile (a far severer test) under them, may be used as an argument
that a large proportion of other animals, now in a state of nature, could easily
be brought to bear widely different climates. We must not, however, push the
foregoing argument too far, on account of the probable origin of some of our
domestic animals from several wild stocks: the blood, for instance, of a
tropical and arctic wolf or wild dog may perhaps be mingled in our domestic
breeds. The rat and mouse cannot be considered as domestic animals, but they
have been transported by man to many parts of the world, and now have a far
wider range than any other rodent, living free under the cold climate of Faroe
in the north and of the Falklands in the south, and on many islands in the
torrid zones. Hence I am inclined to look at adaptation to any special climate
as a quality readily grafted on an innate wide flexibility of constitution,
which is common to most animals. On this view, the capacity of enduring the most
different climates by man himself and by his domestic animals, and such facts as
that former species of the elephant and rhinoceros were capable of enduring a
glacial climate, whereas the living species are now all tropical or sub-tropical
in their habits, ought not to be looked at as anomalies, but merely as examples
of a very common flexibility of constitution, brought, under peculiar
circumstances, into play. How much of the acclimatisation of species to any
peculiar climate is due to mere habit, and how much to the natural selection of
varieties having different innate constitutions, and how much to both means
combined, is a very obscure question. That habit or custom has some influence I
must believe, both from analogy, and from the incessant advice given in
agricultural works, even in the ancient Encyclopaedias of China, to be very
cautious in transposing animals from one district to another; for it is not
likely that man should have succeeded in selecting so many breeds and sub-breeds
with constitutions specially fitted for their own districts: the result must, I
think, be due to habit. On the other hand, I can see no reason to doubt that
natural selection will continually tend to preserve those individuals which are
born with constitutions best adapted to their native countries. In treatises on
many kinds of cultivated plants, certain varieties are said to withstand certain
climates better than others: this is very strikingly shown in works on fruit
trees published in the United States, in which certain varieties are habitually
recommended for the northern, and others for the southern States; and as most of
these varieties are of recent origin, they cannot owe their constitutional
differences to habit. The case of the Jerusalem artichoke, which is never
propagated by seed, and of which consequently new varieties have not been
produced, has even been advanced--for it is now as tender as ever it was--as
proving that acclimatisation cannot be effected! The case, also, of the
kidney-bean has been often cited for a similar purpose, and with much greater
weight; but until some one will sow, during a score of generations, his
kidney-beans so early that a very large proportion are destroyed by frost, and
then collect seed from the few survivors, with care to prevent accidental
crosses, and then again get seed from these seedlings, with the same
precautions, the experiment cannot be said to have been even tried. Nor let it
be supposed that no differences in the constitution of seedling kidney-beans
ever appear, for an account has been published how much more hardy some
seedlings appeared to be than others. On the whole, I think we may conclude that
habit, use, and disuse, have, in some cases, played a considerable part in the
modification of the constitution, and of the structure of various organs; but
that the effects of use and disuse have often been largely combined with, and
sometimes overmastered by, the natural selection of innate differences.
CORRELATION OF GROWTH. I mean by this expression that the whole organisation is
so tied together during its growth and development, that when slight variations
in any one part occur, and are accumulated through natural selection, other
parts become modified. This is a very important subject, most imperfectly
understood. The most obvious case is, that modifications accumulated solely for
the good of the young or larva, will, it may safely be concluded, affect the
structure of the adult; in the same manner as any malconformation affecting the
early embryo, seriously affects the whole organisation of the adult. The several
parts of the body which are homologous, and which, at an early embryonic period,
are alike, seem liable to vary in an allied manner: we see this in the right and
left sides of the body varying in the same manner; in the front and hind legs,
and even in the jaws and limbs, varying together, for the lower jaw is believed
to be homologous with the limbs. These tendencies, I do not doubt, may be
mastered more or less completely by natural selection: thus a family of stags
once existed with an antler only on one side; and if this had been of any great
use to the breed it might probably have been rendered permanent by natural
selection. Homologous parts, as has been remarked by some authors, tend to
cohere; this is often seen in monstrous plants; and nothing is more common than
the union of homologous parts in normal structures, as the union of the petals
of the corolla into a tube. Hard parts seem to affect the form of adjoining soft
parts; it is believed by some authors that the diversity in the shape of the
pelvis in birds causes the remarkable diversity in the shape of their kidneys.
Others believe that the shape of the pelvis in the human mother influences by
pressure the shape of the head of the child. In snakes, according to Schlegel,
the shape of the body and the manner of swallowing determine the position of
several of the most important viscera. The nature of the bond of correlation is
very frequently quite obscure. M. Is. Geoffroy St. Hilaire has forcibly
remarked, that certain malconformations very frequently, and that others rarely
coexist, without our being able to assign any reason. What can be more singular
than the relation between blue eyes and deafness in cats, and the tortoise-shell
colour with the female sex; the feathered feet and skin between the outer toes
in pigeons, and the presence of more or less down on the young birds when first
hatched, with the future colour of their plumage; or, again, the relation
between the hair and teeth in the naked Turkish dog, though here probably
homology comes into play? With respect to this latter case of correlation, I
think it can hardly be accidental, that if we pick out the two orders of
mammalia which are most abnormal in their dermal coverings, viz. Cetacea
(whales) and Edentata (armadilloes, scaly ant-eaters, etc.), that these are
likewise the most abnormal in their teeth. I know of no case better adapted to
show the importance of the laws of correlation in modifying important
structures, independently of utility and, therefore, of natural selection, than
that of the difference between the outer and inner flowers in some Compositous
and Umbelliferous plants. Every one knows the difference in the ray and central
florets of, for instance, the daisy, and this difference is often accompanied
with the abortion of parts of the flower. But, in some Compositous plants, the
seeds also differ in shape and sculpture; and even the ovary itself, with its
accessory parts, differs, as has been described by Cassini. These differences
have been attributed by some authors to pressure, and the shape of the seeds in
the ray-florets in some Compositae countenances this idea; but, in the case of
the corolla of the Umbelliferae, it is by no means, as Dr. Hooker informs me, in
species with the densest heads that the inner and outer flowers most frequently
differ. It might have been thought that the development of the ray-petals by
drawing nourishment from certain other parts of the flower had caused their
abortion; but in some Compositae there is a difference in the seeds of the outer
and inner florets without any difference in the corolla. Possibly, these several
differences may be connected with some difference in the flow of nutriment
towards the central and external flowers: we know, at least, that in irregular
flowers, those nearest to the axis are oftenest subject to peloria, and become
regular. I may add, as an instance of this, and of a striking case of
correlation, that I have recently observed in some garden pelargoniums, that the
central flower of the truss often loses the patches of darker colour in the two
upper petals; and that when this occurs, the adherent nectary is quite aborted;
when the colour is absent from only one of the two upper petals, the nectary is
only much shortened. With respect to the difference in the corolla of the
central and exterior flowers of a head or umbel, I do not feel at all sure that
C. C. Sprengel's idea that the ray-florets serve to attract insects, whose
agency is highly advantageous in the fertilisation of plants of these two
orders, is so far-fetched, as it may at first appear: and if it be advantageous,
natural selection may have come into play. But in regard to the differences both
in the internal and external structure of the seeds, which are not always
correlated with any differences in the flowers, it seems impossible that they
can be in any way advantageous to the plant: yet in the Umbelliferae these
differences are of such apparent importance--the seeds being in some cases,
according to Tausch, orthospermous in the exterior flowers and coelospermous in
the central flowers,--that the elder De Candolle founded his main divisions of
the order on analogous differences. Hence we see that modifications of
structure, viewed by systematists as of high value, may be wholly due to unknown
laws of correlated growth, and without being, as far as we can see, of the
slightest service to the species. We may often falsely attribute to correlation
of growth, structures which are common to whole groups of species, and which in
truth are simply due to inheritance; for an ancient progenitor may have acquired
through natural selection some one modification in structure, and, after
thousands of generations, some other and independent modification; and these two
modifications, having been transmitted to a whole group of descendants with
diverse habits, would naturally be thought to be correlated in some necessary
manner. So, again, I do not doubt that some apparent correlations, occurring
throughout whole orders, are entirely due to the manner alone in which natural
selection can act. For instance, Alph. De Candolle has remarked that winged
seeds are never found in fruits which do not open: I should explain the rule by
the fact that seeds could not gradually become winged through natural selection,
except in fruits which opened; so that the individual plants producing seeds
which were a little better fitted to be wafted further, might get an advantage
over those producing seed less fitted for dispersal; and this process could not
possibly go on in fruit which did not open. The elder Geoffroy and Goethe
propounded, at about the same period, their law of compensation or balancement
of growth; or, as Goethe expressed it, "in order to spend on one side, nature is
forced to economise on the other side." I think this holds true to a certain
extent with our domestic productions: if nourishment flows to one part or organ
in excess, it rarely flows, at least in excess, to another part; thus it is
difficult to get a cow to give much milk and to fatten readily. The same
varieties of the cabbage do not yield abundant and nutritious foliage and a
copious supply of oil-bearing seeds. When the seeds in our fruits become
atrophied, the fruit itself gains largely in size and quality. In our poultry, a
large tuft of feathers on the head is generally accompanied by a diminished
comb, and a large beard by diminished wattles. With species in a state of nature
it can hardly be maintained that the law is of universal application; but many
good observers, more especially botanists, believe in its truth. I will not,
however, here give any instances, for I see hardly any way of distinguishing
between the effects, on the one hand, of a part being largely developed through
natural selection and another and adjoining part being reduced by this same
process or by disuse, and, on the other hand, the actual withdrawal of nutriment
from one part owing to the excess of growth in another and adjoining part. I
suspect, also, that some of the cases of compensation which have been advanced,
and likewise some other facts, may be merged under a more general principle,
namely, that natural selection is continually trying to economise in every part
of the organisation. If under changed conditions of life a structure before
useful becomes less useful, any diminution, however slight, in its development,
will be seized on by natural selection, for it will profit the individual not to
have its nutriment wasted in building up an useless structure. I can thus only
understand a fact with which I was much struck when examining cirripedes, and of
which many other instances could be given: namely, that when a cirripede is
parasitic within another and is thus protected, it loses more or less completely
its own shell or carapace. This is the case with the male Ibla, and in a truly
extraordinary manner with the Proteolepas: for the carapace in all other
cirripedes consists of the three highly-important anterior segments of the head
enormously developed, and furnished with great nerves and muscles; but in the
parasitic and protected Proteolepas, the whole anterior part of the head is
reduced to the merest rudiment attached to the bases of the prehensile antennae.
Now the saving of a large and complex structure, when rendered superfluous by
the parasitic habits of the Proteolepas, though effected by slow steps, would be
a decided advantage to each successive individual of the species; for in the
struggle for life to which every animal is exposed, each individual Proteolepas
would have a better chance of supporting itself, by less nutriment being wasted
in developing a structure now become useless. Thus, as I believe, natural
selection will always succeed in the long run in reducing and saving every part
of the organisation, as soon as it is rendered superfluous, without by any means
causing some other part to be largely developed in a corresponding degree. And,
conversely, that natural selection may perfectly well succeed in largely
developing any organ, without requiring as a necessary compensation the
reduction of some adjoining part. It seems to be a rule, as remarked by Is.
Geoffroy St. Hilaire, both in varieties and in species, that when any part or
organ is repeated many times in the structure of the same individual (as the
vertebrae in snakes, and the stamens in polyandrous flowers) the number is
variable; whereas the number of the same part or organ, when it occurs in lesser
numbers, is constant. The same author and some botanists have further remarked
that multiple parts are also very liable to variation in structure. Inasmuch as
this "vegetative repetition," to use Professor Owen's expression, seems to be a
sign of low organisation; the foregoing remark seems connected with the very
general opinion of naturalists, that beings low in the scale of nature are more
variable than those which are higher. I presume that lowness in this case means
that the several parts of the organisation have been but little specialised for
particular functions; and as long as the same part has to perform diversified
work, we can perhaps see why it should remain variable, that is, why natural
selection should have preserved or rejected each little deviation of form less
carefully than when the part has to serve for one special purpose alone. In the
same way that a knife which has to cut all sorts of things may be of almost any
shape; whilst a tool for some particular object had better be of some particular
shape. Natural selection, it should never be forgotten, can act on each part of
each being, solely through and for its advantage. Rudimentary parts, it has been
stated by some authors, and I believe with truth, are apt to be highly variable.
We shall have to recur to the general subject of rudimentary and aborted organs;
and I will here only add that their variability seems to be owing to their
uselessness, and therefore to natural selection having no power to check
deviations in their structure. Thus rudimentary parts are left to the free play
of the various laws of growth, to the effects of long-continued disuse, and to
the tendency to reversion. A PART DEVELOPED IN ANY SPECIES IN AN EXTRAORDINARY
DEGREE OR MANNER, IN COMPARISON WITH THE SAME PART IN ALLIED SPECIES, TENDS TO
BE HIGHLY VARIABLE. Several years ago I was much struck with a remark, nearly to
the above effect, published by Mr. Waterhouse. I infer also from an observation
made by Professor Owen, with respect to the length of the arms of the
ourang-outang, that he has come to a nearly similar conclusion. It is hopeless
to attempt to convince any one of the truth of this proposition without giving
the long array of facts which I have collected, and which cannot possibly be
here introduced. I can only state my conviction that it is a rule of high
generality. I am aware of several causes of error, but I hope that I have made
due allowance for them. It should be understood that the rule by no means
applies to any part, however unusually developed, unless it be unusually
developed in comparison with the same part in closely allied species. Thus, the
bat's wing is a most abnormal structure in the class mammalia; but the rule
would not here apply, because there is a whole group of bats having wings; it
would apply only if some one species of bat had its wings developed in some
remarkable manner in comparison with the other species of the same genus. The
rule applies very strongly in the case of secondary sexual characters, when
displayed in any unusual manner. The term, secondary sexual characters, used by
Hunter, applies to characters which are attached to one sex, but are not
directly connected with the act of reproduction. The rule applies to males and
females; but as females more rarely offer remarkable secondary sexual
characters, it applies more rarely to them. The rule being so plainly applicable
in the case of secondary sexual characters, may be due to the great variability
of these characters, whether or not displayed in any unusual manner--of which
fact I think there can be little doubt. But that our rule is not confined to
secondary sexual characters is clearly shown in the case of hermaphrodite
cirripedes; and I may here add, that I particularly attended to Mr. Waterhouse's
remark, whilst investigating this Order, and I am fully convinced that the rule
almost invariably holds good with cirripedes. I shall, in my future work, give a
list of the more remarkable cases; I will here only briefly give one, as it
illustrates the rule in its largest application. The opercular valves of sessile
cirripedes (rock barnacles) are, in every sense of the word, very important
structures, and they differ extremely little even in different genera; but in
the several species of one genus, Pyrgoma, these valves present a marvellous
amount of diversification: the homologous valves in the different species being
sometimes wholly unlike in shape; and the amount of variation in the individuals
of several of the species is so great, that it is no exaggeration to state that
the varieties differ more from each other in the characters of these important
valves than do other species of distinct genera. As birds within the same
country vary in a remarkably small degree, I have particularly attended to them,
and the rule seems to me certainly to hold good in this class. I cannot make out
that it applies to plants, and this would seriously have shaken my belief in its
truth, had not the great variability in plants made it particularly difficult to
compare their relative degrees of variability. When we see any part or organ
developed in a remarkable degree or manner in any species, the fair presumption
is that it is of high importance to that species; nevertheless the part in this
case is eminently liable to variation. Why should this be so? On the view that
each species has been independently created, with all its parts as we now see
them, I can see no explanation. But on the view that groups of species have
descended from other species, and have been modified through natural selection,
I think we can obtain some light. In our domestic animals, if any part, or the
whole animal, be neglected and no selection be applied, that part (for instance,
the comb in the Dorking fowl) or the whole breed will cease to have a nearly
uniform character. The breed will then be said to have degenerated. In
rudimentary organs, and in those which have been but little specialised for any
particular purpose, and perhaps in polymorphic groups, we see a nearly parallel
natural case; for in such cases natural selection either has not or cannot come
into full play, and thus the organisation is left in a fluctuating condition.
But what here more especially concerns us is, that in our domestic animals those
points, which at the present time are undergoing rapid change by continued
selection, are also eminently liable to variation. Look at the breeds of the
pigeon; see what a prodigious amount of difference there is in the beak of the
different tumblers, in the beak and wattle of the different carriers, in the
carriage and tail of our fantails, etc., these being the points now mainly
attended to by English fanciers. Even in the sub-breeds, as in the short-faced
tumbler, it is notoriously difficult to breed them nearly to perfection, and
frequently individuals are born which depart widely from the standard. There may
be truly said to be a constant struggle going on between, on the one hand, the
tendency to reversion to a less modified state, as well as an innate tendency to
further variability of all kinds, and, on the other hand, the power of steady
selection to keep the breed true. In the long run selection gains the day, and
we do not expect to fail so far as to breed a bird as coarse as a common tumbler
from a good short-faced strain. But as long as selection is rapidly going on,
there may always be expected to be much variability in the structure undergoing
modification. It further deserves notice that these variable characters,
produced by man's selection, sometimes become attached, from causes quite
unknown to us, more to one sex than to the other, generally to the male sex, as
with the wattle of carriers and the enlarged crop of pouters. Now let us turn to
nature. When a part has been developed in an extraordinary manner in any one
species, compared with the other species of the same genus, we may conclude that
this part has undergone an extraordinary amount of modification, since the
period when the species branched off from the common progenitor of the genus.
This period will seldom be remote in any extreme degree, as species very rarely
endure for more than one geological period. An extraordinary amount of
modification implies an unusually large and long-continued amount of
variability, which has continually been accumulated by natural selection for the
benefit of the species. But as the variability of the extraordinarily-developed
part or organ has been so great and long-continued within a period not
excessively remote, we might, as a general rule, expect still to find more
variability in such parts than in other parts of the organisation, which have
remained for a much longer period nearly constant. And this, I am convinced, is
the case. That the struggle between natural selection on the one hand, and the
tendency to reversion and variability on the other hand, will in the course of
time cease; and that the most abnormally developed organs may be made constant,
I can see no reason to doubt. Hence when an organ, however abnormal it may be,
has been transmitted in approximately the same condition to many modified
descendants, as in the case of the wing of the bat, it must have existed,
according to my theory, for an immense period in nearly the same state; and thus
it comes to be no more variable than any other structure. It is only in those
cases in which the modification has been comparatively recent and
extraordinarily great that we ought to find the GENERATIVE VARIABILITY, as it
may be called, still present in a high degree. For in this case the variability
will seldom as yet have been fixed by the continued selection of the individuals
varying in the required manner and degree, and by the continued rejection of
those tending to revert to a former and less modified condition. The principle
included in these remarks may be extended. It is notorious that specific
characters are more variable than generic. To explain by a simple example what
is meant. If some species in a large genus of plants had blue flowers and some
had red, the colour would be only a specific character, and no one would be
surprised at one of the blue species varying into red, or conversely; but if all
the species had blue flowers, the colour would become a generic character, and
its variation would be a more unusual circumstance. I have chosen this example
because an explanation is not in this case applicable, which most naturalists
would advance, namely, that specific characters are more variable than generic,
because they are taken from parts of less physiological importance than those
commonly used for classing genera. I believe this explanation is partly, yet
only indirectly, true; I shall, however, have to return to this subject in our
chapter on Classification. It would be almost superfluous to adduce evidence in
support of the above statement, that specific characters are more variable than
generic; but I have repeatedly noticed in works on natural history, that when an
author has remarked with surprise that some IMPORTANT organ or part, which is
generally very constant throughout large groups of species, has DIFFERED
considerably in closely-allied species, that it has, also, been VARIABLE in the
individuals of some of the species. And this fact shows that a character, which
is generally of generic value, when it sinks in value and becomes only of
specific value, often becomes variable, though its physiological importance may
remain the same. Something of the same kind applies to monstrosities: at least
Is. Geoffroy St. Hilaire seems to entertain no doubt, that the more an organ
normally differs in the different species of the same group, the more subject it
is to individual anomalies. On the ordinary view of each species having been
independently created, why should that part of the structure, which differs from
the same part in other independently-created species of the same genus, be more
variable than those parts which are closely alike in the several species? I do
not see that any explanation can be given. But on the view of species being only
strongly marked and fixed varieties, we might surely expect to find them still
often continuing to vary in those parts of their structure which have varied
within a moderately recent period, and which have thus come to differ. Or to
state the case in another manner:--the points in which all the species of a
genus resemble each other, and in which they differ from the species of some
other genus, are called generic characters; and these characters in common I
attribute to inheritance from a common progenitor, for it can rarely have
happened that natural selection will have modified several species, fitted to
more or less widely-different habits, in exactly the same manner: and as these
so-called generic characters have been inherited from a remote period, since
that period when the species first branched off from their common progenitor,
and subsequently have not varied or come to differ in any degree, or only in a
slight degree, it is not probable that they should vary at the present day. On
the other hand, the points in which species differ from other species of the
same genus, are called specific characters; and as these specific characters
have varied and come to differ within the period of the branching off of the
species from a common progenitor, it is probable that they should still often be
in some degree variable,--at least more variable than those parts of the
organisation which have for a very long period remained constant. In connexion
with the present subject, I will make only two other remarks. I think it will be
admitted, without my entering on details, that secondary sexual characters are
very variable; I think it also will be admitted that species of the same group
differ from each other more widely in their secondary sexual characters, than in
other parts of their organisation; compare, for instance, the amount of
difference between the males of gallinaceous birds, in which secondary sexual
characters are strongly displayed, with the amount of difference between their
females; and the truth of this proposition will be granted. The cause of the
original variability of secondary sexual characters is not manifest; but we can
see why these characters should not have been rendered as constant and uniform
as other parts of the organisation; for secondary sexual characters have been
accumulated by sexual selection, which is less rigid in its action than ordinary
selection, as it does not entail death, but only gives fewer offspring to the
less favoured males. Whatever the cause may be of the variability of secondary
sexual characters, as they are highly variable, sexual selection will have had a
wide scope for action, and may thus readily have succeeded in giving to the
species of the same group a greater amount of difference in their sexual
characters, than in other parts of their structure. It is a remarkable fact,
that the secondary sexual differences between the two sexes of the same species
are generally displayed in the very same parts of the organisation in which the
different species of the same genus differ from each other. Of this fact I will
give in illustration two instances, the first which happen to stand on my list;
and as the differences in these cases are of a very unusual nature, the relation
can hardly be accidental. The same number of joints in the tarsi is a character
generally common to very large groups of beetles, but in the Engidae, as
Westwood has remarked, the number varies greatly; and the number likewise
differs in the two sexes of the same species: again in fossorial hymenoptera,
the manner of neuration of the wings is a character of the highest importance,
because common to large groups; but in certain genera the neuration differs in
the different species, and likewise in the two sexes of the same species. This
relation has a clear meaning on my view of the subject: I look at all the
species of the same genus as having as certainly descended from the same
progenitor, as have the two sexes of any one of the species. Consequently,
whatever part of the structure of the common progenitor, or of its early
descendants, became variable; variations of this part would it is highly
probable, be taken advantage of by natural and sexual selection, in order to fit
the several species to their several places in the economy of nature, and
likewise to fit the two sexes of the same species to each other, or to fit the
males and females to different habits of life, or the males to struggle with
other males for the possession of the females. Finally, then, I conclude that
the greater variability of specific characters, or those which distinguish
species from species, than of generic characters, or those which the species
possess in common;--that the frequent extreme variability of any part which is
developed in a species in an extraordinary manner in comparison with the same
part in its congeners; and the not great degree of variability in a part,
however extraordinarily it may be developed, if it be common to a whole group of
species;--that the great variability of secondary sexual characters, and the
great amount of difference in these same characters between closely allied
species;--that secondary sexual and ordinary specific differences are generally
displayed in the same parts of the organisation,--are all principles closely
connected together. All being mainly due to the species of the same group having
descended from a common progenitor, from whom they have inherited much in
common,--to parts which have recently and largely varied being more likely still
to go on varying than parts which have long been inherited and have not
varied,--to natural selection having more or less completely, according to the
lapse of time, overmastered the tendency to reversion and to further
variability,--to sexual selection being less rigid than ordinary selection,--and
to variations in the same parts having been accumulated by natural and sexual
selection, and thus adapted for secondary sexual, and for ordinary specific
purposes. DISTINCT SPECIES PRESENT ANALOGOUS VARIATIONS; AND A VARIETY OF ONE
SPECIES OFTEN ASSUMES SOME OF THE CHARACTERS OF AN ALLIED SPECIES, OR REVERTS TO
SOME OF THE CHARACTERS OF AN EARLY PROGENITOR. These propositions will be most
readily understood by looking to our domestic races. The most distinct breeds of
pigeons, in countries most widely apart, present sub-varieties with reversed
feathers on the head and feathers on the feet,--characters not possessed by the
aboriginal rock-pigeon; these then are analogous variations in two or more
distinct races. The frequent presence of fourteen or even sixteen tail-feathers
in the pouter, may be considered as a variation representing the normal
structure of another race, the fantail. I presume that no one will doubt that
all such analogous variations are due to the several races of the pigeon having
inherited from a common parent the same constitution and tendency to variation,
when acted on by similar unknown influences. In the vegetable kingdom we have a
case of analogous variation, in the enlarged stems, or roots as commonly called,
of the Swedish turnip and Ruta baga, plants which several botanists rank as
varieties produced by cultivation from a common parent: if this be not so, the
case will then be one of analogous variation in two so-called distinct species;
and to these a third may be added, namely, the common turnip. According to the
ordinary view of each species having been independently created, we should have
to attribute this similarity in the enlarged stems of these three plants, not to
the vera causa of community of descent, and a consequent tendency to vary in a
like manner, but to three separate yet closely related acts of creation. With
pigeons, however, we have another case, namely, the occasional appearance in all
the breeds, of slaty-blue birds with two black bars on the wings, a white rump,
a bar at the end of the tail, with the outer feathers externally edged near
their bases with white. As all these marks are characteristic of the parent
rock-pigeon, I presume that no one will doubt that this is a case of reversion,
and not of a new yet analogous variation appearing in the several breeds. We may
I think confidently come to this conclusion, because, as we have seen, these
coloured marks are eminently liable to appear in the crossed offspring of two
distinct and differently coloured breeds; and in this case there is nothing in
the external conditions of life to cause the reappearance of the slaty-blue,
with the several marks, beyond the influence of the mere act of crossing on the
laws of inheritance. No doubt it is a very surprising fact that characters
should reappear after having been lost for many, perhaps for hundreds of
generations. But when a breed has been crossed only once by some other breed,
the offspring occasionally show a tendency to revert in character to the foreign
breed for many generations--some say, for a dozen or even a score of
generations. After twelve generations, the proportion of blood, to use a common
expression, of any one ancestor, is only 1 in 2048; and yet, as we see, it is
generally believed that a tendency to reversion is retained by this very small
proportion of foreign blood. In a breed which has not been crossed, but in which
BOTH parents have lost some character which their progenitor possessed, the
tendency, whether strong or weak, to reproduce the lost character might be, as
was formerly remarked, for all that we can see to the contrary, transmitted for
almost any number of generations. When a character which has been lost in a
breed, reappears after a great number of generations, the most probable
hypothesis is, not that the offspring suddenly takes after an ancestor some
hundred generations distant, but that in each successive generation there has
been a tendency to reproduce the character in question, which at last, under
unknown favourable conditions, gains an ascendancy. For instance, it is probable
that in each generation of the barb-pigeon, which produces most rarely a blue
and black-barred bird, there has been a tendency in each generation in the
plumage to assume this colour. This view is hypothetical, but could be supported
by some facts; and I can see no more abstract improbability in a tendency to
produce any character being inherited for an endless number of generations, than
in quite useless or rudimentary organs being, as we all know them to be, thus
inherited. Indeed, we may sometimes observe a mere tendency to produce a
rudiment inherited: for instance, in the common snapdragon (Antirrhinum) a
rudiment of a fifth stamen so often appears, that this plant must have an
inherited tendency to produce it. As all the species of the same genus are
supposed, on my theory, to have descended from a common parent, it might be
expected that they would occasionally vary in an analogous manner; so that a
variety of one species would resemble in some of its characters another species;
this other species being on my view only a well-marked and permanent variety.
But characters thus gained would probably be of an unimportant nature, for the
presence of all important characters will be governed by natural selection, in
accordance with the diverse habits of the species, and will not be left to the
mutual action of the conditions of life and of a similar inherited constitution.
It might further be expected that the species of the same genus would
occasionally exhibit reversions to lost ancestral characters. As, however, we
never know the exact character of the common ancestor of a group, we could not
distinguish these two cases: if, for instance, we did not know that the
rock-pigeon was not feather-footed or turn-crowned, we could not have told,
whether these characters in our domestic breeds were reversions or only
analogous variations; but we might have inferred that the blueness was a case of
reversion, from the number of the markings, which are correlated with the blue
tint, and which it does not appear probable would all appear together from
simple variation. More especially we might have inferred this, from the blue
colour and marks so often appearing when distinct breeds of diverse colours are
crossed. Hence, though under nature it must generally be left doubtful, what
cases are reversions to an anciently existing character, and what are new but
analogous variations, yet we ought, on my theory, sometimes to find the varying
offspring of a species assuming characters (either from reversion or from
analogous variation) which already occur in some other members of the same
group. And this undoubtedly is the case in nature. A considerable part of the
difficulty in recognising a variable species in our systematic works, is due to
its varieties mocking, as it were, some of the other species of the same genus.
A considerable catalogue, also, could be given of forms intermediate between two
other forms, which themselves must be doubtfully ranked as either varieties or
species; and this shows, unless all these forms be considered as independently
created species, that the one in varying has assumed some of the characters of
the other, so as to produce the intermediate form. But the best evidence is
afforded by parts or organs of an important and uniform nature occasionally
varying so as to acquire, in some degree, the character of the same part or
organ in an allied species. I have collected a long list of such cases; but
here, as before, I lie under a great disadvantage in not being able to give
them. I can only repeat that such cases certainly do occur, and seem to me very
remarkable. I will, however, give one curious and complex case, not indeed as
affecting any important character, but from occurring in several species of the
same genus, partly under domestication and partly under nature. It is a case
apparently of reversion. The ass not rarely has very distinct transverse bars on
its legs, like those on the legs of a zebra: it has been asserted that these are
plainest in the foal, and from inquiries which I have made, I believe this to be
true. It has also been asserted that the stripe on each shoulder is sometimes
double. The shoulder stripe is certainly very variable in length and outline. A
white ass, but NOT an albino, has been described without either spinal or
shoulder-stripe; and these stripes are sometimes very obscure, or actually quite
lost, in dark-coloured asses. The koulan of Pallas is said to have been seen
with a double shoulder-stripe. The hemionus has no shoulder-stripe; but traces
of it, as stated by Mr. Blyth and others, occasionally appear: and I have been
informed by Colonel Poole that the foals of this species are generally striped
on the legs, and faintly on the shoulder. The quagga, though so plainly barred
like a zebra over the body, is without bars on the legs; but Dr. Gray has
figured one specimen with very distinct zebra-like bars on the hocks. With
respect to the horse, I have collected cases in England of the spinal stripe in
horses of the most distinct breeds, and of ALL colours; transverse bars on the
legs are not rare in duns, mouse-duns, and in one instance in a chestnut: a
faint shoulder-stripe may sometimes be seen in duns, and I have seen a trace in
a bay horse. My son made a careful examination and sketch for me of a dun
Belgian cart-horse with a double stripe on each shoulder and with leg-stripes;
and a man, whom I can implicitly trust, has examined for me a small dun Welch
pony with THREE short parallel stripes on each shoulder. In the north-west part
of India the Kattywar breed of horses is so generally striped, that, as I hear
from Colonel Poole, who examined the breed for the Indian Government, a horse
without stripes is not considered as purely-bred. The spine is always striped;
the legs are generally barred; and the shoulder-stripe, which is sometimes
double and sometimes treble, is common; the side of the face, moreover, is
sometimes striped. The stripes are plainest in the foal; and sometimes quite
disappear in old horses. Colonel Poole has seen both gray and bay Kattywar
horses striped when first foaled. I have, also, reason to suspect, from
information given me by Mr. W. W. Edwards, that with the English race-horse the
spinal stripe is much commoner in the foal than in the full-grown animal.
Without here entering on further details, I may state that I have collected
cases of leg and shoulder stripes in horses of very different breeds, in various
countries from Britain to Eastern China; and from Norway in the north to the
Malay Archipelago in the south. In all parts of the world these stripes occur
far oftenest in duns and mouse-duns; by the term dun a large range of colour is
included, from one between brown and black to a close approach to cream-colour.
I am aware that Colonel Hamilton Smith, who has written on this subject,
believes that the several breeds of the horse have descended from several
aboriginal species--one of which, the dun, was striped; and that the
above-described appearances are all due to ancient crosses with the dun stock.
But I am not at all satisfied with this theory, and should be loth to apply it
to breeds so distinct as the heavy Belgian cart-horse, Welch ponies, cobs, the
lanky Kattywar race, etc., inhabiting the most distant parts of the world. Now
let us turn to the effects of crossing the several species of the horse-genus.
Rollin asserts, that the common mule from the ass and horse is particularly apt
to have bars on its legs. I once saw a mule with its legs so much striped that
any one at first would have thought that it must have been the product of a
zebra; and Mr. W. C. Martin, in his excellent treatise on the horse, has given a
figure of a similar mule. In four coloured drawings, which I have seen, of
hybrids between the ass and zebra, the legs were much more plainly barred than
the rest of the body; and in one of them there was a double shoulder-stripe. In
Lord Moreton's famous hybrid from a chestnut mare and male quagga, the hybrid,
and even the pure offspring subsequently produced from the mare by a black
Arabian sire, were much more plainly barred across the legs than is even the
pure quagga. Lastly, and this is another most remarkable case, a hybrid has been
figured by Dr. Gray (and he informs me that he knows of a second case) from the
ass and the hemionus; and this hybrid, though the ass seldom has stripes on its
legs and the hemionus has none and has not even a shoulder-stripe, nevertheless
had all four legs barred, and had three short shoulder-stripes, like those on
the dun Welch pony, and even had some zebra-like stripes on the sides of its
face. With respect to this last fact, I was so convinced that not even a stripe
of colour appears from what would commonly be called an accident, that I was led
solely from the occurrence of the face-stripes on this hybrid from the ass and
hemionus, to ask Colonel Poole whether such face-stripes ever occur in the
eminently striped Kattywar breed of horses, and was, as we have seen, answered
in the affirmative. What now are we to say to these several facts? We see
several very distinct species of the horse-genus becoming, by simple variation,
striped on the legs like a zebra, or striped on the shoulders like an ass. In
the horse we see this tendency strong whenever a dun tint appears--a tint which
approaches to that of the general colouring of the other species of the genus.
The appearance of the stripes is not accompanied by any change of form or by any
other new character. We see this tendency to become striped most strongly
displayed in hybrids from between several of the most distinct species. Now
observe the case of the several breeds of pigeons: they are descended from a
pigeon (including two or three sub-species or geographical races) of a bluish
colour, with certain bars and other marks; and when any breed assumes by simple
variation a bluish tint, these bars and other marks invariably reappear; but
without any other change of form or character. When the oldest and truest breeds
of various colours are crossed, we see a strong tendency for the blue tint and
bars and marks to reappear in the mongrels. I have stated that the most probable
hypothesis to account for the reappearance of very ancient characters, is--that
there is a TENDENCY in the young of each successive generation to produce the
long-lost character, and that this tendency, from unknown causes, sometimes
prevails. And we have just seen that in several species of the horse-genus the
stripes are either plainer or appear more commonly in the young than in the old.
Call the breeds of pigeons, some of which have bred true for centuries, species;
and how exactly parallel is the case with that of the species of the
horse-genus! For myself, I venture confidently to look back thousands on
thousands of generations, and I see an animal striped like a zebra, but perhaps
otherwise very differently constructed, the common parent of our domestic horse,
whether or not it be descended from one or more wild stocks, of the ass, the
hemionus, quagga, and zebra. He who believes that each equine species was
independently created, will, I presume, assert that each species has been
created with a tendency to vary, both under nature and under domestication, in
this particular manner, so as often to become striped like other species of the
genus; and that each has been created with a strong tendency, when crossed with
species inhabiting distant quarters of the world, to produce hybrids resembling
in their stripes, not their own parents, but other species of the genus. To
admit this view is, as it seems to me, to reject a real for an unreal, or at
least for an unknown, cause. It makes the works of God a mere mockery and
deception; I would almost as soon believe with the old and ignorant
cosmogonists, that fossil shells had never lived, but had been created in stone
so as to mock the shells now living on the sea-shore. SUMMARY. Our ignorance of
the laws of variation is profound. Not in one case out of a hundred can we
pretend to assign any reason why this or that part differs, more or less, from
the same part in the parents. But whenever we have the means of instituting a
comparison, the same laws appear to have acted in producing the lesser
differences between varieties of the same species, and the greater differences
between species of the same genus. The external conditions of life, as climate
and food, etc., seem to have induced some slight modifications. Habit in
producing constitutional differences, and use in strengthening, and disuse in
weakening and diminishing organs, seem to have been more potent in their
effects. Homologous parts tend to vary in the same way, and homologous parts
tend to cohere. Modifications in hard parts and in external parts sometimes
affect softer and internal parts. When one part is largely developed, perhaps it
tends to draw nourishment from the adjoining parts; and every part of the
structure which can be saved without detriment to the individual, will be saved.
Changes of structure at an early age will generally affect parts subsequently
developed; and there are very many other correlations of growth, the nature of
which we are utterly unable to understand. Multiple parts are variable in number
and in structure, perhaps arising from such parts not having been closely
specialised to any particular function, so that their modifications have not
been closely checked by natural selection. It is probably from this same cause
that organic beings low in the scale of nature are more variable than those
which have their whole organisation more specialised, and are higher in the
scale. Rudimentary organs, from being useless, will be disregarded by natural
selection, and hence probably are variable. Specific characters--that is, the
characters which have come to differ since the several species of the same genus
branched off from a common parent--are more variable than generic characters, or
those which have long been inherited, and have not differed within this same
period. In these remarks we have referred to special parts or organs being still
variable, because they have recently varied and thus come to differ; but we have
also seen in the second Chapter that the same principle applies to the whole
individual; for in a district where many species of any genus are found--that
is, where there has been much former variation and differentiation, or where the
manufactory of new specific forms has been actively at work--there, on an
average, we now find most varieties or incipient species. Secondary sexual
characters are highly variable, and such characters differ much in the species
of the same group. Variability in the same parts of the organisation has
generally been taken advantage of in giving secondary sexual differences to the
sexes of the same species, and specific differences to the several species of
the same genus. Any part or organ developed to an extraordinary size or in an
extraordinary manner, in comparison with the same part or organ in the allied
species, must have gone through an extraordinary amount of modification since
the genus arose; and thus we can understand why it should often still be
variable in a much higher degree than other parts; for variation is a
long-continued and slow process, and natural selection will in such cases not as
yet have had time to overcome the tendency to further variability and to
reversion to a less modified state. But when a species with any
extraordinarily-developed organ has become the parent of many modified
descendants--which on my view must be a very slow process, requiring a long
lapse of time--in this case, natural selection may readily have succeeded in
giving a fixed character to the organ, in however extraordinary a manner it may
be developed. Species inheriting nearly the same constitution from a common
parent and exposed to similar influences will naturally tend to present
analogous variations, and these same species may occasionally revert to some of
the characters of their ancient progenitors. Although new and important
modifications may not arise from reversion and analogous variation, such
modifications will add to the beautiful and harmonious diversity of nature.
Whatever the cause may be of each slight difference in the offspring from their
parents--and a cause for each must exist--it is the steady accumulation, through
natural selection, of such differences, when beneficial to the individual, that
gives rise to all the more important modifications of structure, by which the
innumerable beings on the face of this earth are enabled to struggle with each
other, and the best adapted to survive. CHAPTER 6. DIFFICULTIES ON THEORY.
Difficulties on the theory of descent with modification. Transitions. Absence or
rarity of transitional varieties. Transitions in habits of life. Diversified
habits in the same species. Species with habits widely different from those of
their allies. Organs of extreme perfection. Means of transition. Cases of
difficulty. Natura non facit saltum. Organs of small importance. Organs not in
all cases absolutely perfect. The law of Unity of Type and of the Conditions of
Existence embraced by the theory of Natural Selection. Long before having
arrived at this part of my work, a crowd of difficulties will have occurred to
the reader. Some of them are so grave that to this day I can never reflect on
them without being staggered; but, to the best of my judgment, the greater
number are only apparent, and those that are real are not, I think, fatal to my
theory. These difficulties and objections may be classed under the following
heads:-- Firstly, why, if species have descended from other species by
insensibly fine gradations, do we not everywhere see innumerable transitional
forms? Why is not all nature in confusion instead of the species being, as we
see them, well defined? Secondly, is it possible that an animal having, for
instance, the structure and habits of a bat, could have been formed by the
modification of some animal with wholly different habits? Can we believe that
natural selection could produce, on the one hand, organs of trifling importance,
such as the tail of a giraffe, which serves as a fly-flapper, and, on the other
hand, organs of such wonderful structure, as the eye, of which we hardly as yet
fully understand the inimitable perfection? Thirdly, can instincts be acquired
and modified through natural selection? What shall we say to so marvellous an
instinct as that which leads the bee to make cells, which have practically
anticipated the discoveries of profound mathematicians? Fourthly, how can we
account for species, when crossed, being sterile and producing sterile
offspring, whereas, when varieties are crossed, their fertility is unimpaired?
The two first heads shall be here discussed--Instinct and Hybridism in separate
chapters. ON THE ABSENCE OR RARITY OF TRANSITIONAL VARIETIES. As natural
selection acts solely by the preservation of profitable modifications, each new
form will tend in a fully-stocked country to take the place of, and finally to
exterminate, its own less improved parent or other less-favoured forms with
which it comes into competition. Thus extinction and natural selection will, as
we have seen, go hand in hand. Hence, if we look at each species as descended
from some other unknown form, both the parent and all the transitional varieties
will generally have been exterminated by the very process of formation and
perfection of the new form. But, as by this theory innumerable transitional
forms must have existed, why do we not find them embedded in countless numbers
in the crust of the earth? It will be much more convenient to discuss this
question in the chapter on the Imperfection of the geological record; and I will
here only state that I believe the answer mainly lies in the record being
incomparably less perfect than is generally supposed; the imperfection of the
record being chiefly due to organic beings not inhabiting profound depths of the
sea, and to their remains being embedded and preserved to a future age only in
masses of sediment sufficiently thick and extensive to withstand an enormous
amount of future degradation; and such fossiliferous masses can be accumulated
only where much sediment is deposited on the shallow bed of the sea, whilst it
slowly subsides. These contingencies will concur only rarely, and after
enormously long intervals. Whilst the bed of the sea is stationary or is rising,
or when very little sediment is being deposited, there will be blanks in our
geological history. The crust of the earth is a vast museum; but the natural
collections have been made only at intervals of time immensely remote. But it
may be urged that when several closely-allied species inhabit the same territory
we surely ought to find at the present time many transitional forms. Let us take
a simple case: in travelling from north to south over a continent, we generally
meet at successive intervals with closely allied or representative species,
evidently filling nearly the same place in the natural economy of the land.
These representative species often meet and interlock; and as the one becomes
rarer and rarer, the other becomes more and more frequent, till the one replaces
the other. But if we compare these species where they intermingle, they are
generally as absolutely distinct from each other in every detail of structure as
are specimens taken from the metropolis inhabited by each. By my theory these
allied species have descended from a common parent; and during the process of
modification, each has become adapted to the conditions of life of its own
region, and has supplanted and exterminated its original parent and all the
transitional varieties between its past and present states. Hence we ought not
to expect at the present time to meet with numerous transitional varieties in
each region, though they must have existed there, and may be embedded there in a
fossil condition. But in the intermediate region, having intermediate conditions
of life, why do we not now find closely-linking intermediate varieties? This
difficulty for a long time quite confounded me. But I think it can be in large
part explained. In the first place we should be extremely cautious in inferring,
because an area is now continuous, that it has been continuous during a long
period. Geology would lead us to believe that almost every continent has been
broken up into islands even during the later tertiary periods; and in such
islands distinct species might have been separately formed without the
possibility of intermediate varieties existing in the intermediate zones. By
changes in the form of the land and of climate, marine areas now continuous must
often have existed within recent times in a far less continuous and uniform
condition than at present. But I will pass over this way of escaping from the
difficulty; for I believe that many perfectly defined species have been formed
on strictly continuous areas; though I do not doubt that the formerly broken
condition of areas now continuous has played an important part in the formation
of new species, more especially with freely-crossing and wandering animals. In
looking at species as they are now distributed over a wide area, we generally
find them tolerably numerous over a large territory, then becoming somewhat
abruptly rarer and rarer on the confines, and finally disappearing. Hence the
neutral territory between two representative species is generally narrow in
comparison with the territory proper to each. We see the same fact in ascending
mountains, and sometimes it is quite remarkable how abruptly, as Alph. De
Candolle has observed, a common alpine species disappears. The same fact has
been noticed by Forbes in sounding the depths of the sea with the dredge. To
those who look at climate and the physical conditions of life as the
all-important elements of distribution, these facts ought to cause surprise, as
climate and height or depth graduate away insensibly. But when we bear in mind
that almost every species, even in its metropolis, would increase immensely in
numbers, were it not for other competing species; that nearly all either prey on
or serve as prey for others; in short, that each organic being is either
directly or indirectly related in the most important manner to other organic
beings, we must see that the range of the inhabitants of any country by no means
exclusively depends on insensibly changing physical conditions, but in large
part on the presence of other species, on which it depends, or by which it is
destroyed, or with which it comes into competition; and as these species are
already defined objects (however they may have become so), not blending one into
another by insensible gradations, the range of any one species, depending as it
does on the range of others, will tend to be sharply defined. Moreover, each
species on the confines of its range, where it exists in lessened numbers, will,
during fluctuations in the number of its enemies or of its prey, or in the
seasons, be extremely liable to utter extermination; and thus its geographical
range will come to be still more sharply defined. If I am right in believing
that allied or representative species, when inhabiting a continuous area, are
generally so distributed that each has a wide range, with a comparatively narrow
neutral territory between them, in which they become rather suddenly rarer and
rarer; then, as varieties do not essentially differ from species, the same rule
will probably apply to both; and if we in imagination adapt a varying species to
a very large area, we shall have to adapt two varieties to two large areas, and
a third variety to a narrow intermediate zone. The intermediate variety,
consequently, will exist in lesser numbers from inhabiting a narrow and lesser
area; and practically, as far as I can make out, this rule holds good with
varieties in a state of nature. I have met with striking instances of the rule
in the case of varieties intermediate between well-marked varieties in the genus
Balanus. And it would appear from information given me by Mr. Watson, Dr. Asa
Gray, and Mr. Wollaston, that generally when varieties intermediate between two
other forms occur, they are much rarer numerically than the forms which they
connect. Now, if we may trust these facts and inferences, and therefore conclude
that varieties linking two other varieties together have generally existed in
lesser numbers than the forms which they connect, then, I think, we can
understand why intermediate varieties should not endure for very long
periods;--why as a general rule they should be exterminated and disappear,
sooner than the forms which they originally linked together. For any form
existing in lesser numbers would, as already remarked, run a greater chance of
being exterminated than one existing in large numbers; and in this particular
case the intermediate form would be eminently liable to the inroads of closely
allied forms existing on both sides of it. But a far more important
consideration, as I believe, is that, during the process of further
modification, by which two varieties are supposed on my theory to be converted
and perfected into two distinct species, the two which exist in larger numbers
from inhabiting larger areas, will have a great advantage over the intermediate
variety, which exists in smaller numbers in a narrow and intermediate zone. For
forms existing in larger numbers will always have a better chance, within any
given period, of presenting further favourable variations for natural selection
to seize on, than will the rarer forms which exist in lesser numbers. Hence, the
more common forms, in the race for life, will tend to beat and supplant the less
common forms, for these will be more slowly modified and improved. It is the
same principle which, as I believe, accounts for the common species in each
country, as shown in the second chapter, presenting on an average a greater
number of well-marked varieties than do the rarer species. I may illustrate what
I mean by supposing three varieties of sheep to be kept, one adapted to an
extensive mountainous region; a second to a comparatively narrow, hilly tract;
and a third to wide plains at the base; and that the inhabitants are all trying
with equal steadiness and skill to improve their stocks by selection; the
chances in this case will be strongly in favour of the great holders on the
mountains or on the plains improving their breeds more quickly than the small
holders on the intermediate narrow, hilly tract; and consequently the improved
mountain or plain breed will soon take the place of the less improved hill
breed; and thus the two breeds, which originally existed in greater numbers,
will come into close contact with each other, without the interposition of the
supplanted, intermediate hill-variety. To sum up, I believe that species come to
be tolerably well-defined objects, and do not at any one period present an
inextricable chaos of varying and intermediate links: firstly, because new
varieties are very slowly formed, for variation is a very slow process, and
natural selection can do nothing until favourable variations chance to occur,
and until a place in the natural polity of the country can be better filled by
some modification of some one or more of its inhabitants. And such new places
will depend on slow changes of climate, or on the occasional immigration of new
inhabitants, and, probably, in a still more important degree, on some of the old
inhabitants becoming slowly modified, with the new forms thus produced and the
old ones acting and reacting on each other. So that, in any one region and at
any one time, we ought only to see a few species presenting slight modifications
of structure in some degree permanent; and this assuredly we do see. Secondly,
areas now continuous must often have existed within the recent period in
isolated portions, in which many forms, more especially amongst the classes
which unite for each birth and wander much, may have separately been rendered
sufficiently distinct to rank as representative species. In this case,
intermediate varieties between the several representative species and their
common parent, must formerly have existed in each broken portion of the land,
but these links will have been supplanted and exterminated during the process of
natural selection, so that they will no longer exist in a living state. Thirdly,
when two or more varieties have been formed in different portions of a strictly
continuous area, intermediate varieties will, it is probable, at first have been
formed in the intermediate zones, but they will generally have had a short
duration. For these intermediate varieties will, from reasons already assigned
(namely from what we know of the actual distribution of closely allied or
representative species, and likewise of acknowledged varieties), exist in the
intermediate zones in lesser numbers than the varieties which they tend to
connect. From this cause alone the intermediate varieties will be liable to
accidental extermination; and during the process of further modification through
natural selection, they will almost certainly be beaten and supplanted by the
forms which they connect; for these from existing in greater numbers will, in
the aggregate, present more variation, and thus be further improved through
natural selection and gain further advantages. Lastly, looking not to any one
time, but to all time, if my theory be true, numberless intermediate varieties,
linking most closely all the species of the same group together, must assuredly
have existed; but the very process of natural selection constantly tends, as has
been so often remarked, to exterminate the parent forms and the intermediate
links. Consequently evidence of their former existence could be found only
amongst fossil remains, which are preserved, as we shall in a future chapter
attempt to show, in an extremely imperfect and intermittent record. ON THE
ORIGIN AND TRANSITIONS OF ORGANIC BEINGS WITH PECULIAR HABITS AND STRUCTURE. It
has been asked by the opponents of such views as I hold, how, for instance, a
land carnivorous animal could have been converted into one with aquatic habits;
for how could the animal in its transitional state have subsisted? It would be
easy to show that within the same group carnivorous animals exist having every
intermediate grade between truly aquatic and strictly terrestrial habits; and as
each exists by a struggle for life, it is clear that each is well adapted in its
habits to its place in nature. Look at the Mustela vison of North America, which
has webbed feet and which resembles an otter in its fur, short legs, and form of
tail; during summer this animal dives for and preys on fish, but during the long
winter it leaves the frozen waters, and preys like other polecats on mice and
land animals. If a different case had been taken, and it had been asked how an
insectivorous quadruped could possibly have been converted into a flying bat,
the question would have been far more difficult, and I could have given no
answer. Yet I think such difficulties have very little weight. Here, as on other
occasions, I lie under a heavy disadvantage, for out of the many striking cases
which I have collected, I can give only one or two instances of transitional
habits and structures in closely allied species of the same genus; and of
diversified habits, either constant or occasional, in the same species. And it
seems to me that nothing less than a long list of such cases is sufficient to
lessen the difficulty in any particular case like that of the bat. Look at the
family of squirrels; here we have the finest gradation from animals with their
tails only slightly flattened, and from others, as Sir J. Richardson has
remarked, with the posterior part of their bodies rather wide and with the skin
on their flanks rather full, to the so-called flying squirrels; and flying
squirrels have their limbs and even the base of the tail united by a broad
expanse of skin, which serves as a parachute and allows them to glide through
the air to an astonishing distance from tree to tree. We cannot doubt that each
structure is of use to each kind of squirrel in its own country, by enabling it
to escape birds or beasts of prey, or to collect food more quickly, or, as there
is reason to believe, by lessening the danger from occasional falls. But it does
not follow from this fact that the structure of each squirrel is the best that
it is possible to conceive under all natural conditions. Let the climate and
vegetation change, let other competing rodents or new beasts of prey immigrate,
or old ones become modified, and all analogy would lead us to believe that some
at least of the squirrels would decrease in numbers or become exterminated,
unless they also became modified and improved in structure in a corresponding
manner. Therefore, I can see no difficulty, more especially under changing
conditions of life, in the continued preservation of individuals with fuller and
fuller flank-membranes, each modification being useful, each being propagated,
until by the accumulated effects of this process of natural selection, a perfect
so-called flying squirrel was produced. Now look at the Galeopithecus or flying
lemur, which formerly was falsely ranked amongst bats. It has an extremely wide
flank-membrane, stretching from the corners of the jaw to the tail, and
including the limbs and the elongated fingers: the flank membrane is, also,
furnished with an extensor muscle. Although no graduated links of structure,
fitted for gliding through the air, now connect the Galeopithecus with the other
Lemuridae, yet I can see no difficulty in supposing that such links formerly
existed, and that each had been formed by the same steps as in the case of the
less perfectly gliding squirrels; and that each grade of structure had been
useful to its possessor. Nor can I see any insuperable difficulty in further
believing it possible that the membrane-connected fingers and fore-arm of the
Galeopithecus might be greatly lengthened by natural selection; and this, as far
as the organs of flight are concerned, would convert it into a bat. In bats
which have the wing-membrane extended from the top of the shoulder to the tail,
including the hind-legs, we perhaps see traces of an apparatus originally
constructed for gliding through the air rather than for flight. If about a dozen
genera of birds had become extinct or were unknown, who would have ventured to
have surmised that birds might have existed which used their wings solely as
flappers, like the logger-headed duck (Micropterus of Eyton); as fins in the
water and front legs on the land, like the penguin; as sails, like the ostrich;
and functionally for no purpose, like the Apteryx. Yet the structure of each of
these birds is good for it, under the conditions of life to which it is exposed,
for each has to live by a struggle; but it is not necessarily the best possible
under all possible conditions. It must not be inferred from these remarks that
any of the grades of wing-structure here alluded to, which perhaps may all have
resulted from disuse, indicate the natural steps by which birds have acquired
their perfect power of flight; but they serve, at least, to show what
diversified means of transition are possible. Seeing that a few members of such
water-breathing classes as the Crustacea and Mollusca are adapted to live on the
land, and seeing that we have flying birds and mammals, flying insects of the
most diversified types, and formerly had flying reptiles, it is conceivable that
flying-fish, which now glide far through the air, slightly rising and turning by
the aid of their fluttering fins, might have been modified into perfectly winged
animals. If this had been effected, who would have ever imagined that in an
early transitional state they had been inhabitants of the open ocean, and had
used their incipient organs of flight exclusively, as far as we know, to escape
being devoured by other fish? When we see any structure highly perfected for any
particular habit, as the wings of a bird for flight, we should bear in mind that
animals displaying early transitional grades of the structure will seldom
continue to exist to the present day, for they will have been supplanted by the
very process of perfection through natural selection. Furthermore, we may
conclude that transitional grades between structures fitted for very different
habits of life will rarely have been developed at an early period in great
numbers and under many subordinate forms. Thus, to return to our imaginary
illustration of the flying-fish, it does not seem probable that fishes capable
of true flight would have been developed under many subordinate forms, for
taking prey of many kinds in many ways, on the land and in the water, until
their organs of flight had come to a high stage of perfection, so as to have
given them a decided advantage over other animals in the battle for life. Hence
the chance of discovering species with transitional grades of structure in a
fossil condition will always be less, from their having existed in lesser
numbers, than in the case of species with fully developed structures. I will now
give two or three instances of diversified and of changed habits in the
individuals of the same species. When either case occurs, it would be easy for
natural selection to fit the animal, by some modification of its structure, for
its changed habits, or exclusively for one of its several different habits. But
it is difficult to tell, and immaterial for us, whether habits generally change
first and structure afterwards; or whether slight modifications of structure
lead to changed habits; both probably often change almost simultaneously. Of
cases of changed habits it will suffice merely to allude to that of the many
British insects which now feed on exotic plants, or exclusively on artificial
substances. Of diversified habits innumerable instances could be given: I have
often watched a tyrant flycatcher (Saurophagus sulphuratus) in South America,
hovering over one spot and then proceeding to another, like a kestrel, and at
other times standing stationary on the margin of water, and then dashing like a
kingfisher at a fish. In our own country the larger titmouse (Parus major) may
be seen climbing branches, almost like a creeper; it often, like a shrike, kills
small birds by blows on the head; and I have many times seen and heard it
hammering the seeds of the yew on a branch, and thus breaking them like a
nuthatch. In North America the black bear was seen by Hearne swimming for hours
with widely open mouth, thus catching, like a whale, insects in the water. Even
in so extreme a case as this, if the supply of insects were constant, and if
better adapted competitors did not already exist in the country, I can see no
difficulty in a race of bears being rendered, by natural selection, more and
more aquatic in their structure and habits, with larger and larger mouths, till
a creature was produced as monstrous as a whale. As we sometimes see individuals
of a species following habits widely different from those both of their own
species and of the other species of the same genus, we might expect, on my
theory, that such individuals would occasionally have given rise to new species,
having anomalous habits, and with their structure either slightly or
considerably modified from that of their proper type. And such instances do
occur in nature. Can a more striking instance of adaptation be given than that
of a woodpecker for climbing trees and for seizing insects in the chinks of the
bark? Yet in North America there are woodpeckers which feed largely on fruit,
and others with elongated wings which chase insects on the wing; and on the
plains of La Plata, where not a tree grows, there is a woodpecker, which in
every essential part of its organisation, even in its colouring, in the harsh
tone of its voice, and undulatory flight, told me plainly of its close
blood-relationship to our common species; yet it is a woodpecker which never
climbs a tree! Petrels are the most aerial and oceanic of birds, yet in the
quiet Sounds of Tierra del Fuego, the Puffinuria berardi, in its general habits,
in its astonishing power of diving, its manner of swimming, and of flying when
unwillingly it takes flight, would be mistaken by any one for an auk or grebe;
nevertheless, it is essentially a petrel, but with many parts of its
organisation profoundly modified. On the other hand, the acutest observer by
examining the dead body of the water-ouzel would never have suspected its
sub-aquatic habits; yet this anomalous member of the strictly terrestrial thrush
family wholly subsists by diving,--grasping the stones with its feet and using
its wings under water. He who believes that each being has been created as we
now see it, must occasionally have felt surprise when he has met with an animal
having habits and structure not at all in agreement. What can be plainer than
that the webbed feet of ducks and geese are formed for swimming? yet there are
upland geese with webbed feet which rarely or never go near the water; and no
one except Audubon has seen the frigate-bird, which has all its four toes
webbed, alight on the surface of the sea. On the other hand, grebes and coots
are eminently aquatic, although their toes are only bordered by membrane. What
seems plainer than that the long toes of grallatores are formed for walking over
swamps and floating plants, yet the water-hen is nearly as aquatic as the coot;
and the landrail nearly as terrestrial as the quail or partridge. In such cases,
and many others could be given, habits have changed without a corresponding
change of structure. The webbed feet of the upland goose may be said to have
become rudimentary in function, though not in structure. In the frigate-bird,
the deeply-scooped membrane between the toes shows that structure has begun to
change. He who believes in separate and innumerable acts of creation will say,
that in these cases it has pleased the Creator to cause a being of one type to
take the place of one of another type; but this seems to me only restating the
fact in dignified language. He who believes in the struggle for existence and in
the principle of natural selection, will acknowledge that every organic being is
constantly endeavouring to increase in numbers; and that if any one being vary
ever so little, either in habits or structure, and thus gain an advantage over
some other inhabitant of the country, it will seize on the place of that
inhabitant, however different it may be from its own place. Hence it will cause
him no surprise that there should be geese and frigate-birds with webbed feet,
either living on the dry land or most rarely alighting on the water; that there
should be long-toed corncrakes living in meadows instead of in swamps; that
there should be woodpeckers where not a tree grows; that there should be diving
thrushes, and petrels with the habits of auks. ORGANS OF EXTREME PERFECTION AND
COMPLICATION. To suppose that the eye, with all its inimitable contrivances for
adjusting the focus to different distances, for admitting different amounts of
light, and for the correction of spherical and chromatic aberration, could have
been formed by natural selection, seems, I freely confess, absurd in the highest
possible degree. Yet reason tells me, that if numerous gradations from a perfect
and complex eye to one very imperfect and simple, each grade being useful to its
possessor, can be shown to exist; if further, the eye does vary ever so
slightly, and the variations be inherited, which is certainly the case; and if
any variation or modification in the organ be ever useful to an animal under
changing conditions of life, then the difficulty of believing that a perfect and
complex eye could be formed by natural selection, though insuperable by our
imagination, can hardly be considered real. How a nerve comes to be sensitive to
light, hardly concerns us more than how life itself first originated; but I may
remark that several facts make me suspect that any sensitive nerve may be
rendered sensitive to light, and likewise to those coarser vibrations of the air
which produce sound. In looking for the gradations by which an organ in any
species has been perfected, we ought to look exclusively to its lineal
ancestors; but this is scarcely ever possible, and we are forced in each case to
look to species of the same group, that is to the collateral descendants from
the same original parent-form, in order to see what gradations are possible, and
for the chance of some gradations having been transmitted from the earlier
stages of descent, in an unaltered or little altered condition. Amongst existing
Vertebrata, we find but a small amount of gradation in the structure of the eye,
and from fossil species we can learn nothing on this head. In this great class
we should probably have to descend far beneath the lowest known fossiliferous
stratum to discover the earlier stages, by which the eye has been perfected. In
the Articulata we can commence a series with an optic nerve merely coated with
pigment, and without any other mechanism; and from this low stage, numerous
gradations of structure, branching off in two fundamentally different lines, can
be shown to exist, until we reach a moderately high stage of perfection. In
certain crustaceans, for instance, there is a double cornea, the inner one
divided into facets, within each of which there is a lens-shaped swelling. In
other crustaceans the transparent cones which are coated by pigment, and which
properly act only by excluding lateral pencils of light, are convex at their
upper ends and must act by convergence; and at their lower ends there seems to
be an imperfect vitreous substance. With these facts, here far too briefly and
imperfectly given, which show that there is much graduated diversity in the eyes
of living crustaceans, and bearing in mind how small the number of living
animals is in proportion to those which have become extinct, I can see no very
great difficulty (not more than in the case of many other structures) in
believing that natural selection has converted the simple apparatus of an optic
nerve merely coated with pigment and invested by transparent membrane, into an
optical instrument as perfect as is possessed by any member of the great
Articulate class. He who will go thus far, if he find on finishing this treatise
that large bodies of facts, otherwise inexplicable, can be explained by the
theory of descent, ought not to hesitate to go further, and to admit that a
structure even as perfect as the eye of an eagle might be formed by natural
selection, although in this case he does not know any of the transitional
grades. His reason ought to conquer his imagination; though I have felt the
difficulty far too keenly to be surprised at any degree of hesitation in
extending the principle of natural selection to such startling lengths. It is
scarcely possible to avoid comparing the eye to a telescope. We know that this
instrument has been perfected by the long-continued efforts of the highest human
intellects; and we naturally infer that the eye has been formed by a somewhat
analogous process. But may not this inference be presumptuous? Have we any right
to assume that the Creator works by intellectual powers like those of man? If we
must compare the eye to an optical instrument, we ought in imagination to take a
thick layer of transparent tissue, with a nerve sensitive to light beneath, and
then suppose every part of this layer to be continually changing slowly in
density, so as to separate into layers of different densities and thicknesses,
placed at different distances from each other, and with the surfaces of each
layer slowly changing in form. Further we must suppose that there is a power
always intently watching each slight accidental alteration in the transparent
layers; and carefully selecting each alteration which, under varied
circumstances, may in any way, or in any degree, tend to produce a distincter
image. We must suppose each new state of the instrument to be multiplied by the
million; and each to be preserved till a better be produced, and then the old
ones to be destroyed. In living bodies, variation will cause the slight
alterations, generation will multiply them almost infinitely, and natural
selection will pick out with unerring skill each improvement. Let this process
go on for millions on millions of years; and during each year on millions of
individuals of many kinds; and may we not believe that a living optical
instrument might thus be formed as superior to one of glass, as the works of the
Creator are to those of man? If it could be demonstrated that any complex organ
existed, which could not possibly have been formed by numerous, successive,
slight modifications, my theory would absolutely break down. But I can find out
no such case. No doubt many organs exist of which we do not know the
transitional grades, more especially if we look to much-isolated species, round
which, according to my theory, there has been much extinction. Or again, if we
look to an organ common to all the members of a large class, for in this latter
case the organ must have been first formed at an extremely remote period, since
which all the many members of the class have been developed; and in order to
discover the early transitional grades through which the organ has passed, we
should have to look to very ancient ancestral forms, long since become extinct.
We should be extremely cautious in concluding that an organ could not have been
formed by transitional gradations of some kind. Numerous cases could be given
amongst the lower animals of the same organ performing at the same time wholly
distinct functions; thus the alimentary canal respires, digests, and excretes in
the larva of the dragon-fly and in the fish Cobites. In the Hydra, the animal
may be turned inside out, and the exterior surface will then digest and the
stomach respire. In such cases natural selection might easily specialise, if any
advantage were thus gained, a part or organ, which had performed two functions,
for one function alone, and thus wholly change its nature by insensible steps.
Two distinct organs sometimes perform simultaneously the same function in the
same individual; to give one instance, there are fish with gills or branchiae
that breathe the air dissolved in the water, at the same time that they breathe
free air in their swimbladders, this latter organ having a ductus pneumaticus
for its supply, and being divided by highly vascular partitions. In these cases,
one of the two organs might with ease be modified and perfected so as to perform
all the work by itself, being aided during the process of modification by the
other organ; and then this other organ might be modified for some other and
quite distinct purpose, or be quite obliterated. The illustration of the
swimbladder in fishes is a good one, because it shows us clearly the highly
important fact that an organ originally constructed for one purpose, namely
flotation, may be converted into one for a wholly different purpose, namely
respiration. The swimbladder has, also, been worked in as an accessory to the
auditory organs of certain fish, or, for I do not know which view is now
generally held, a part of the auditory apparatus has been worked in as a
complement to the swimbladder. All physiologists admit that the swimbladder is
homologous, or "ideally similar," in position and structure with the lungs of
the higher vertebrate animals: hence there seems to me to be no great difficulty
in believing that natural selection has actually converted a swimbladder into a
lung, or organ used exclusively for respiration. I can, indeed, hardly doubt
that all vertebrate animals having true lungs have descended by ordinary
generation from an ancient prototype, of which we know nothing, furnished with a
floating apparatus or swimbladder. We can thus, as I infer from Professor Owen's
interesting description of these parts, understand the strange fact that every
particle of food and drink which we swallow has to pass over the orifice of the
trachea, with some risk of falling into the lungs, notwithstanding the beautiful
contrivance by which the glottis is closed. In the higher Vertebrata the
branchiae have wholly disappeared--the slits on the sides of the neck and the
loop-like course of the arteries still marking in the embryo their former
position. But it is conceivable that the now utterly lost branchiae might have
been gradually worked in by natural selection for some quite distinct purpose:
in the same manner as, on the view entertained by some naturalists that the
branchiae and dorsal scales of Annelids are homologous with the wings and
wing-covers of insects, it is probable that organs which at a very ancient
period served for respiration have been actually converted into organs of
flight. In considering transitions of organs, it is so important to bear in mind
the probability of conversion from one function to another, that I will give one
more instance. Pedunculated cirripedes have two minute folds of skin, called by
me the ovigerous frena, which serve, through the means of a sticky secretion, to
retain the eggs until they are hatched within the sack. These cirripedes have no
branchiae, the whole surface of the body and sack, including the small frena,
serving for respiration. The Balanidae or sessile cirripedes, on the other hand,
have no ovigerous frena, the eggs lying loose at the bottom of the sack, in the
well-enclosed shell; but they have large folded branchiae. Now I think no one
will dispute that the ovigerous frena in the one family are strictly homologous
with the branchiae of the other family; indeed, they graduate into each other.
Therefore I do not doubt that little folds of skin, which originally served as
ovigerous frena, but which, likewise, very slightly aided the act of
respiration, have been gradually converted by natural selection into branchiae,
simply through an increase in their size and the obliteration of their adhesive
glands. If all pedunculated cirripedes had become extinct, and they have already
suffered far more extinction than have sessile cirripedes, who would ever have
imagined that the branchiae in this latter family had originally existed as
organs for preventing the ova from being washed out of the sack? Although we
must be extremely cautious in concluding that any organ could not possibly have
been produced by successive transitional gradations, yet, undoubtedly, grave
cases of difficulty occur, some of which will be discussed in my future work.
One of the gravest is that of neuter insects, which are often very differently
constructed from either the males or fertile females; but this case will be
treated of in the next chapter. The electric organs of fishes offer another case
of special difficulty; it is impossible to conceive by what steps these wondrous
organs have been produced; but, as Owen and others have remarked, their intimate
structure closely resembles that of common muscle; and as it has lately been
shown that Rays have an organ closely analogous to the electric apparatus, and
yet do not, as Matteuchi asserts, discharge any electricity, we must own that we
are far too ignorant to argue that no transition of any kind is possible. The
electric organs offer another and even more serious difficulty; for they occur
in only about a dozen fishes, of which several are widely remote in their
affinities. Generally when the same organ appears in several members of the same
class, especially if in members having very different habits of life, we may
attribute its presence to inheritance from a common ancestor; and its absence in
some of the members to its loss through disuse or natural selection. But if the
electric organs had been inherited from one ancient progenitor thus provided, we
might have expected that all electric fishes would have been specially related
to each other. Nor does geology at all lead to the belief that formerly most
fishes had electric organs, which most of their modified descendants have lost.
The presence of luminous organs in a few insects, belonging to different
families and orders, offers a parallel case of difficulty. Other cases could be
given; for instance in plants, the very curious contrivance of a mass of
pollen-grains, borne on a foot-stalk with a sticky gland at the end, is the same
in Orchis and Asclepias,--genera almost as remote as possible amongst flowering
plants. In all these cases of two very distinct species furnished with
apparently the same anomalous organ, it should be observed that, although the
general appearance and function of the organ may be the same, yet some
fundamental difference can generally be detected. I am inclined to believe that
in nearly the same way as two men have sometimes independently hit on the very
same invention, so natural selection, working for the good of each being and
taking advantage of analogous variations, has sometimes modified in very nearly
the same manner two parts in two organic beings, which owe but little of their
structure in common to inheritance from the same ancestor. Although in many
cases it is most difficult to conjecture by what transitions an organ could have
arrived at its present state; yet, considering that the proportion of living and
known forms to the extinct and unknown is very small, I have been astonished how
rarely an organ can be named, towards which no transitional grade is known to
lead. The truth of this remark is indeed shown by that old canon in natural
history of "Natura non facit saltum." We meet with this admission in the
writings of almost every experienced naturalist; or, as Milne Edwards has well
expressed it, nature is prodigal in variety, but niggard in innovation. Why, on
the theory of Creation, should this be so? Why should all the parts and organs
of many independent beings, each supposed to have been separately created for
its proper place in nature, be so invariably linked together by graduated steps?
Why should not Nature have taken a leap from structure to structure? On the
theory of natural selection, we can clearly understand why she should not; for
natural selection can act only by taking advantage of slight successive
variations; she can never take a leap, but must advance by the shortest and
slowest steps. ORGANS OF LITTLE APPARENT IMPORTANCE. As natural selection acts
by life and death,--by the preservation of individuals with any favourable
variation, and by the destruction of those with any unfavourable deviation of
structure,--I have sometimes felt much difficulty in understanding the origin of
simple parts, of which the importance does not seem sufficient to cause the
preservation of successively varying individuals. I have sometimes felt as much
difficulty, though of a very different kind, on this head, as in the case of an
organ as perfect and complex as the eye. In the first place, we are much too
ignorant in regard to the whole economy of any one organic being, to say what
slight modifications would be of importance or not. In a former chapter I have
given instances of most trifling characters, such as the down on fruit and the
colour of the flesh, which, from determining the attacks of insects or from
being correlated with constitutional differences, might assuredly be acted on by
natural selection. The tail of the giraffe looks like an artificially
constructed fly-flapper; and it seems at first incredible that this could have
been adapted for its present purpose by successive slight modifications, each
better and better, for so trifling an object as driving away flies; yet we
should pause before being too positive even in this case, for we know that the
distribution and existence of cattle and other animals in South America
absolutely depends on their power of resisting the attacks of insects: so that
individuals which could by any means defend themselves from these small enemies,
would be able to range into new pastures and thus gain a great advantage. It is
not that the larger quadrupeds are actually destroyed (except in some rare
cases) by the flies, but they are incessantly harassed and their strength
reduced, so that they are more subject to disease, or not so well enabled in a
coming dearth to search for food, or to escape from beasts of prey. Organs now
of trifling importance have probably in some cases been of high importance to an
early progenitor, and, after having been slowly perfected at a former period,
have been transmitted in nearly the same state, although now become of very
slight use; and any actually injurious deviations in their structure will always
have been checked by natural selection. Seeing how important an organ of
locomotion the tail is in most aquatic animals, its general presence and use for
many purposes in so many land animals, which in their lungs or modified
swim-bladders betray their aquatic origin, may perhaps be thus accounted for. A
well-developed tail having been formed in an aquatic animal, it might
subsequently come to be worked in for all sorts of purposes, as a fly-flapper,
an organ of prehension, or as an aid in turning, as with the dog, though the aid
must be slight, for the hare, with hardly any tail, can double quickly enough.
In the second place, we may sometimes attribute importance to characters which
are really of very little importance, and which have originated from quite
secondary causes, independently of natural selection. We should remember that
climate, food, etc., probably have some little direct influence on the
organisation; that characters reappear from the law of reversion; that
correlation of growth will have had a most important influence in modifying
various structures; and finally, that sexual selection will often have largely
modified the external characters of animals having a will, to give one male an
advantage in fighting with another or in charming the females. Moreover when a
modification of structure has primarily arisen from the above or other unknown
causes, it may at first have been of no advantage to the species, but may
subsequently have been taken advantage of by the descendants of the species
under new conditions of life and with newly acquired habits. To give a few
instances to illustrate these latter remarks. If green woodpeckers alone had
existed, and we did not know that there were many black and pied kinds, I dare
say that we should have thought that the green colour was a beautiful adaptation
to hide this tree-frequenting bird from its enemies; and consequently that it
was a character of importance and might have been acquired through natural
selection; as it is, I have no doubt that the colour is due to some quite
distinct cause, probably to sexual selection. A trailing bamboo in the Malay
Archipelago climbs the loftiest trees by the aid of exquisitely constructed
hooks clustered around the ends of the branches, and this contrivance, no doubt,
is of the highest service to the plant; but as we see nearly similar hooks on
many trees which are not climbers, the hooks on the bamboo may have arisen from
unknown laws of growth, and have been subsequently taken advantage of by the
plant undergoing further modification and becoming a climber. The naked skin on
the head of a vulture is generally looked at as a direct adaptation for
wallowing in putridity; and so it may be, or it may possibly be due to the
direct action of putrid matter; but we should be very cautious in drawing any
such inference, when we see that the skin on the head of the clean-feeding male
turkey is likewise naked. The sutures in the skulls of young mammals have been
advanced as a beautiful adaptation for aiding parturition, and no doubt they
facilitate, or may be indispensable for this act; but as sutures occur in the
skulls of young birds and reptiles, which have only to escape from a broken egg,
we may infer that this structure has arisen from the laws of growth, and has
been taken advantage of in the parturition of the higher animals. We are
profoundly ignorant of the causes producing slight and unimportant variations;
and we are immediately made conscious of this by reflecting on the differences
in the breeds of our domesticated animals in different countries,--more
especially in the less civilized countries where there has been but little
artificial selection. Careful observers are convinced that a damp climate
affects the growth of the hair, and that with the hair the horns are correlated.
Mountain breeds always differ from lowland breeds; and a mountainous country
would probably affect the hind limbs from exercising them more, and possibly
even the form of the pelvis; and then by the law of homologous variation, the
front limbs and even the head would probably be affected. The shape, also, of
the pelvis might affect by pressure the shape of the head of the young in the
womb. The laborious breathing necessary in high regions would, we have some
reason to believe, increase the size of the chest; and again correlation would
come into play. Animals kept by savages in different countries often have to
struggle for their own subsistence, and would be exposed to a certain extent to
natural selection, and individuals with slightly different constitutions would
succeed best under different climates; and there is reason to believe that
constitution and colour are correlated. A good observer, also, states that in
cattle susceptibility to the attacks of flies is correlated with colour, as is
the liability to be poisoned by certain plants; so that colour would be thus
subjected to the action of natural selection. But we are far too ignorant to
speculate on the relative importance of the several known and unknown laws of
variation; and I have here alluded to them only to show that, if we are unable
to account for the characteristic differences of our domestic breeds, which
nevertheless we generally admit to have arisen through ordinary generation, we
ought not to lay too much stress on our ignorance of the precise cause of the
slight analogous differences between species. I might have adduced for this same
purpose the differences between the races of man, which are so strongly marked;
I may add that some little light can apparently be thrown on the origin of these
differences, chiefly through sexual selection of a particular kind, but without
here entering on copious details my reasoning would appear frivolous. The
foregoing remarks lead me to say a few words on the protest lately made by some
naturalists, against the utilitarian doctrine that every detail of structure has
been produced for the good of its possessor. They believe that very many
structures have been created for beauty in the eyes of man, or for mere variety.
This doctrine, if true, would be absolutely fatal to my theory. Yet I fully
admit that many structures are of no direct use to their possessors. Physical
conditions probably have had some little effect on structure, quite
independently of any good thus gained. Correlation of growth has no doubt played
a most important part, and a useful modification of one part will often have
entailed on other parts diversified changes of no direct use. So again
characters which formerly were useful, or which formerly had arisen from
correlation of growth, or from other unknown cause, may reappear from the law of
reversion, though now of no direct use. The effects of sexual selection, when
displayed in beauty to charm the females, can be called useful only in rather a
forced sense. But by far the most important consideration is that the chief part
of the organisation of every being is simply due to inheritance; and
consequently, though each being assuredly is well fitted for its place in
nature, many structures now have no direct relation to the habits of life of
each species. Thus, we can hardly believe that the webbed feet of the upland
goose or of the frigate-bird are of special use to these birds; we cannot
believe that the same bones in the arm of the monkey, in the fore leg of the
horse, in the wing of the bat, and in the flipper of the seal, are of special
use to these animals. We may safely attribute these structures to inheritance.
But to the progenitor of the upland goose and of the frigate-bird, webbed feet
no doubt were as useful as they now are to the most aquatic of existing birds.
So we may believe that the progenitor of the seal had not a flipper, but a foot
with five toes fitted for walking or grasping; and we may further venture to
believe that the several bones in the limbs of the monkey, horse, and bat, which
have been inherited from a common progenitor, were formerly of more special use
to that progenitor, or its progenitors, than they now are to these animals
having such widely diversified habits. Therefore we may infer that these several
bones might have been acquired through natural selection, subjected formerly, as
now, to the several laws of inheritance, reversion, correlation of growth, etc.
Hence every detail of structure in every living creature (making some little
allowance for the direct action of physical conditions) may be viewed, either as
having been of special use to some ancestral form, or as being now of special
use to the descendants of this form--either directly, or indirectly through the
complex laws of growth. Natural selection cannot possibly produce any
modification in any one species exclusively for the good of another species;
though throughout nature one species incessantly takes advantage of, and profits
by, the structure of another. But natural selection can and does often produce
structures for the direct injury of other species, as we see in the fang of the
adder, and in the ovipositor of the ichneumon, by which its eggs are deposited
in the living bodies of other insects. If it could be proved that any part of
the structure of any one species had been formed for the exclusive good of
another species, it would annihilate my theory, for such could not have been
produced through natural selection. Although many statements may be found in
works on natural history to this effect, I cannot find even one which seems to
me of any weight. It is admitted that the rattlesnake has a poison-fang for its
own defence and for the destruction of its prey; but some authors suppose that
at the same time this snake is furnished with a rattle for its own injury,
namely, to warn its prey to escape. I would almost as soon believe that the cat
curls the end of its tail when preparing to spring, in order to warn the doomed
mouse. But I have not space here to enter on this and other such cases. Natural
selection will never produce in a being anything injurious to itself, for
natural selection acts solely by and for the good of each. No organ will be
formed, as Paley has remarked, for the purpose of causing pain or for doing an
injury to its possessor. If a fair balance be struck between the good and evil
caused by each part, each will be found on the whole advantageous. After the
lapse of time, under changing conditions of life, if any part comes to be
injurious, it will be modified; or if it be not so, the being will become
extinct, as myriads have become extinct. Natural selection tends only to make
each organic being as perfect as, or slightly more perfect than, the other
inhabitants of the same country with which it has to struggle for existence. And
we see that this is the degree of perfection attained under nature. The endemic
productions of New Zealand, for instance, are perfect one compared with another;
but they are now rapidly yielding before the advancing legions of plants and
animals introduced from Europe. Natural selection will not produce absolute
perfection, nor do we always meet, as far as we can judge, with this high
standard under nature. The correction for the aberration of light is said, on
high authority, not to be perfect even in that most perfect organ, the eye. If
our reason leads us to admire with enthusiasm a multitude of inimitable
contrivances in nature, this same reason tells us, though we may easily err on
both sides, that some other contrivances are less perfect. Can we consider the
sting of the wasp or of the bee as perfect, which, when used against many
attacking animals, cannot be withdrawn, owing to the backward serratures, and so
inevitably causes the death of the insect by tearing out its viscera? If we look
at the sting of the bee, as having originally existed in a remote progenitor as
a boring and serrated instrument, like that in so many members of the same great
order, and which has been modified but not perfected for its present purpose,
with the poison originally adapted to cause galls subsequently intensified, we
can perhaps understand how it is that the use of the sting should so often cause
the insect's own death: for if on the whole the power of stinging be useful to
the community, it will fulfil all the requirements of natural selection, though
it may cause the death of some few members. If we admire the truly wonderful
power of scent by which the males of many insects find their females, can we
admire the production for this single purpose of thousands of drones, which are
utterly useless to the community for any other end, and which are ultimately
slaughtered by their industrious and sterile sisters? It may be difficult, but
we ought to admire the savage instinctive hatred of the queen-bee, which urges
her instantly to destroy the young queens her daughters as soon as born, or to
perish herself in the combat; for undoubtedly this is for the good of the
community; and maternal love or maternal hatred, though the latter fortunately
is most rare, is all the same to the inexorable principle of natural selection.
If we admire the several ingenious contrivances, by which the flowers of the
orchis and of many other plants are fertilised through insect agency, can we
consider as equally perfect the elaboration by our fir-trees of dense clouds of
pollen, in order that a few granules may be wafted by a chance breeze on to the
ovules? SUMMARY OF CHAPTER. We have in this chapter discussed some of the
difficulties and objections which may be urged against my theory. Many of them
are very grave; but I think that in the discussion light has been thrown on
several facts, which on the theory of independent acts of creation are utterly
obscure. We have seen that species at any one period are not indefinitely
variable, and are not linked together by a multitude of intermediate gradations,
partly because the process of natural selection will always be very slow, and
will act, at any one time, only on a very few forms; and partly because the very
process of natural selection almost implies the continual supplanting and
extinction of preceding and intermediate gradations. Closely allied species, now
living on a continuous area, must often have been formed when the area was not
continuous, and when the conditions of life did not insensibly graduate away
from one part to another. When two varieties are formed in two districts of a
continuous area, an intermediate variety will often be formed, fitted for an
intermediate zone; but from reasons assigned, the intermediate variety will
usually exist in lesser numbers than the two forms which it connects;
consequently the two latter, during the course of further modification, from
existing in greater numbers, will have a great advantage over the less numerous
intermediate variety, and will thus generally succeed in supplanting and
exterminating it. We have seen in this chapter how cautious we should be in
concluding that the most different habits of life could not graduate into each
other; that a bat, for instance, could not have been formed by natural selection
from an animal which at first could only glide through the air. We have seen
that a species may under new conditions of life change its habits, or have
diversified habits, with some habits very unlike those of its nearest congeners.
Hence we can understand, bearing in mind that each organic being is trying to
live wherever it can live, how it has arisen that there are upland geese with
webbed feet, ground woodpeckers, diving thrushes, and petrels with the habits of
auks. Although the belief that an organ so perfect as the eye could have been
formed by natural selection, is more than enough to stagger any one; yet in the
case of any organ, if we know of a long series of gradations in complexity, each
good for its possessor, then, under changing conditions of life, there is no
logical impossibility in the acquirement of any conceivable degree of perfection
through natural selection. In the cases in which we know of no intermediate or
transitional states, we should be very cautious in concluding that none could
have existed, for the homologies of many organs and their intermediate states
show that wonderful metamorphoses in function are at least possible. For
instance, a swim-bladder has apparently been converted into an air-breathing
lung. The same organ having performed simultaneously very different functions,
and then having been specialised for one function; and two very distinct organs
having performed at the same time the same function, the one having been
perfected whilst aided by the other, must often have largely facilitated
transitions. We are far too ignorant, in almost every case, to be enabled to
assert that any part or organ is so unimportant for the welfare of a species,
that modifications in its structure could not have been slowly accumulated by
means of natural selection. But we may confidently believe that many
modifications, wholly due to the laws of growth, and at first in no way
advantageous to a species, have been subsequently taken advantage of by the
still further modified descendants of this species. We may, also, believe that a
part formerly of high importance has often been retained (as the tail of an
aquatic animal by its terrestrial descendants), though it has become of such
small importance that it could not, in its present state, have been acquired by
natural selection,--a power which acts solely by the preservation of profitable
variations in the struggle for life. Natural selection will produce nothing in
one species for the exclusive good or injury of another; though it may well
produce parts, organs, and excretions highly useful or even indispensable, or
highly injurious to another species, but in all cases at the same time useful to
the owner. Natural selection in each well-stocked country, must act chiefly
through the competition of the inhabitants one with another, and consequently
will produce perfection, or strength in the battle for life, only according to
the standard of that country. Hence the inhabitants of one country, generally
the smaller one, will often yield, as we see they do yield, to the inhabitants
of another and generally larger country. For in the larger country there will
have existed more individuals, and more diversified forms, and the competition
will have been severer, and thus the standard of perfection will have been
rendered higher. Natural selection will not necessarily produce absolute
perfection; nor, as far as we can judge by our limited faculties, can absolute
perfection be everywhere found. On the theory of natural selection we can
clearly understand the full meaning of that old canon in natural history,
"Natura non facit saltum." This canon, if we look only to the present
inhabitants of the world, is not strictly correct, but if we include all those
of past times, it must by my theory be strictly true. It is generally
acknowledged that all organic beings have been formed on two great laws--Unity
of Type, and the Conditions of Existence. By unity of type is meant that
fundamental agreement in structure, which we see in organic beings of the same
class, and which is quite independent of their habits of life. On my theory,
unity of type is explained by unity of descent. The expression of conditions of
existence, so often insisted on by the illustrious Cuvier, is fully embraced by
the principle of natural selection. For natural selection acts by either now
adapting the varying parts of each being to its organic and inorganic conditions
of life; or by having adapted them during long-past periods of time: the
adaptations being aided in some cases by use and disuse, being slightly affected
by the direct action of the external conditions of life, and being in all cases
subjected to the several laws of growth. Hence, in fact, the law of the
Conditions of Existence is the higher law; as it includes, through the
inheritance of former adaptations, that of Unity of Type. CHAPTER 7. INSTINCT.
Instincts comparable with habits, but different in their origin. Instincts
graduated. Aphides and ants. Instincts variable. Domestic instincts, their
origin. Natural instincts of the cuckoo, ostrich, and parasitic bees.
Slave-making ants. Hive-bee, its cell-making instinct. Difficulties on the
theory of the Natural Selection of instincts. Neuter or sterile insects.
Summary. The subject of instinct might have been worked into the previous
chapters; but I have thought that it would be more convenient to treat the
subject separately, especially as so wonderful an instinct as that of the
hive-bee making its cells will probably have occurred to many readers, as a
difficulty sufficient to overthrow my whole theory. I must premise, that I have
nothing to do with the origin of the primary mental powers, any more than I have
with that of life itself. We are concerned only with the diversities of instinct
and of the other mental qualities of animals within the same class. I will not
attempt any definition of instinct. It would be easy to show that several
distinct mental actions are commonly embraced by this term; but every one
understands what is meant, when it is said that instinct impels the cuckoo to
migrate and to lay her eggs in other birds' nests. An action, which we ourselves
should require experience to enable us to perform, when performed by an animal,
more especially by a very young one, without any experience, and when performed
by many individuals in the same way, without their knowing for what purpose it
is performed, is usually said to be instinctive. But I could show that none of
these characters of instinct are universal. A little dose, as Pierre Huber
expresses it, of judgment or reason, often comes into play, even in animals very
low in the scale of nature. Frederick Cuvier and several of the older
metaphysicians have compared instinct with habit. This comparison gives, I
think, a remarkably accurate notion of the frame of mind under which an
instinctive action is performed, but not of its origin. How unconsciously many
habitual actions are performed, indeed not rarely in direct opposition to our
conscious will! yet they may be modified by the will or reason. Habits easily
become associated with other habits, and with certain periods of time and states
of the body. When once acquired, they often remain constant throughout life.
Several other points of resemblance between instincts and habits could be
pointed out. As in repeating a well-known song, so in instincts, one action
follows another by a sort of rhythm; if a person be interrupted in a song, or in
repeating anything by rote, he is generally forced to go back to recover the
habitual train of thought: so P. Huber found it was with a caterpillar, which
makes a very complicated hammock; for if he took a caterpillar which had
completed its hammock up to, say, the sixth stage of construction, and put it
into a hammock completed up only to the third stage, the caterpillar simply
re-performed the fourth, fifth, and sixth stages of construction. If, however, a
caterpillar were taken out of a hammock made up, for instance, to the third
stage, and were put into one finished up to the sixth stage, so that much of its
work was already done for it, far from feeling the benefit of this, it was much
embarrassed, and, in order to complete its hammock, seemed forced to start from
the third stage, where it had left off, and thus tried to complete the already
finished work. If we suppose any habitual action to become inherited--and I
think it can be shown that this does sometimes happen--then the resemblance
between what originally was a habit and an instinct becomes so close as not to
be distinguished. If Mozart, instead of playing the pianoforte at three years
old with wonderfully little practice, had played a tune with no practice at all,
he might truly be said to have done so instinctively. But it would be the most
serious error to suppose that the greater number of instincts have been acquired
by habit in one generation, and then transmitted by inheritance to succeeding
generations. It can be clearly shown that the most wonderful instincts with
which we are acquainted, namely, those of the hive-bee and of many ants, could
not possibly have been thus acquired. It will be universally admitted that
instincts are as important as corporeal structure for the welfare of each
species, under its present conditions of life. Under changed conditions of life,
it is at least possible that slight modifications of instinct might be
profitable to a species; and if it can be shown that instincts do vary ever so
little, then I can see no difficulty in natural selection preserving and
continually accumulating variations of instinct to any extent that may be
profitable. It is thus, as I believe, that all the most complex and wonderful
instincts have originated. As modifications of corporeal structure arise from,
and are increased by, use or habit, and are diminished or lost by disuse, so I
do not doubt it has been with instincts. But I believe that the effects of habit
are of quite subordinate importance to the effects of the natural selection of
what may be called accidental variations of instincts;--that is of variations
produced by the same unknown causes which produce slight deviations of bodily
structure. No complex instinct can possibly be produced through natural
selection, except by the slow and gradual accumulation of numerous, slight, yet
profitable, variations. Hence, as in the case of corporeal structures, we ought
to find in nature, not the actual transitional gradations by which each complex
instinct has been acquired--for these could be found only in the lineal
ancestors of each species--but we ought to find in the collateral lines of
descent some evidence of such gradations; or we ought at least to be able to
show that gradations of some kind are possible; and this we certainly can do. I
have been surprised to find, making allowance for the instincts of animals
having been but little observed except in Europe and North America, and for no
instinct being known amongst extinct species, how very generally gradations,
leading to the most complex instincts, can be discovered. The canon of "Natura
non facit saltum" applies with almost equal force to instincts as to bodily
organs. Changes of instinct may sometimes be facilitated by the same species
having different instincts at different periods of life, or at different seasons
of the year, or when placed under different circumstances, etc.; in which case
either one or the other instinct might be preserved by natural selection. And
such instances of diversity of instinct in the same species can be shown to
occur in nature. Again as in the case of corporeal structure, and conformably
with my theory, the instinct of each species is good for itself, but has never,
as far as we can judge, been produced for the exclusive good of others. One of
the strongest instances of an animal apparently performing an action for the
sole good of another, with which I am acquainted, is that of aphides voluntarily
yielding their sweet excretion to ants: that they do so voluntarily, the
following facts show. I removed all the ants from a group of about a dozen
aphides on a dock-plant, and prevented their attendance during several hours.
After this interval, I felt sure that the aphides would want to excrete. I
watched them for some time through a lens, but not one excreted; I then tickled
and stroked them with a hair in the same manner, as well as I could, as the ants
do with their antennae; but not one excreted. Afterwards I allowed an ant to
visit them, and it immediately seemed, by its eager way of running about, to be
well aware what a rich flock it had discovered; it then began to play with its
antennae on the abdomen first of one aphis and then of another; and each aphis,
as soon as it felt the antennae, immediately lifted up its abdomen and excreted
a limpid drop of sweet juice, which was eagerly devoured by the ant. Even the
quite young aphides behaved in this manner, showing that the action was
instinctive, and not the result of experience. But as the excretion is extremely
viscid, it is probably a convenience to the aphides to have it removed; and
therefore probably the aphides do not instinctively excrete for the sole good of
the ants. Although I do not believe that any animal in the world performs an
action for the exclusive good of another of a distinct species, yet each species
tries to take advantage of the instincts of others, as each takes advantage of
the weaker bodily structure of others. So again, in some few cases, certain
instincts cannot be considered as absolutely perfect; but as details on this and
other such points are not indispensable, they may be here passed over. As some
degree of variation in instincts under a state of nature, and the inheritance of
such variations, are indispensable for the action of natural selection, as many
instances as possible ought to have been here given; but want of space prevents
me. I can only assert, that instincts certainly do vary--for instance, the
migratory instinct, both in extent and direction, and in its total loss. So it
is with the nests of birds, which vary partly in dependence on the situations
chosen, and on the nature and temperature of the country inhabited, but often
from causes wholly unknown to us: Audubon has given several remarkable cases of
differences in nests of the same species in the northern and southern United
States. Fear of any particular enemy is certainly an instinctive quality, as may
be seen in nestling birds, though it is strengthened by experience, and by the
sight of fear of the same enemy in other animals. But fear of man is slowly
acquired, as I have elsewhere shown, by various animals inhabiting desert
islands; and we may see an instance of this, even in England, in the greater
wildness of all our large birds than of our small birds; for the large birds
have been most persecuted by man. We may safely attribute the greater wildness
of our large birds to this cause; for in uninhabited islands large birds are not
more fearful than small; and the magpie, so wary in England, is tame in Norway,
as is the hooded crow in Egypt. That the general disposition of individuals of
the same species, born in a state of nature, is extremely diversified, can be
shown by a multitude of facts. Several cases also, could be given, of occasional
and strange habits in certain species, which might, if advantageous to the
species, give rise, through natural selection, to quite new instincts. But I am
well aware that these general statements, without facts given in detail, can
produce but a feeble effect on the reader's mind. I can only repeat my
assurance, that I do not speak without good evidence. The possibility, or even
probability, of inherited variations of instinct in a state of nature will be
strengthened by briefly considering a few cases under domestication. We shall
thus also be enabled to see the respective parts which habit and the selection
of so-called accidental variations have played in modifying the mental qualities
of our domestic animals. A number of curious and authentic instances could be
given of the inheritance of all shades of disposition and tastes, and likewise
of the oddest tricks, associated with certain frames of mind or periods of time.
But let us look to the familiar case of the several breeds of dogs: it cannot be
doubted that young pointers (I have myself seen a striking instance) will
sometimes point and even back other dogs the very first time that they are taken
out; retrieving is certainly in some degree inherited by retrievers; and a
tendency to run round, instead of at, a flock of sheep, by shepherd-dogs. I
cannot see that these actions, performed without experience by the young, and in
nearly the same manner by each individual, performed with eager delight by each
breed, and without the end being known,--for the young pointer can no more know
that he points to aid his master, than the white butterfly knows why she lays
her eggs on the leaf of the cabbage,--I cannot see that these actions differ
essentially from true instincts. If we were to see one kind of wolf, when young
and without any training, as soon as it scented its prey, stand motionless like
a statue, and then slowly crawl forward with a peculiar gait; and another kind
of wolf rushing round, instead of at, a herd of deer, and driving them to a
distant point, we should assuredly call these actions instinctive. Domestic
instincts, as they may be called, are certainly far less fixed or invariable
than natural instincts; but they have been acted on by far less rigorous
selection, and have been transmitted for an incomparably shorter period, under
less fixed conditions of life. How strongly these domestic instincts, habits,
and dispositions are inherited, and how curiously they become mingled, is well
shown when different breeds of dogs are crossed. Thus it is known that a cross
with a bull-dog has affected for many generations the courage and obstinacy of
greyhounds; and a cross with a greyhound has given to a whole family of
shepherd-dogs a tendency to hunt hares. These domestic instincts, when thus
tested by crossing, resemble natural instincts, which in a like manner become
curiously blended together, and for a long period exhibit traces of the
instincts of either parent: for example, Le Roy describes a dog, whose
great-grandfather was a wolf, and this dog showed a trace of its wild parentage
only in one way, by not coming in a straight line to his master when called.
Domestic instincts are sometimes spoken of as actions which have become
inherited solely from long-continued and compulsory habit, but this, I think, is
not true. No one would ever have thought of teaching, or probably could have
taught, the tumbler-pigeon to tumble,--an action which, as I have witnessed, is
performed by young birds, that have never seen a pigeon tumble. We may believe
that some one pigeon showed a slight tendency to this strange habit, and that
the long-continued selection of the best individuals in successive generations
made tumblers what they now are; and near Glasgow there are house-tumblers, as I
hear from Mr. Brent, which cannot fly eighteen inches high without going head
over heels. It may be doubted whether any one would have thought of training a
dog to point, had not some one dog naturally shown a tendency in this line; and
this is known occasionally to happen, as I once saw in a pure terrier. When the
first tendency was once displayed, methodical selection and the inherited
effects of compulsory training in each successive generation would soon complete
the work; and unconscious selection is still at work, as each man tries to
procure, without intending to improve the breed, dogs which will stand and hunt
best. On the other hand, habit alone in some cases has sufficed; no animal is
more difficult to tame than the young of the wild rabbit; scarcely any animal is
tamer than the young of the tame rabbit; but I do not suppose that domestic
rabbits have ever been selected for tameness; and I presume that we must
attribute the whole of the inherited change from extreme wildness to extreme
tameness, simply to habit and long-continued close confinement. Natural
instincts are lost under domestication: a remarkable instance of this is seen in
those breeds of fowls which very rarely or never become "broody," that is, never
wish to sit on their eggs. Familiarity alone prevents our seeing how universally
and largely the minds of our domestic animals have been modified by
domestication. It is scarcely possible to doubt that the love of man has become
instinctive in the dog. All wolves, foxes, jackals, and species of the cat
genus, when kept tame, are most eager to attack poultry, sheep, and pigs; and
this tendency has been found incurable in dogs which have been brought home as
puppies from countries, such as Tierra del Fuego and Australia, where the
savages do not keep these domestic animals. How rarely, on the other hand, do
our civilised dogs, even when quite young, require to be taught not to attack
poultry, sheep, and pigs! No doubt they occasionally do make an attack, and are
then beaten; and if not cured, they are destroyed; so that habit, with some
degree of selection, has probably concurred in civilising by inheritance our
dogs. On the other hand, young chickens have lost, wholly by habit, that fear of
the dog and cat which no doubt was originally instinctive in them, in the same
way as it is so plainly instinctive in young pheasants, though reared under a
hen. It is not that chickens have lost all fear, but fear only of dogs and cats,
for if the hen gives the danger-chuckle, they will run (more especially young
turkeys) from under her, and conceal themselves in the surrounding grass or
thickets; and this is evidently done for the instinctive purpose of allowing, as
we see in wild ground-birds, their mother to fly away. But this instinct
retained by our chickens has become useless under domestication, for the
mother-hen has almost lost by disuse the power of flight. Hence, we may
conclude, that domestic instincts have been acquired and natural instincts have
been lost partly by habit, and partly by man selecting and accumulating during
successive generations, peculiar mental habits and actions, which at first
appeared from what we must in our ignorance call an accident. In some cases
compulsory habit alone has sufficed to produce such inherited mental changes; in
other cases compulsory habit has done nothing, and all has been the result of
selection, pursued both methodically and unconsciously; but in most cases,
probably, habit and selection have acted together. We shall, perhaps, best
understand how instincts in a state of nature have become modified by selection,
by considering a few cases. I will select only three, out of the several which I
shall have to discuss in my future work,--namely, the instinct which leads the
cuckoo to lay her eggs in other birds' nests; the slave-making instinct of
certain ants; and the comb-making power of the hive-bee: these two latter
instincts have generally, and most justly, been ranked by naturalists as the
most wonderful of all known instincts. It is now commonly admitted that the more
immediate and final cause of the cuckoo's instinct is, that she lays her eggs,
not daily, but at intervals of two or three days; so that, if she were to make
her own nest and sit on her own eggs, those first laid would have to be left for
some time unincubated, or there would be eggs and young birds of different ages
in the same nest. If this were the case, the process of laying and hatching
might be inconveniently long, more especially as she has to migrate at a very
early period; and the first hatched young would probably have to be fed by the
male alone. But the American cuckoo is in this predicament; for she makes her
own nest and has eggs and young successively hatched, all at the same time. It
has been asserted that the American cuckoo occasionally lays her eggs in other
birds' nests; but I hear on the high authority of Dr. Brewer, that this is a
mistake. Nevertheless, I could give several instances of various birds which
have been known occasionally to lay their eggs in other birds' nests. Now let us
suppose that the ancient progenitor of our European cuckoo had the habits of the
American cuckoo; but that occasionally she laid an egg in another bird's nest.
If the old bird profited by this occasional habit, or if the young were made
more vigorous by advantage having been taken of the mistaken maternal instinct
of another bird, than by their own mother's care, encumbered as she can hardly
fail to be by having eggs and young of different ages at the same time; then the
old birds or the fostered young would gain an advantage. And analogy would lead
me to believe, that the young thus reared would be apt to follow by inheritance
the occasional and aberrant habit of their mother, and in their turn would be
apt to lay their eggs in other birds' nests, and thus be successful in rearing
their young. By a continued process of this nature, I believe that the strange
instinct of our cuckoo could be, and has been, generated. I may add that,
according to Dr. Gray and to some other observers, the European cuckoo has not
utterly lost all maternal love and care for her own offspring. The occasional
habit of birds laying their eggs in other birds' nests, either of the same or of
a distinct species, is not very uncommon with the Gallinaceae; and this perhaps
explains the origin of a singular instinct in the allied group of ostriches. For
several hen ostriches, at least in the case of the American species, unite and
lay first a few eggs in one nest and then in another; and these are hatched by
the males. This instinct may probably be accounted for by the fact of the hens
laying a large number of eggs; but, as in the case of the cuckoo, at intervals
of two or three days. This instinct, however, of the American ostrich has not as
yet been perfected; for a surprising number of eggs lie strewed over the plains,
so that in one day's hunting I picked up no less than twenty lost and wasted
eggs. Many bees are parasitic, and always lay their eggs in the nests of bees of
other kinds. This case is more remarkable than that of the cuckoo; for these
bees have not only their instincts but their structure modified in accordance
with their parasitic habits; for they do not possess the pollen-collecting
apparatus which would be necessary if they had to store food for their own
young. Some species, likewise, of Sphegidae (wasp-like insects) are parasitic on
other species; and M. Fabre has lately shown good reason for believing that
although the Tachytes nigra generally makes its own burrow and stores it with
paralysed prey for its own larvae to feed on, yet that when this insect finds a
burrow already made and stored by another sphex, it takes advantage of the
prize, and becomes for the occasion parasitic. In this case, as with the
supposed case of the cuckoo, I can see no difficulty in natural selection making
an occasional habit permanent, if of advantage to the species, and if the insect
whose nest and stored food are thus feloniously appropriated, be not thus
exterminated. SLAVE-MAKING INSTINCT. This remarkable instinct was first
discovered in the Formica (Polyerges) rufescens by Pierre Huber, a better
observer even than his celebrated father. This ant is absolutely dependent on
its slaves; without their aid, the species would certainly become extinct in a
single year. The males and fertile females do no work. The workers or sterile
females, though most energetic and courageous in capturing slaves, do no other
work. They are incapable of making their own nests, or of feeding their own
larvae. When the old nest is found inconvenient, and they have to migrate, it is
the slaves which determine the migration, and actually carry their masters in
their jaws. So utterly helpless are the masters, that when Huber shut up thirty
of them without a slave, but with plenty of the food which they like best, and
with their larvae and pupae to stimulate them to work, they did nothing; they
could not even feed themselves, and many perished of hunger. Huber then
introduced a single slave (F. fusca), and she instantly set to work, fed and
saved the survivors; made some cells and tended the larvae, and put all to
rights. What can be more extraordinary than these well-ascertained facts? If we
had not known of any other slave-making ant, it would have been hopeless to have
speculated how so wonderful an instinct could have been perfected. Formica
sanguinea was likewise first discovered by P. Huber to be a slave-making ant.
This species is found in the southern parts of England, and its habits have been
attended to by Mr. F. Smith, of the British Museum, to whom I am much indebted
for information on this and other subjects. Although fully trusting to the
statements of Huber and Mr. Smith, I tried to approach the subject in a
sceptical frame of mind, as any one may well be excused for doubting the truth
of so extraordinary and odious an instinct as that of making slaves. Hence I
will give the observations which I have myself made, in some little detail. I
opened fourteen nests of F. sanguinea, and found a few slaves in all. Males and
fertile females of the slave-species are found only in their own proper
communities, and have never been observed in the nests of F. sanguinea. The
slaves are black and not above half the size of their red masters, so that the
contrast in their appearance is very great. When the nest is slightly disturbed,
the slaves occasionally come out, and like their masters are much agitated and
defend the nest: when the nest is much disturbed and the larvae and pupae are
exposed, the slaves work energetically with their masters in carrying them away
to a place of safety. Hence, it is clear, that the slaves feel quite at home.
During the months of June and July, on three successive years, I have watched
for many hours several nests in Surrey and Sussex, and never saw a slave either
leave or enter a nest. As, during these months, the slaves are very few in
number, I thought that they might behave differently when more numerous; but Mr.
Smith informs me that he has watched the nests at various hours during May, June
and August, both in Surrey and Hampshire, and has never seen the slaves, though
present in large numbers in August, either leave or enter the nest. Hence he
considers them as strictly household slaves. The masters, on the other hand, may
be constantly seen bringing in materials for the nest, and food of all kinds.
During the present year, however, in the month of July, I came across a
community with an unusually large stock of slaves, and I observed a few slaves
mingled with their masters leaving the nest, and marching along the same road to
a tall Scotch-fir-tree, twenty-five yards distant, which they ascended together,
probably in search of aphides or cocci. According to Huber, who had ample
opportunities for observation, in Switzerland the slaves habitually work with
their masters in making the nest, and they alone open and close the doors in the
morning and evening; and, as Huber expressly states, their principal office is
to search for aphides. This difference in the usual habits of the masters and
slaves in the two countries, probably depends merely on the slaves being
captured in greater numbers in Switzerland than in England. One day I
fortunately chanced to witness a migration from one nest to another, and it was
a most interesting spectacle to behold the masters carefully carrying, as Huber
has described, their slaves in their jaws. Another day my attention was struck
by about a score of the slave-makers haunting the same spot, and evidently not
in search of food; they approached and were vigorously repulsed by an
independent community of the slave species (F. fusca); sometimes as many as
three of these ants clinging to the legs of the slave-making F. sanguinea. The
latter ruthlessly killed their small opponents, and carried their dead bodies as
food to their nest, twenty-nine yards distant; but they were prevented from
getting any pupae to rear as slaves. I then dug up a small parcel of the pupae
of F. fusca from another nest, and put them down on a bare spot near the place
of combat; they were eagerly seized, and carried off by the tyrants, who perhaps
fancied that, after all, they had been victorious in their late combat. At the
same time I laid on the same place a small parcel of the pupae of another
species, F. flava, with a few of these little yellow ants still clinging to the
fragments of the nest. This species is sometimes, though rarely, made into
slaves, as has been described by Mr. Smith. Although so small a species, it is
very courageous, and I have seen it ferociously attack other ants. In one
instance I found to my surprise an independent community of F. flava under a
stone beneath a nest of the slave-making F. sanguinea; and when I had
accidentally disturbed both nests, the little ants attacked their big neighbours
with surprising courage. Now I was curious to ascertain whether F. sanguinea
could distinguish the pupae of F. fusca, which they habitually make into slaves,
from those of the little and furious F. flava, which they rarely capture, and it
was evident that they did at once distinguish them: for we have seen that they
eagerly and instantly seized the pupae of F. fusca, whereas they were much
terrified when they came across the pupae, or even the earth from the nest of F.
flava, and quickly ran away; but in about a quarter of an hour, shortly after
all the little yellow ants had crawled away, they took heart and carried off the
pupae. One evening I visited another community of F. sanguinea, and found a
number of these ants entering their nest, carrying the dead bodies of F. fusca
(showing that it was not a migration) and numerous pupae. I traced the returning
file burthened with booty, for about forty yards, to a very thick clump of
heath, whence I saw the last individual of F. sanguinea emerge, carrying a pupa;
but I was not able to find the desolated nest in the thick heath. The nest,
however, must have been close at hand, for two or three individuals of F. fusca
were rushing about in the greatest agitation, and one was perched motionless
with its own pupa in its mouth on the top of a spray of heath over its ravaged
home. Such are the facts, though they did not need confirmation by me, in regard
to the wonderful instinct of making slaves. Let it be observed what a contrast
the instinctive habits of F. sanguinea present with those of the F. rufescens.
The latter does not build its own nest, does not determine its own migrations,
does not collect food for itself or its young, and cannot even feed itself: it
is absolutely dependent on its numerous slaves. Formica sanguinea, on the other
hand, possesses much fewer slaves, and in the early part of the summer extremely
few. The masters determine when and where a new nest shall be formed, and when
they migrate, the masters carry the slaves. Both in Switzerland and England the
slaves seem to have the exclusive care of the larvae, and the masters alone go
on slave-making expeditions. In Switzerland the slaves and masters work
together, making and bringing materials for the nest: both, but chiefly the
slaves, tend, and milk as it may be called, their aphides; and thus both collect
food for the community. In England the masters alone usually leave the nest to
collect building materials and food for themselves, their slaves and larvae. So
that the masters in this country receive much less service from their slaves
than they do in Switzerland. By what steps the instinct of F. sanguinea
originated I will not pretend to conjecture. But as ants, which are not
slave-makers, will, as I have seen, carry off pupae of other species, if
scattered near their nests, it is possible that pupae originally stored as food
might become developed; and the ants thus unintentionally reared would then
follow their proper instincts, and do what work they could. If their presence
proved useful to the species which had seized them--if it were more advantageous
to this species to capture workers than to procreate them--the habit of
collecting pupae originally for food might by natural selection be strengthened
and rendered permanent for the very different purpose of raising slaves. When
the instinct was once acquired, if carried out to a much less extent even than
in our British F. sanguinea, which, as we have seen, is less aided by its slaves
than the same species in Switzerland, I can see no difficulty in natural
selection increasing and modifying the instinct--always supposing each
modification to be of use to the species--until an ant was formed as abjectly
dependent on its slaves as is the Formica rufescens. CELL-MAKING INSTINCT OF THE
HIVE-BEE. I will not here enter on minute details on this subject, but will
merely give an outline of the conclusions at which I have arrived. He must be a
dull man who can examine the exquisite structure of a comb, so beautifully
adapted to its end, without enthusiastic admiration. We hear from mathematicians
that bees have practically solved a recondite problem, and have made their cells
of the proper shape to hold the greatest possible amount of honey, with the
least possible consumption of precious wax in their construction. It has been
remarked that a skilful workman, with fitting tools and measures, would find it
very difficult to make cells of wax of the true form, though this is perfectly
effected by a crowd of bees working in a dark hive. Grant whatever instincts you
please, and it seems at first quite inconceivable how they can make all the
necessary angles and planes, or even perceive when they are correctly made. But
the difficulty is not nearly so great as it at first appears: all this beautiful
work can be shown, I think, to follow from a few very simple instincts. I was
led to investigate this subject by Mr. Waterhouse, who has shown that the form
of the cell stands in close relation to the presence of adjoining cells; and the
following view may, perhaps, be considered only as a modification of his theory.
Let us look to the great principle of gradation, and see whether Nature does not
reveal to us her method of work. At one end of a short series we have
humble-bees, which use their old cocoons to hold honey, sometimes adding to them
short tubes of wax, and likewise making separate and very irregular rounded
cells of wax. At the other end of the series we have the cells of the hive-bee,
placed in a double layer: each cell, as is well known, is an hexagonal prism,
with the basal edges of its six sides bevelled so as to join on to a pyramid,
formed of three rhombs. These rhombs have certain angles, and the three which
form the pyramidal base of a single cell on one side of the comb, enter into the
composition of the bases of three adjoining cells on the opposite side. In the
series between the extreme perfection of the cells of the hive-bee and the
simplicity of those of the humble-bee, we have the cells of the Mexican Melipona
domestica, carefully described and figured by Pierre Huber. The Melipona itself
is intermediate in structure between the hive and humble bee, but more nearly
related to the latter: it forms a nearly regular waxen comb of cylindrical
cells, in which the young are hatched, and, in addition, some large cells of wax
for holding honey. These latter cells are nearly spherical and of nearly equal
sizes, and are aggregated into an irregular mass. But the important point to
notice, is that these cells are always made at that degree of nearness to each
other, that they would have intersected or broken into each other, if the
spheres had been completed; but this is never permitted, the bees building
perfectly flat walls of wax between the spheres which thus tend to intersect.
Hence each cell consists of an outer spherical portion and of two, three, or
more perfectly flat surfaces, according as the cell adjoins two, three or more
other cells. When one cell comes into contact with three other cells, which,
from the spheres being nearly of the same size, is very frequently and
necessarily the case, the three flat surfaces are united into a pyramid; and
this pyramid, as Huber has remarked, is manifestly a gross imitation of the
three-sided pyramidal basis of the cell of the hive-bee. As in the cells of the
hive-bee, so here, the three plane surfaces in any one cell necessarily enter
into the construction of three adjoining cells. It is obvious that the Melipona
saves wax by this manner of building; for the flat walls between the adjoining
cells are not double, but are of the same thickness as the outer spherical
portions, and yet each flat portion forms a part of two cells. Reflecting on
this case, it occurred to me that if the Melipona had made its spheres at some
given distance from each other, and had made them of equal sizes and had
arranged them symmetrically in a double layer, the resulting structure would
probably have been as perfect as the comb of the hive-bee. Accordingly I wrote
to Professor Miller, of Cambridge, and this geometer has kindly read over the
following statement, drawn up from his information, and tells me that it is
strictly correct:-- If a number of equal spheres be described with their centres
placed in two parallel layers; with the centre of each sphere at the distance of
radius x the square root of 2 or radius x 1.41421 (or at some lesser distance),
from the centres of the six surrounding spheres in the same layer; and at the
same distance from the centres of the adjoining spheres in the other and
parallel layer; then, if planes of intersection between the several spheres in
both layers be formed, there will result a double layer of hexagonal prisms
united together by pyramidal bases formed of three rhombs; and the rhombs and
the sides of the hexagonal prisms will have every angle identically the same
with the best measurements which have been made of the cells of the hive-bee.
Hence we may safely conclude that if we could slightly modify the instincts
already possessed by the Melipona, and in themselves not very wonderful, this
bee would make a structure as wonderfully perfect as that of the hive-bee. We
must suppose the Melipona to make her cells truly spherical, and of equal sizes;
and this would not be very surprising, seeing that she already does so to a
certain extent, and seeing what perfectly cylindrical burrows in wood many
insects can make, apparently by turning round on a fixed point. We must suppose
the Melipona to arrange her cells in level layers, as she already does her
cylindrical cells; and we must further suppose, and this is the greatest
difficulty, that she can somehow judge accurately at what distance to stand from
her fellow-labourers when several are making their spheres; but she is already
so far enabled to judge of distance, that she always describes her spheres so as
to intersect largely; and then she unites the points of intersection by
perfectly flat surfaces. We have further to suppose, but this is no difficulty,
that after hexagonal prisms have been formed by the intersection of adjoining
spheres in the same layer, she can prolong the hexagon to any length requisite
to hold the stock of honey; in the same way as the rude humble-bee adds
cylinders of wax to the circular mouths of her old cocoons. By such
modifications of instincts in themselves not very wonderful,--hardly more
wonderful than those which guide a bird to make its nest,--I believe that the
hive-bee has acquired, through natural selection, her inimitable architectural
powers. But this theory can be tested by experiment. Following the example of
Mr. Tegetmeier, I separated two combs, and put between them a long, thick,
square strip of wax: the bees instantly began to excavate minute circular pits
in it; and as they deepened these little pits, they made them wider and wider
until they were converted into shallow basins, appearing to the eye perfectly
true or parts of a sphere, and of about the diameter of a cell. It was most
interesting to me to observe that wherever several bees had begun to excavate
these basins near together, they had begun their work at such a distance from
each other, that by the time the basins had acquired the above stated width
(i.e. about the width of an ordinary cell), and were in depth about one sixth of
the diameter of the sphere of which they formed a part, the rims of the basins
intersected or broke into each other. As soon as this occurred, the bees ceased
to excavate, and began to build up flat walls of wax on the lines of
intersection between the basins, so that each hexagonal prism was built upon the
festooned edge of a smooth basin, instead of on the straight edges of a
three-sided pyramid as in the case of ordinary cells. I then put into the hive,
instead of a thick, square piece of wax, a thin and narrow, knife-edged ridge,
coloured with vermilion. The bees instantly began on both sides to excavate
little basins near to each other, in the same way as before; but the ridge of
wax was so thin, that the bottoms of the basins, if they had been excavated to
the same depth as in the former experiment, would have broken into each other
from the opposite sides. The bees, however, did not suffer this to happen, and
they stopped their excavations in due time; so that the basins, as soon as they
had been a little deepened, came to have flat bottoms; and these flat bottoms,
formed by thin little plates of the vermilion wax having been left ungnawed,
were situated, as far as the eye could judge, exactly along the planes of
imaginary intersection between the basins on the opposite sides of the ridge of
wax. In parts, only little bits, in other parts, large portions of a rhombic
plate had been left between the opposed basins, but the work, from the unnatural
state of things, had not been neatly performed. The bees must have worked at
very nearly the same rate on the opposite sides of the ridge of vermilion wax,
as they circularly gnawed away and deepened the basins on both sides, in order
to have succeeded in thus leaving flat plates between the basins, by stopping
work along the intermediate planes or planes of intersection. Considering how
flexible thin wax is, I do not see that there is any difficulty in the bees,
whilst at work on the two sides of a strip of wax, perceiving when they have
gnawed the wax away to the proper thinness, and then stopping their work. In
ordinary combs it has appeared to me that the bees do not always succeed in
working at exactly the same rate from the opposite sides; for I have noticed
half-completed rhombs at the base of a just-commenced cell, which were slightly
concave on one side, where I suppose that the bees had excavated too quickly,
and convex on the opposed side, where the bees had worked less quickly. In one
well-marked instance, I put the comb back into the hive, and allowed the bees to
go on working for a short time, and again examined the cell, and I found that
the rhombic plate had been completed, and had become PERFECTLY FLAT: it was
absolutely impossible, from the extreme thinness of the little rhombic plate,
that they could have effected this by gnawing away the convex side; and I
suspect that the bees in such cases stand in the opposed cells and push and bend
the ductile and warm wax (which as I have tried is easily done) into its proper
intermediate plane, and thus flatten it. From the experiment of the ridge of
vermilion wax, we can clearly see that if the bees were to build for themselves
a thin wall of wax, they could make their cells of the proper shape, by standing
at the proper distance from each other, by excavating at the same rate, and by
endeavouring to make equal spherical hollows, but never allowing the spheres to
break into each other. Now bees, as may be clearly seen by examining the edge of
a growing comb, do make a rough, circumferential wall or rim all round the comb;
and they gnaw into this from the opposite sides, always working circularly as
they deepen each cell. They do not make the whole three-sided pyramidal base of
any one cell at the same time, but only the one rhombic plate which stands on
the extreme growing margin, or the two plates, as the case may be; and they
never complete the upper edges of the rhombic plates, until the hexagonal walls
are commenced. Some of these statements differ from those made by the justly
celebrated elder Huber, but I am convinced of their accuracy; and if I had
space, I could show that they are conformable with my theory. Huber's statement
that the very first cell is excavated out of a little parallel-sided wall of
wax, is not, as far as I have seen, strictly correct; the first commencement
having always been a little hood of wax; but I will not here enter on these
details. We see how important a part excavation plays in the construction of the
cells; but it would be a great error to suppose that the bees cannot build up a
rough wall of wax in the proper position--that is, along the plane of
intersection between two adjoining spheres. I have several specimens showing
clearly that they can do this. Even in the rude circumferential rim or wall of
wax round a growing comb, flexures may sometimes be observed, corresponding in
position to the planes of the rhombic basal plates of future cells. But the
rough wall of wax has in every case to be finished off, by being largely gnawed
away on both sides. The manner in which the bees build is curious; they always
make the first rough wall from ten to twenty times thicker than the excessively
thin finished wall of the cell, which will ultimately be left. We shall
understand how they work, by supposing masons first to pile up a broad ridge of
cement, and then to begin cutting it away equally on both sides near the ground,
till a smooth, very thin wall is left in the middle; the masons always piling up
the cut-away cement, and adding fresh cement, on the summit of the ridge. We
shall thus have a thin wall steadily growing upward; but always crowned by a
gigantic coping. From all the cells, both those just commenced and those
completed, being thus crowned by a strong coping of wax, the bees can cluster
and crawl over the comb without injuring the delicate hexagonal walls, which are
only about one four-hundredth of an inch in thickness; the plates of the
pyramidal basis being about twice as thick. By this singular manner of building,
strength is continually given to the comb, with the utmost ultimate economy of
wax. It seems at first to add to the difficulty of understanding how the cells
are made, that a multitude of bees all work together; one bee after working a
short time at one cell going to another, so that, as Huber has stated, a score
of individuals work even at the commencement of the first cell. I was able
practically to show this fact, by covering the edges of the hexagonal walls of a
single cell, or the extreme margin of the circumferential rim of a growing comb,
with an extremely thin layer of melted vermilion wax; and I invariably found
that the colour was most delicately diffused by the bees--as delicately as a
painter could have done with his brush--by atoms of the coloured wax having been
taken from the spot on which it had been placed, and worked into the growing
edges of the cells all round. The work of construction seems to be a sort of
balance struck between many bees, all instinctively standing at the same
relative distance from each other, all trying to sweep equal spheres, and then
building up, or leaving ungnawed, the planes of intersection between these
spheres. It was really curious to note in cases of difficulty, as when two
pieces of comb met at an angle, how often the bees would entirely pull down and
rebuild in different ways the same cell, sometimes recurring to a shape which
they had at first rejected. When bees have a place on which they can stand in
their proper positions for working,--for instance, on a slip of wood, placed
directly under the middle of a comb growing downwards so that the comb has to be
built over one face of the slip--in this case the bees can lay the foundations
of one wall of a new hexagon, in its strictly proper place, projecting beyond
the other completed cells. It suffices that the bees should be enabled to stand
at their proper relative distances from each other and from the walls of the
last completed cells, and then, by striking imaginary spheres, they can build up
a wall intermediate between two adjoining spheres; but, as far as I have seen,
they never gnaw away and finish off the angles of a cell till a large part both
of that cell and of the adjoining cells has been built. This capacity in bees of
laying down under certain circumstances a rough wall in its proper place between
two just-commenced cells, is important, as it bears on a fact, which seems at
first quite subversive of the foregoing theory; namely, that the cells on the
extreme margin of wasp-combs are sometimes strictly hexagonal; but I have not
space here to enter on this subject. Nor does there seem to me any great
difficulty in a single insect (as in the case of a queen-wasp) making hexagonal
cells, if she work alternately on the inside and outside of two or three cells
commenced at the same time, always standing at the proper relative distance from
the parts of the cells just begun, sweeping spheres or cylinders, and building
up intermediate planes. It is even conceivable that an insect might, by fixing
on a point at which to commence a cell, and then moving outside, first to one
point, and then to five other points, at the proper relative distances from the
central point and from each other, strike the planes of intersection, and so
make an isolated hexagon: but I am not aware that any such case has been
observed; nor would any good be derived from a single hexagon being built, as in
its construction more materials would be required than for a cylinder. As
natural selection acts only by the accumulation of slight modifications of
structure or instinct, each profitable to the individual under its conditions of
life, it may reasonably be asked, how a long and graduated succession of
modified architectural instincts, all tending towards the present perfect plan
of construction, could have profited the progenitors of the hive-bee? I think
the answer is not difficult: it is known that bees are often hard pressed to get
sufficient nectar; and I am informed by Mr. Tegetmeier that it has been
experimentally found that no less than from twelve to fifteen pounds of dry
sugar are consumed by a hive of bees for the secretion of each pound of wax; so
that a prodigious quantity of fluid nectar must be collected and consumed by the
bees in a hive for the secretion of the wax necessary for the construction of
their combs. Moreover, many bees have to remain idle for many days during the
process of secretion. A large store of honey is indispensable to support a large
stock of bees during the winter; and the security of the hive is known mainly to
depend on a large number of bees being supported. Hence the saving of wax by
largely saving honey must be a most important element of success in any family
of bees. Of course the success of any species of bee may be dependent on the
number of its parasites or other enemies, or on quite distinct causes, and so be
altogether independent of the quantity of honey which the bees could collect.
But let us suppose that this latter circumstance determined, as it probably
often does determine, the numbers of a humble-bee which could exist in a
country; and let us further suppose that the community lived throughout the
winter, and consequently required a store of honey: there can in this case be no
doubt that it would be an advantage to our humble-bee, if a slight modification
of her instinct led her to make her waxen cells near together, so as to
intersect a little; for a wall in common even to two adjoining cells, would save
some little wax. Hence it would continually be more and more advantageous to our
humble-bee, if she were to make her cells more and more regular, nearer
together, and aggregated into a mass, like the cells of the Melipona; for in
this case a large part of the bounding surface of each cell would serve to bound
other cells, and much wax would be saved. Again, from the same cause, it would
be advantageous to the Melipona, if she were to make her cells closer together,
and more regular in every way than at present; for then, as we have seen, the
spherical surfaces would wholly disappear, and would all be replaced by plane
surfaces; and the Melipona would make a comb as perfect as that of the hive-bee.
Beyond this stage of perfection in architecture, natural selection could not
lead; for the comb of the hive-bee, as far as we can see, is absolutely perfect
in economising wax. Thus, as I believe, the most wonderful of all known
instincts, that of the hive-bee, can be explained by natural selection having
taken advantage of numerous, successive, slight modifications of simpler
instincts; natural selection having by slow degrees, more and more perfectly,
led the bees to sweep equal spheres at a given distance from each other in a
double layer, and to build up and excavate the wax along the planes of
intersection. The bees, of course, no more knowing that they swept their spheres
at one particular distance from each other, than they know what are the several
angles of the hexagonal prisms and of the basal rhombic plates. The motive power
of the process of natural selection having been economy of wax; that individual
swarm which wasted least honey in the secretion of wax, having succeeded best,
and having transmitted by inheritance its newly acquired economical instinct to
new swarms, which in their turn will have had the best chance of succeeding in
the struggle for existence. No doubt many instincts of very difficult
explanation could be opposed to the theory of natural selection,--cases, in
which we cannot see how an instinct could possibly have originated; cases, in
which no intermediate gradations are known to exist; cases of instinct of
apparently such trifling importance, that they could hardly have been acted on
by natural selection; cases of instincts almost identically the same in animals
so remote in the scale of nature, that we cannot account for their similarity by
inheritance from a common parent, and must therefore believe that they have been
acquired by independent acts of natural selection. I will not here enter on
these several cases, but will confine myself to one special difficulty, which at
first appeared to me insuperable, and actually fatal to my whole theory. I
allude to the neuters or sterile females in insect-communities: for these
neuters often differ widely in instinct and in structure from both the males and
fertile females, and yet, from being sterile, they cannot propagate their kind.
The subject well deserves to be discussed at great length, but I will here take
only a single case, that of working or sterile ants. How the workers have been
rendered sterile is a difficulty; but not much greater than that of any other
striking modification of structure; for it can be shown that some insects and
other articulate animals in a state of nature occasionally become sterile; and
if such insects had been social, and it had been profitable to the community
that a number should have been annually born capable of work, but incapable of
procreation, I can see no very great difficulty in this being effected by
natural selection. But I must pass over this preliminary difficulty. The great
difficulty lies in the working ants differing widely from both the males and the
fertile females in structure, as in the shape of the thorax and in being
destitute of wings and sometimes of eyes, and in instinct. As far as instinct
alone is concerned, the prodigious difference in this respect between the
workers and the perfect females, would have been far better exemplified by the
hive-bee. If a working ant or other neuter insect had been an animal in the
ordinary state, I should have unhesitatingly assumed that all its characters had
been slowly acquired through natural selection; namely, by an individual having
been born with some slight profitable modification of structure, this being
inherited by its offspring, which again varied and were again selected, and so
onwards. But with the working ant we have an insect differing greatly from its
parents, yet absolutely sterile; so that it could never have transmitted
successively acquired modifications of structure or instinct to its progeny. It
may well be asked how is it possible to reconcile this case with the theory of
natural selection? First, let it be remembered that we have innumerable
instances, both in our domestic productions and in those in a state of nature,
of all sorts of differences of structure which have become correlated to certain
ages, and to either sex. We have differences correlated not only to one sex, but
to that short period alone when the reproductive system is active, as in the
nuptial plumage of many birds, and in the hooked jaws of the male salmon. We
have even slight differences in the horns of different breeds of cattle in
relation to an artificially imperfect state of the male sex; for oxen of certain
breeds have longer horns than in other breeds, in comparison with the horns of
the bulls or cows of these same breeds. Hence I can see no real difficulty in
any character having become correlated with the sterile condition of certain
members of insect-communities: the difficulty lies in understanding how such
correlated modifications of structure could have been slowly accumulated by
natural selection. This difficulty, though appearing insuperable, is lessened,
or, as I believe, disappears, when it is remembered that selection may be
applied to the family, as well as to the individual, and may thus gain the
desired end. Thus, a well-flavoured vegetable is cooked, and the individual is
destroyed; but the horticulturist sows seeds of the same stock, and confidently
expects to get nearly the same variety; breeders of cattle wish the flesh and
fat to be well marbled together; the animal has been slaughtered, but the
breeder goes with confidence to the same family. I have such faith in the powers
of selection, that I do not doubt that a breed of cattle, always yielding oxen
with extraordinarily long horns, could be slowly formed by carefully watching
which individual bulls and cows, when matched, produced oxen with the longest
horns; and yet no one ox could ever have propagated its kind. Thus I believe it
has been with social insects: a slight modification of structure, or instinct,
correlated with the sterile condition of certain members of the community, has
been advantageous to the community: consequently the fertile males and females
of the same community flourished, and transmitted to their fertile offspring a
tendency to produce sterile members having the same modification. And I believe
that this process has been repeated, until that prodigious amount of difference
between the fertile and sterile females of the same species has been produced,
which we see in many social insects. But we have not as yet touched on the
climax of the difficulty; namely, the fact that the neuters of several ants
differ, not only from the fertile females and males, but from each other,
sometimes to an almost incredible degree, and are thus divided into two or even
three castes. The castes, moreover, do not generally graduate into each other,
but are perfectly well defined; being as distinct from each other, as are any
two species of the same genus, or rather as any two genera of the same family.
Thus in Eciton, there are working and soldier neuters, with jaws and instincts
extraordinarily different: in Cryptocerus, the workers of one caste alone carry
a wonderful sort of shield on their heads, the use of which is quite unknown: in
the Mexican Myrmecocystus, the workers of one caste never leave the nest; they
are fed by the workers of another caste, and they have an enormously developed
abdomen which secretes a sort of honey, supplying the place of that excreted by
the aphides, or the domestic cattle as they may be called, which our European
ants guard or imprison. It will indeed be thought that I have an overweening
confidence in the principle of natural selection, when I do not admit that such
wonderful and well-established facts at once annihilate my theory. In the
simpler case of neuter insects all of one caste or of the same kind, which have
been rendered by natural selection, as I believe to be quite possible, different
from the fertile males and females,--in this case, we may safely conclude from
the analogy of ordinary variations, that each successive, slight, profitable
modification did not probably at first appear in all the individual neuters in
the same nest, but in a few alone; and that by the long-continued selection of
the fertile parents which produced most neuters with the profitable
modification, all the neuters ultimately came to have the desired character. On
this view we ought occasionally to find neuter-insects of the same species, in
the same nest, presenting gradations of structure; and this we do find, even
often, considering how few neuter-insects out of Europe have been carefully
examined. Mr. F. Smith has shown how surprisingly the neuters of several British
ants differ from each other in size and sometimes in colour; and that the
extreme forms can sometimes be perfectly linked together by individuals taken
out of the same nest: I have myself compared perfect gradations of this kind. It
often happens that the larger or the smaller sized workers are the most
numerous; or that both large and small are numerous, with those of an
intermediate size scanty in numbers. Formica flava has larger and smaller
workers, with some of intermediate size; and, in this species, as Mr. F. Smith
has observed, the larger workers have simple eyes (ocelli), which though small
can be plainly distinguished, whereas the smaller workers have their ocelli
rudimentary. Having carefully dissected several specimens of these workers, I
can affirm that the eyes are far more rudimentary in the smaller workers than
can be accounted for merely by their proportionally lesser size; and I fully
believe, though I dare not assert so positively, that the workers of
intermediate size have their ocelli in an exactly intermediate condition. So
that we here have two bodies of sterile workers in the same nest, differing not
only in size, but in their organs of vision, yet connected by some few members
in an intermediate condition. I may digress by adding, that if the smaller
workers had been the most useful to the community, and those males and females
had been continually selected, which produced more and more of the smaller
workers, until all the workers had come to be in this condition; we should then
have had a species of ant with neuters very nearly in the same condition with
those of Myrmica. For the workers of Myrmica have not even rudiments of ocelli,
though the male and female ants of this genus have well-developed ocelli. I may
give one other case: so confidently did I expect to find gradations in important
points of structure between the different castes of neuters in the same species,
that I gladly availed myself of Mr. F. Smith's offer of numerous specimens from
the same nest of the driver ant (Anomma) of West Africa. The reader will perhaps
best appreciate the amount of difference in these workers, by my giving not the
actual measurements, but a strictly accurate illustration: the difference was
the same as if we were to see a set of workmen building a house of whom many
were five feet four inches high, and many sixteen feet high; but we must suppose
that the larger workmen had heads four instead of three times as big as those of
the smaller men, and jaws nearly five times as big. The jaws, moreover, of the
working ants of the several sizes differed wonderfully in shape, and in the form
and number of the teeth. But the important fact for us is, that though the
workers can be grouped into castes of different sizes, yet they graduate
insensibly into each other, as does the widely-different structure of their
jaws. I speak confidently on this latter point, as Mr. Lubbock made drawings for
me with the camera lucida of the jaws which I had dissected from the workers of
the several sizes. With these facts before me, I believe that natural selection,
by acting on the fertile parents, could form a species which should regularly
produce neuters, either all of large size with one form of jaw, or all of small
size with jaws having a widely different structure; or lastly, and this is our
climax of difficulty, one set of workers of one size and structure, and
simultaneously another set of workers of a different size and structure;--a
graduated series having been first formed, as in the case of the driver ant, and
then the extreme forms, from being the most useful to the community, having been
produced in greater and greater numbers through the natural selection of the
parents which generated them; until none with an intermediate structure were
produced. Thus, as I believe, the wonderful fact of two distinctly defined
castes of sterile workers existing in the same nest, both widely different from
each other and from their parents, has originated. We can see how useful their
production may have been to a social community of insects, on the same principle
that the division of labour is useful to civilised man. As ants work by
inherited instincts and by inherited tools or weapons, and not by acquired
knowledge and manufactured instruments, a perfect division of labour could be
effected with them only by the workers being sterile; for had they been fertile,
they would have intercrossed, and their instincts and structure would have
become blended. And nature has, as I believe, effected this admirable division
of labour in the communities of ants, by the means of natural selection. But I
am bound to confess, that, with all my faith in this principle, I should never
have anticipated that natural selection could have been efficient in so high a
degree, had not the case of these neuter insects convinced me of the fact. I
have, therefore, discussed this case, at some little but wholly insufficient
length, in order to show the power of natural selection, and likewise because
this is by far the most serious special difficulty, which my theory has
encountered. The case, also, is very interesting, as it proves that with
animals, as with plants, any amount of modification in structure can be effected
by the accumulation of numerous, slight, and as we must call them accidental,
variations, which are in any manner profitable, without exercise or habit having
come into play. For no amount of exercise, or habit, or volition, in the utterly
sterile members of a community could possibly have affected the structure or
instincts of the fertile members, which alone leave descendants. I am surprised
that no one has advanced this demonstrative case of neuter insects, against the
well-known doctrine of Lamarck. SUMMARY. I have endeavoured briefly in this
chapter to show that the mental qualities of our domestic animals vary, and that
the variations are inherited. Still more briefly I have attempted to show that
instincts vary slightly in a state of nature. No one will dispute that instincts
are of the highest importance to each animal. Therefore I can see no difficulty,
under changing conditions of life, in natural selection accumulating slight
modifications of instinct to any extent, in any useful direction. In some cases
habit or use and disuse have probably come into play. I do not pretend that the
facts given in this chapter strengthen in any great degree my theory; but none
of the cases of difficulty, to the best of my judgment, annihilate it. On the
other hand, the fact that instincts are not always absolutely perfect and are
liable to mistakes;--that no instinct has been produced for the exclusive good
of other animals, but that each animal takes advantage of the instincts of
others;--that the canon in natural history, of "natura non facit saltum" is
applicable to instincts as well as to corporeal structure, and is plainly
explicable on the foregoing views, but is otherwise inexplicable,--all tend to
corroborate the theory of natural selection. This theory is, also, strengthened
by some few other facts in regard to instincts; as by that common case of
closely allied, but certainly distinct, species, when inhabiting distant parts
of the world and living under considerably different conditions of life, yet
often retaining nearly the same instincts. For instance, we can understand on
the principle of inheritance, how it is that the thrush of South America lines
its nest with mud, in the same peculiar manner as does our British thrush: how
it is that the male wrens (Troglodytes) of North America, build "cock-nests," to
roost in, like the males of our distinct Kitty-wrens,--a habit wholly unlike
that of any other known bird. Finally, it may not be a logical deduction, but to
my imagination it is far more satisfactory to look at such instincts as the
young cuckoo ejecting its foster-brothers,--ants making slaves,--the larvae of
ichneumonidae feeding within the live bodies of caterpillars,--not as specially
endowed or created instincts, but as small consequences of one general law,
leading to the advancement of all organic beings, namely, multiply, vary, let
the strongest live and the weakest die. CHAPTER 8. HYBRIDISM. Distinction
between the sterility of first crosses and of hybrids. Sterility various in
degree, not universal, affected by close interbreeding, removed by
domestication. Laws governing the sterility of hybrids. Sterility not a special
endowment, but incidental on other differences. Causes of the sterility of first
crosses and of hybrids. Parallelism between the effects of changed conditions of
life and crossing. Fertility of varieties when crossed and of their mongrel
offspring not universal. Hybrids and mongrels compared independently of their
fertility. Summary. The view generally entertained by naturalists is that
species, when intercrossed, have been specially endowed with the quality of
sterility, in order to prevent the confusion of all organic forms. This view
certainly seems at first probable, for species within the same country could
hardly have kept distinct had they been capable of crossing freely. The
importance of the fact that hybrids are very generally sterile, has, I think,
been much underrated by some late writers. On the theory of natural selection
the case is especially important, inasmuch as the sterility of hybrids could not
possibly be of any advantage to them, and therefore could not have been acquired
by the continued preservation of successive profitable degrees of sterility. I
hope, however, to be able to show that sterility is not a specially acquired or
endowed quality, but is incidental on other acquired differences. In treating
this subject, two classes of facts, to a large extent fundamentally different,
have generally been confounded together; namely, the sterility of two species
when first crossed, and the sterility of the hybrids produced from them. Pure
species have of course their organs of reproduction in a perfect condition, yet
when intercrossed they produce either few or no offspring. Hybrids, on the other
hand, have their reproductive organs functionally impotent, as may be clearly
seen in the state of the male element in both plants and animals; though the
organs themselves are perfect in structure, as far as the microscope reveals. In
the first case the two sexual elements which go to form the embryo are perfect;
in the second case they are either not at all developed, or are imperfectly
developed. This distinction is important, when the cause of the sterility, which
is common to the two cases, has to be considered. The distinction has probably
been slurred over, owing to the sterility in both cases being looked on as a
special endowment, beyond the province of our reasoning powers. The fertility of
varieties, that is of the forms known or believed to have descended from common
parents, when intercrossed, and likewise the fertility of their mongrel
offspring, is, on my theory, of equal importance with the sterility of species;
for it seems to make a broad and clear distinction between varieties and
species. First, for the sterility of species when crossed and of their hybrid
offspring. It is impossible to study the several memoirs and works of those two
conscientious and admirable observers, Kolreuter and Gartner, who almost devoted
their lives to this subject, without being deeply impressed with the high
generality of some degree of sterility. Kolreuter makes the rule universal; but
then he cuts the knot, for in ten cases in which he found two forms, considered
by most authors as distinct species, quite fertile together, he unhesitatingly
ranks them as varieties. Gartner, also, makes the rule equally universal; and he
disputes the entire fertility of Kolreuter's ten cases. But in these and in many
other cases, Gartner is obliged carefully to count the seeds, in order to show
that there is any degree of sterility. He always compares the maximum number of
seeds produced by two species when crossed and by their hybrid offspring, with
the average number produced by both pure parent-species in a state of nature.
But a serious cause of error seems to me to be here introduced: a plant to be
hybridised must be castrated, and, what is often more important, must be
secluded in order to prevent pollen being brought to it by insects from other
plants. Nearly all the plants experimentised on by Gartner were potted, and
apparently were kept in a chamber in his house. That these processes are often
injurious to the fertility of a plant cannot be doubted; for Gartner gives in
his table about a score of cases of plants which he castrated, and artificially
fertilised with their own pollen, and (excluding all cases such as the
Leguminosae, in which there is an acknowledged difficulty in the manipulation)
half of these twenty plants had their fertility in some degree impaired.
Moreover, as Gartner during several years repeatedly crossed the primrose and
cowslip, which we have such good reason to believe to be varieties, and only
once or twice succeeded in getting fertile seed; as he found the common red and
blue pimpernels (Anagallis arvensis and coerulea), which the best botanists rank
as varieties, absolutely sterile together; and as he came to the same conclusion
in several other analogous cases; it seems to me that we may well be permitted
to doubt whether many other species are really so sterile, when intercrossed, as
Gartner believes. It is certain, on the one hand, that the sterility of various
species when crossed is so different in degree and graduates away so insensibly,
and, on the other hand, that the fertility of pure species is so easily affected
by various circumstances, that for all practical purposes it is most difficult
to say where perfect fertility ends and sterility begins. I think no better
evidence of this can be required than that the two most experienced observers
who have ever lived, namely, Kolreuter and Gartner, should have arrived at
diametrically opposite conclusions in regard to the very same species. It is
also most instructive to compare--but I have not space here to enter on
details--the evidence advanced by our best botanists on the question whether
certain doubtful forms should be ranked as species or varieties, with the
evidence from fertility adduced by different hybridisers, or by the same author,
from experiments made during different years. It can thus be shown that neither
sterility nor fertility affords any clear distinction between species and
varieties; but that the evidence from this source graduates away, and is
doubtful in the same degree as is the evidence derived from other constitutional
and structural differences. In regard to the sterility of hybrids in successive
generations; though Gartner was enabled to rear some hybrids, carefully guarding
them from a cross with either pure parent, for six or seven, and in one case for
ten generations, yet he asserts positively that their fertility never increased,
but generally greatly decreased. I do not doubt that this is usually the case,
and that the fertility often suddenly decreases in the first few generations.
Nevertheless I believe that in all these experiments the fertility has been
diminished by an independent cause, namely, from close interbreeding. I have
collected so large a body of facts, showing that close interbreeding lessens
fertility, and, on the other hand, that an occasional cross with a distinct
individual or variety increases fertility, that I cannot doubt the correctness
of this almost universal belief amongst breeders. Hybrids are seldom raised by
experimentalists in great numbers; and as the parent-species, or other allied
hybrids, generally grow in the same garden, the visits of insects must be
carefully prevented during the flowering season: hence hybrids will generally be
fertilised during each generation by their own individual pollen; and I am
convinced that this would be injurious to their fertility, already lessened by
their hybrid origin. I am strengthened in this conviction by a remarkable
statement repeatedly made by Gartner, namely, that if even the less fertile
hybrids be artificially fertilised with hybrid pollen of the same kind, their
fertility, notwithstanding the frequent ill effects of manipulation, sometimes
decidedly increases, and goes on increasing. Now, in artificial fertilisation
pollen is as often taken by chance (as I know from my own experience) from the
anthers of another flower, as from the anthers of the flower itself which is to
be fertilised; so that a cross between two flowers, though probably on the same
plant, would be thus effected. Moreover, whenever complicated experiments are in
progress, so careful an observer as Gartner would have castrated his hybrids,
and this would have insured in each generation a cross with the pollen from a
distinct flower, either from the same plant or from another plant of the same
hybrid nature. And thus, the strange fact of the increase of fertility in the
successive generations of ARTIFICIALLY FERTILISED hybrids may, I believe, be
accounted for by close interbreeding having been avoided. Now let us turn to the
results arrived at by the third most experienced hybridiser, namely, the
Honourable and Reverend W. Herbert. He is as emphatic in his conclusion that
some hybrids are perfectly fertile--as fertile as the pure parent-species--as
are Kolreuter and Gartner that some degree of sterility between distinct species
is a universal law of nature. He experimentised on some of the very same species
as did Gartner. The difference in their results may, I think, be in part
accounted for by Herbert's great horticultural skill, and by his having
hothouses at his command. Of his many important statements I will here give only
a single one as an example, namely, that "every ovule in a pod of Crinum capense
fertilised by C. revolutum produced a plant, which (he says) I never saw to
occur in a case of its natural fecundation." So that we here have perfect, or
even more than commonly perfect, fertility in a first cross between two distinct
species. This case of the Crinum leads me to refer to a most singular fact,
namely, that there are individual plants, as with certain species of Lobelia,
and with all the species of the genus Hippeastrum, which can be far more easily
fertilised by the pollen of another and distinct species, than by their own
pollen. For these plants have been found to yield seed to the pollen of a
distinct species, though quite sterile with their own pollen, notwithstanding
that their own pollen was found to be perfectly good, for it fertilised distinct
species. So that certain individual plants and all the individuals of certain
species can actually be hybridised much more readily than they can be
self-fertilised! For instance, a bulb of Hippeastrum aulicum produced four
flowers; three were fertilised by Herbert with their own pollen, and the fourth
was subsequently fertilised by the pollen of a compound hybrid descended from
three other and distinct species: the result was that "the ovaries of the three
first flowers soon ceased to grow, and after a few days perished entirely,
whereas the pod impregnated by the pollen of the hybrid made vigorous growth and
rapid progress to maturity, and bore good seed, which vegetated freely." In a
letter to me, in 1839, Mr. Herbert told me that he had then tried the experiment
during five years, and he continued to try it during several subsequent years,
and always with the same result. This result has, also, been confirmed by other
observers in the case of Hippeastrum with its sub-genera, and in the case of
some other genera, as Lobelia, Passiflora and Verbascum. Although the plants in
these experiments appeared perfectly healthy, and although both the ovules and
pollen of the same flower were perfectly good with respect to other species, yet
as they were functionally imperfect in their mutual self-action, we must infer
that the plants were in an unnatural state. Nevertheless these facts show on
what slight and mysterious causes the lesser or greater fertility of species
when crossed, in comparison with the same species when self-fertilised,
sometimes depends. The practical experiments of horticulturists, though not made
with scientific precision, deserve some notice. It is notorious in how
complicated a manner the species of Pelargonium, Fuchsia, Calceolaria, Petunia,
Rhododendron, etc., have been crossed, yet many of these hybrids seed freely.
For instance, Herbert asserts that a hybrid from Calceolaria integrifolia and
plantaginea, species most widely dissimilar in general habit, "reproduced itself
as perfectly as if it had been a natural species from the mountains of Chile." I
have taken some pains to ascertain the degree of fertility of some of the
complex crosses of Rhododendrons, and I am assured that many of them are
perfectly fertile. Mr. C. Noble, for instance, informs me that he raises stocks
for grafting from a hybrid between Rhododendron Ponticum and Catawbiense, and
that this hybrid "seeds as freely as it is possible to imagine." Had hybrids,
when fairly treated, gone on decreasing in fertility in each successive
generation, as Gartner believes to be the case, the fact would have been
notorious to nurserymen. Horticulturists raise large beds of the same hybrids,
and such alone are fairly treated, for by insect agency the several individuals
of the same hybrid variety are allowed to freely cross with each other, and the
injurious influence of close interbreeding is thus prevented. Any one may
readily convince himself of the efficiency of insect-agency by examining the
flowers of the more sterile kinds of hybrid rhododendrons, which produce no
pollen, for he will find on their stigmas plenty of pollen brought from other
flowers. In regard to animals, much fewer experiments have been carefully tried
than with plants. If our systematic arrangements can be trusted, that is if the
genera of animals are as distinct from each other, as are the genera of plants,
then we may infer that animals more widely separated in the scale of nature can
be more easily crossed than in the case of plants; but the hybrids themselves
are, I think, more sterile. I doubt whether any case of a perfectly fertile
hybrid animal can be considered as thoroughly well authenticated. It should,
however, be borne in mind that, owing to few animals breeding freely under
confinement, few experiments have been fairly tried: for instance, the
canary-bird has been crossed with nine other finches, but as not one of these
nine species breeds freely in confinement, we have no right to expect that the
first crosses between them and the canary, or that their hybrids, should be
perfectly fertile. Again, with respect to the fertility in successive
generations of the more fertile hybrid animals, I hardly know of an instance in
which two families of the same hybrid have been raised at the same time from
different parents, so as to avoid the ill effects of close interbreeding. On the
contrary, brothers and sisters have usually been crossed in each successive
generation, in opposition to the constantly repeated admonition of every
breeder. And in this case, it is not at all surprising that the inherent
sterility in the hybrids should have gone on increasing. If we were to act thus,
and pair brothers and sisters in the case of any pure animal, which from any
cause had the least tendency to sterility, the breed would assuredly be lost in
a very few generations. Although I do not know of any thoroughly
well-authenticated cases of perfectly fertile hybrid animals, I have some reason
to believe that the hybrids from Cervulus vaginalis and Reevesii, and from
Phasianus colchicus with P. torquatus and with P. versicolor are perfectly
fertile. The hybrids from the common and Chinese geese (A. cygnoides), species
which are so different that they are generally ranked in distinct genera, have
often bred in this country with either pure parent, and in one single instance
they have bred inter se. This was effected by Mr. Eyton, who raised two hybrids
from the same parents but from different hatches; and from these two birds he
raised no less than eight hybrids (grandchildren of the pure geese) from one
nest. In India, however, these cross-bred geese must be far more fertile; for I
am assured by two eminently capable judges, namely Mr. Blyth and Capt. Hutton,
that whole flocks of these crossed geese are kept in various parts of the
country; and as they are kept for profit, where neither pure parent-species
exists, they must certainly be highly fertile. A doctrine which originated with
Pallas, has been largely accepted by modern naturalists; namely, that most of
our domestic animals have descended from two or more aboriginal species, since
commingled by intercrossing. On this view, the aboriginal species must either at
first have produced quite fertile hybrids, or the hybrids must have become in
subsequent generations quite fertile under domestication. This latter
alternative seems to me the most probable, and I am inclined to believe in its
truth, although it rests on no direct evidence. I believe, for instance, that
our dogs have descended from several wild stocks; yet, with perhaps the
exception of certain indigenous domestic dogs of South America, all are quite
fertile together; and analogy makes me greatly doubt, whether the several
aboriginal species would at first have freely bred together and have produced
quite fertile hybrids. So again there is reason to believe that our European and
the humped Indian cattle are quite fertile together; but from facts communicated
to me by Mr. Blyth, I think they must be considered as distinct species. On this
view of the origin of many of our domestic animals, we must either give up the
belief of the almost universal sterility of distinct species of animals when
crossed; or we must look at sterility, not as an indelible characteristic, but
as one capable of being removed by domestication. Finally, looking to all the
ascertained facts on the intercrossing of plants and animals, it may be
concluded that some degree of sterility, both in first crosses and in hybrids,
is an extremely general result; but that it cannot, under our present state of
knowledge, be considered as absolutely universal. LAWS GOVERNING THE STERILITY
OF FIRST CROSSES AND OF HYBRIDS. We will now consider a little more in detail
the circumstances and rules governing the sterility of first crosses and of
hybrids. Our chief object will be to see whether or not the rules indicate that
species have specially been endowed with this quality, in order to prevent their
crossing and blending together in utter confusion. The following rules and
conclusions are chiefly drawn up from Gartner's admirable work on the
hybridisation of plants. I have taken much pains to ascertain how far the rules
apply to animals, and considering how scanty our knowledge is in regard to
hybrid animals, I have been surprised to find how generally the same rules apply
to both kingdoms. It has been already remarked, that the degree of fertility,
both of first crosses and of hybrids, graduates from zero to perfect fertility.
It is surprising in how many curious ways this gradation can be shown to exist;
but only the barest outline of the facts can here be given. When pollen from a
plant of one family is placed on the stigma of a plant of a distinct family, it
exerts no more influence than so much inorganic dust. From this absolute zero of
fertility, the pollen of different species of the same genus applied to the
stigma of some one species, yields a perfect gradation in the number of seeds
produced, up to nearly complete or even quite complete fertility; and, as we
have seen, in certain abnormal cases, even to an excess of fertility, beyond
that which the plant's own pollen will produce. So in hybrids themselves, there
are some which never have produced, and probably never would produce, even with
the pollen of either pure parent, a single fertile seed: but in some of these
cases a first trace of fertility may be detected, by the pollen of one of the
pure parent-species causing the flower of the hybrid to wither earlier than it
otherwise would have done; and the early withering of the flower is well known
to be a sign of incipient fertilisation. From this extreme degree of sterility
we have self-fertilised hybrids producing a greater and greater number of seeds
up to perfect fertility. Hybrids from two species which are very difficult to
cross, and which rarely produce any offspring, are generally very sterile; but
the parallelism between the difficulty of making a first cross, and the
sterility of the hybrids thus produced--two classes of facts which are generally
confounded together--is by no means strict. There are many cases, in which two
pure species can be united with unusual facility, and produce numerous
hybrid-offspring, yet these hybrids are remarkably sterile. On the other hand,
there are species which can be crossed very rarely, or with extreme difficulty,
but the hybrids, when at last produced, are very fertile. Even within the limits
of the same genus, for instance in Dianthus, these two opposite cases occur. The
fertility, both of first crosses and of hybrids, is more easily affected by
unfavourable conditions, than is the fertility of pure species. But the degree
of fertility is likewise innately variable; for it is not always the same when
the same two species are crossed under the same circumstances, but depends in
part upon the constitution of the individuals which happen to have been chosen
for the experiment. So it is with hybrids, for their degree of fertility is
often found to differ greatly in the several individuals raised from seed out of
the same capsule and exposed to exactly the same conditions. By the term
systematic affinity is meant, the resemblance between species in structure and
in constitution, more especially in the structure of parts which are of high
physiological importance and which differ little in the allied species. Now the
fertility of first crosses between species, and of the hybrids produced from
them, is largely governed by their systematic affinity. This is clearly shown by
hybrids never having been raised between species ranked by systematists in
distinct families; and on the other hand, by very closely allied species
generally uniting with facility. But the correspondence between systematic
affinity and the facility of crossing is by no means strict. A multitude of
cases could be given of very closely allied species which will not unite, or
only with extreme difficulty; and on the other hand of very distinct species
which unite with the utmost facility. In the same family there may be a genus,
as Dianthus, in which very many species can most readily be crossed; and another
genus, as Silene, in which the most persevering efforts have failed to produce
between extremely close species a single hybrid. Even within the limits of the
same genus, we meet with this same difference; for instance, the many species of
Nicotiana have been more largely crossed than the species of almost any other
genus; but Gartner found that N. acuminata, which is not a particularly distinct
species, obstinately failed to fertilise, or to be fertilised by, no less than
eight other species of Nicotiana. Very many analogous facts could be given. No
one has been able to point out what kind, or what amount, of difference in any
recognisable character is sufficient to prevent two species crossing. It can be
shown that plants most widely different in habit and general appearance, and
having strongly marked differences in every part of the flower, even in the
pollen, in the fruit, and in the cotyledons, can be crossed. Annual and
perennial plants, deciduous and evergreen trees, plants inhabiting different
stations and fitted for extremely different climates, can often be crossed with
ease. By a reciprocal cross between two species, I mean the case, for instance,
of a stallion-horse being first crossed with a female-ass, and then a male-ass
with a mare: these two species may then be said to have been reciprocally
crossed. There is often the widest possible difference in the facility of making
reciprocal crosses. Such cases are highly important, for they prove that the
capacity in any two species to cross is often completely independent of their
systematic affinity, or of any recognisable difference in their whole
organisation. On the other hand, these cases clearly show that the capacity for
crossing is connected with constitutional differences imperceptible by us, and
confined to the reproductive system. This difference in the result of reciprocal
crosses between the same two species was long ago observed by Kolreuter. To give
an instance: Mirabilis jalappa can easily be fertilised by the pollen of M.
longiflora, and the hybrids thus produced are sufficiently fertile; but
Kolreuter tried more than two hundred times, during eight following years, to
fertilise reciprocally M. longiflora with the pollen of M. jalappa, and utterly
failed. Several other equally striking cases could be given. Thuret has observed
the same fact with certain sea-weeds or Fuci. Gartner, moreover, found that this
difference of facility in making reciprocal crosses is extremely common in a
lesser degree. He has observed it even between forms so closely related (as
Matthiola annua and glabra) that many botanists rank them only as varieties. It
is also a remarkable fact, that hybrids raised from reciprocal crosses, though
of course compounded of the very same two species, the one species having first
been used as the father and then as the mother, generally differ in fertility in
a small, and occasionally in a high degree. Several other singular rules could
be given from Gartner: for instance, some species have a remarkable power of
crossing with other species; other species of the same genus have a remarkable
power of impressing their likeness on their hybrid offspring; but these two
powers do not at all necessarily go together. There are certain hybrids which
instead of having, as is usual, an intermediate character between their two
parents, always closely resemble one of them; and such hybrids, though
externally so like one of their pure parent-species, are with rare exceptions
extremely sterile. So again amongst hybrids which are usually intermediate in
structure between their parents, exceptional and abnormal individuals sometimes
are born, which closely resemble one of their pure parents; and these hybrids
are almost always utterly sterile, even when the other hybrids raised from seed
from the same capsule have a considerable degree of fertility. These facts show
how completely fertility in the hybrid is independent of its external
resemblance to either pure parent. Considering the several rules now given,
which govern the fertility of first crosses and of hybrids, we see that when
forms, which must be considered as good and distinct species, are united, their
fertility graduates from zero to perfect fertility, or even to fertility under
certain conditions in excess. That their fertility, besides being eminently
susceptible to favourable and unfavourable conditions, is innately variable.
That it is by no means always the same in degree in the first cross and in the
hybrids produced from this cross. That the fertility of hybrids is not related
to the degree in which they resemble in external appearance either parent. And
lastly, that the facility of making a first cross between any two species is not
always governed by their systematic affinity or degree of resemblance to each
other. This latter statement is clearly proved by reciprocal crosses between the
same two species, for according as the one species or the other is used as the
father or the mother, there is generally some difference, and occasionally the
widest possible difference, in the facility of effecting an union. The hybrids,
moreover, produced from reciprocal crosses often differ in fertility. Now do
these complex and singular rules indicate that species have been endowed with
sterility simply to prevent their becoming confounded in nature? I think not.
For why should the sterility be so extremely different in degree, when various
species are crossed, all of which we must suppose it would be equally important
to keep from blending together? Why should the degree of sterility be innately
variable in the individuals of the same species? Why should some species cross
with facility, and yet produce very sterile hybrids; and other species cross
with extreme difficulty, and yet produce fairly fertile hybrids? Why should
there often be so great a difference in the result of a reciprocal cross between
the same two species? Why, it may even be asked, has the production of hybrids
been permitted? to grant to species the special power of producing hybrids, and
then to stop their further propagation by different degrees of sterility, not
strictly related to the facility of the first union between their parents, seems
to be a strange arrangement. The foregoing rules and facts, on the other hand,
appear to me clearly to indicate that the sterility both of first crosses and of
hybrids is simply incidental or dependent on unknown differences, chiefly in the
reproductive systems, of the species which are crossed. The differences being of
so peculiar and limited a nature, that, in reciprocal crosses between two
species the male sexual element of the one will often freely act on the female
sexual element of the other, but not in a reversed direction. It will be
advisable to explain a little more fully by an example what I mean by sterility
being incidental on other differences, and not a specially endowed quality. As
the capacity of one plant to be grafted or budded on another is so entirely
unimportant for its welfare in a state of nature, I presume that no one will
suppose that this capacity is a SPECIALLY endowed quality, but will admit that
it is incidental on differences in the laws of growth of the two plants. We can
sometimes see the reason why one tree will not take on another, from differences
in their rate of growth, in the hardness of their wood, in the period of the
flow or nature of their sap, etc.; but in a multitude of cases we can assign no
reason whatever. Great diversity in the size of two plants, one being woody and
the other herbaceous, one being evergreen and the other deciduous, and
adaptation to widely different climates, does not always prevent the two
grafting together. As in hybridisation, so with grafting, the capacity is
limited by systematic affinity, for no one has been able to graft trees together
belonging to quite distinct families; and, on the other hand, closely allied
species, and varieties of the same species, can usually, but not invariably, be
grafted with ease. But this capacity, as in hybridisation, is by no means
absolutely governed by systematic affinity. Although many distinct genera within
the same family have been grafted together, in other cases species of the same
genus will not take on each other. The pear can be grafted far more readily on
the quince, which is ranked as a distinct genus, than on the apple, which is a
member of the same genus. Even different varieties of the pear take with
different degrees of facility on the quince; so do different varieties of the
apricot and peach on certain varieties of the plum. As Gartner found that there
was sometimes an innate difference in different INDIVIDUALS of the same two
species in crossing; so Sagaret believes this to be the case with different
individuals of the same two species in being grafted together. As in reciprocal
crosses, the facility of effecting an union is often very far from equal, so it
sometimes is in grafting; the common gooseberry, for instance, cannot be grafted
on the currant, whereas the currant will take, though with difficulty, on the
gooseberry. We have seen that the sterility of hybrids, which have their
reproductive organs in an imperfect condition, is a very different case from the
difficulty of uniting two pure species, which have their reproductive organs
perfect; yet these two distinct cases run to a certain extent parallel.
Something analogous occurs in grafting; for Thouin found that three species of
Robinia, which seeded freely on their own roots, and which could be grafted with
no great difficulty on another species, when thus grafted were rendered barren.
On the other hand, certain species of Sorbus, when grafted on other species,
yielded twice as much fruit as when on their own roots. We are reminded by this
latter fact of the extraordinary case of Hippeastrum, Lobelia, etc., which
seeded much more freely when fertilised with the pollen of distinct species,
than when self-fertilised with their own pollen. We thus see, that although
there is a clear and fundamental difference between the mere adhesion of grafted
stocks, and the union of the male and female elements in the act of
reproduction, yet that there is a rude degree of parallelism in the results of
grafting and of crossing distinct species. And as we must look at the curious
and complex laws governing the facility with which trees can be grafted on each
other as incidental on unknown differences in their vegetative systems, so I
believe that the still more complex laws governing the facility of first
crosses, are incidental on unknown differences, chiefly in their reproductive
systems. These differences, in both cases, follow to a certain extent, as might
have been expected, systematic affinity, by which every kind of resemblance and
dissimilarity between organic beings is attempted to be expressed. The facts by
no means seem to me to indicate that the greater or lesser difficulty of either
grafting or crossing together various species has been a special endowment;
although in the case of crossing, the difficulty is as important for the
endurance and stability of specific forms, as in the case of grafting it is
unimportant for their welfare. CAUSES OF THE STERILITY OF FIRST CROSSES AND OF
HYBRIDS. We may now look a little closer at the probable causes of the sterility
of first crosses and of hybrids. These two cases are fundamentally different,
for, as just remarked, in the union of two pure species the male and female
sexual elements are perfect, whereas in hybrids they are imperfect. Even in
first crosses, the greater or lesser difficulty in effecting a union apparently
depends on several distinct causes. There must sometimes be a physical
impossibility in the male element reaching the ovule, as would be the case with
a plant having a pistil too long for the pollen-tubes to reach the ovarium. It
has also been observed that when pollen of one species is placed on the stigma
of a distantly allied species, though the pollen-tubes protrude, they do not
penetrate the stigmatic surface. Again, the male element may reach the female
element, but be incapable of causing an embryo to be developed, as seems to have
been the case with some of Thuret's experiments on Fuci. No explanation can be
given of these facts, any more than why certain trees cannot be grafted on
others. Lastly, an embryo may be developed, and then perish at an early period.
This latter alternative has not been sufficiently attended to; but I believe,
from observations communicated to me by Mr. Hewitt, who has had great experience
in hybridising gallinaceous birds, that the early death of the embryo is a very
frequent cause of sterility in first crosses. I was at first very unwilling to
believe in this view; as hybrids, when once born, are generally healthy and
long-lived, as we see in the case of the common mule. Hybrids, however, are
differently circumstanced before and after birth: when born and living in a
country where their two parents can live, they are generally placed under
suitable conditions of life. But a hybrid partakes of only half of the nature
and constitution of its mother, and therefore before birth, as long as it is
nourished within its mother's womb or within the egg or seed produced by the
mother, it may be exposed to conditions in some degree unsuitable, and
consequently be liable to perish at an early period; more especially as all very
young beings seem eminently sensitive to injurious or unnatural conditions of
life. In regard to the sterility of hybrids, in which the sexual elements are
imperfectly developed, the case is very different. I have more than once alluded
to a large body of facts, which I have collected, showing that when animals and
plants are removed from their natural conditions, they are extremely liable to
have their reproductive systems seriously affected. This, in fact, is the great
bar to the domestication of animals. Between the sterility thus superinduced and
that of hybrids, there are many points of similarity. In both cases the
sterility is independent of general health, and is often accompanied by excess
of size or great luxuriance. In both cases, the sterility occurs in various
degrees; in both, the male element is the most liable to be affected; but
sometimes the female more than the male. In both, the tendency goes to a certain
extent with systematic affinity, for whole groups of animals and plants are
rendered impotent by the same unnatural conditions; and whole groups of species
tend to produce sterile hybrids. On the other hand, one species in a group will
sometimes resist great changes of conditions with unimpaired fertility; and
certain species in a group will produce unusually fertile hybrids. No one can
tell, till he tries, whether any particular animal will breed under confinement
or any plant seed freely under culture; nor can he tell, till he tries, whether
any two species of a genus will produce more or less sterile hybrids. Lastly,
when organic beings are placed during several generations under conditions not
natural to them, they are extremely liable to vary, which is due, as I believe,
to their reproductive systems having been specially affected, though in a lesser
degree than when sterility ensues. So it is with hybrids, for hybrids in
successive generations are eminently liable to vary, as every experimentalist
has observed. Thus we see that when organic beings are placed under new and
unnatural conditions, and when hybrids are produced by the unnatural crossing of
two species, the reproductive system, independently of the general state of
health, is affected by sterility in a very similar manner. In the one case, the
conditions of life have been disturbed, though often in so slight a degree as to
be inappreciable by us; in the other case, or that of hybrids, the external
conditions have remained the same, but the organisation has been disturbed by
two different structures and constitutions having been blended into one. For it
is scarcely possible that two organisations should be compounded into one,
without some disturbance occurring in the development, or periodical action, or
mutual relation of the different parts and organs one to another, or to the
conditions of life. When hybrids are able to breed inter se, they transmit to
their offspring from generation to generation the same compounded organisation,
and hence we need not be surprised that their sterility, though in some degree
variable, rarely diminishes. It must, however, be confessed that we cannot
understand, excepting on vague hypotheses, several facts with respect to the
sterility of hybrids; for instance, the unequal fertility of hybrids produced
from reciprocal crosses; or the increased sterility in those hybrids which
occasionally and exceptionally resemble closely either pure parent. Nor do I
pretend that the foregoing remarks go to the root of the matter: no explanation
is offered why an organism, when placed under unnatural conditions, is rendered
sterile. All that I have attempted to show, is that in two cases, in some
respects allied, sterility is the common result,--in the one case from the
conditions of life having been disturbed, in the other case from the
organisation having been disturbed by two organisations having been compounded
into one. It may seem fanciful, but I suspect that a similar parallelism extends
to an allied yet very different class of facts. It is an old and almost
universal belief, founded, I think, on a considerable body of evidence, that
slight changes in the conditions of life are beneficial to all living things. We
see this acted on by farmers and gardeners in their frequent exchanges of seed,
tubers, etc., from one soil or climate to another, and back again. During the
convalescence of animals, we plainly see that great benefit is derived from
almost any change in the habits of life. Again, both with plants and animals,
there is abundant evidence, that a cross between very distinct individuals of
the same species, that is between members of different strains or sub-breeds,
gives vigour and fertility to the offspring. I believe, indeed, from the facts
alluded to in our fourth chapter, that a certain amount of crossing is
indispensable even with hermaphrodites; and that close interbreeding continued
during several generations between the nearest relations, especially if these be
kept under the same conditions of life, always induces weakness and sterility in
the progeny. Hence it seems that, on the one hand, slight changes in the
conditions of life benefit all organic beings, and on the other hand, that
slight crosses, that is crosses between the males and females of the same
species which have varied and become slightly different, give vigour and
fertility to the offspring. But we have seen that greater changes, or changes of
a particular nature, often render organic beings in some degree sterile; and
that greater crosses, that is crosses between males and females which have
become widely or specifically different, produce hybrids which are generally
sterile in some degree. I cannot persuade myself that this parallelism is an
accident or an illusion. Both series of facts seem to be connected together by
some common but unknown bond, which is essentially related to the principle of
life. FERTILITY OF VARIETIES WHEN CROSSED, AND OF THEIR MONGREL OFFSPRING. It
may be urged, as a most forcible argument, that there must be some essential
distinction between species and varieties, and that there must be some error in
all the foregoing remarks, inasmuch as varieties, however much they may differ
from each other in external appearance, cross with perfect facility, and yield
perfectly fertile offspring. I fully admit that this is almost invariably the
case. But if we look to varieties produced under nature, we are immediately
involved in hopeless difficulties; for if two hitherto reputed varieties be
found in any degree sterile together, they are at once ranked by most
naturalists as species. For instance, the blue and red pimpernel, the primrose
and cowslip, which are considered by many of our best botanists as varieties,
are said by Gartner not to be quite fertile when crossed, and he consequently
ranks them as undoubted species. If we thus argue in a circle, the fertility of
all varieties produced under nature will assuredly have to be granted. If we
turn to varieties, produced, or supposed to have been produced, under
domestication, we are still involved in doubt. For when it is stated, for
instance, that the German Spitz dog unites more easily than other dogs with
foxes, or that certain South American indigenous domestic dogs do not readily
cross with European dogs, the explanation which will occur to everyone, and
probably the true one, is that these dogs have descended from several
aboriginally distinct species. Nevertheless the perfect fertility of so many
domestic varieties, differing widely from each other in appearance, for instance
of the pigeon or of the cabbage, is a remarkable fact; more especially when we
reflect how many species there are, which, though resembling each other most
closely, are utterly sterile when intercrossed. Several considerations, however,
render the fertility of domestic varieties less remarkable than at first
appears. It can, in the first place, be clearly shown that mere external
dissimilarity between two species does not determine their greater or lesser
degree of sterility when crossed; and we may apply the same rule to domestic
varieties. In the second place, some eminent naturalists believe that a long
course of domestication tends to eliminate sterility in the successive
generations of hybrids, which were at first only slightly sterile; and if this
be so, we surely ought not to expect to find sterility both appearing and
disappearing under nearly the same conditions of life. Lastly, and this seems to
me by far the most important consideration, new races of animals and plants are
produced under domestication by man's methodical and unconscious power of
selection, for his own use and pleasure: he neither wishes to select, nor could
select, slight differences in the reproductive system, or other constitutional
differences correlated with the reproductive system. He supplies his several
varieties with the same food; treats them in nearly the same manner, and does
not wish to alter their general habits of life. Nature acts uniformly and slowly
during vast periods of time on the whole organisation, in any way which may be
for each creature's own good; and thus she may, either directly, or more
probably indirectly, through correlation, modify the reproductive system in the
several descendants from any one species. Seeing this difference in the process
of selection, as carried on by man and nature, we need not be surprised at some
difference in the result. I have as yet spoken as if the varieties of the same
species were invariably fertile when intercrossed. But it seems to me impossible
to resist the evidence of the existence of a certain amount of sterility in the
few following cases, which I will briefly abstract. The evidence is at least as
good as that from which we believe in the sterility of a multitude of species.
The evidence is, also, derived from hostile witnesses, who in all other cases
consider fertility and sterility as safe criterions of specific distinction.
Gartner kept during several years a dwarf kind of maize with yellow seeds, and a
tall variety with red seeds, growing near each other in his garden; and although
these plants have separated sexes, they never naturally crossed. He then
fertilised thirteen flowers of the one with the pollen of the other; but only a
single head produced any seed, and this one head produced only five grains.
Manipulation in this case could not have been injurious, as the plants have
separated sexes. No one, I believe, has suspected that these varieties of maize
are distinct species; and it is important to notice that the hybrid plants thus
raised were themselves PERFECTLY fertile; so that even Gartner did not venture
to consider the two varieties as specifically distinct. Girou de Buzareingues
crossed three varieties of gourd, which like the maize has separated sexes, and
he asserts that their mutual fertilisation is by so much the less easy as their
differences are greater. How far these experiments may be trusted, I know not;
but the forms experimentised on, are ranked by Sagaret, who mainly founds his
classification by the test of infertility, as varieties. The following case is
far more remarkable, and seems at first quite incredible; but it is the result
of an astonishing number of experiments made during many years on nine species
of Verbascum, by so good an observer and so hostile a witness, as Gartner:
namely, that yellow and white varieties of the same species of Verbascum when
intercrossed produce less seed, than do either coloured varieties when
fertilised with pollen from their own coloured flowers. Moreover, he asserts
that when yellow and white varieties of one species are crossed with yellow and
white varieties of a DISTINCT species, more seed is produced by the crosses
between the same coloured flowers, than between those which are differently
coloured. Yet these varieties of Verbascum present no other difference besides
the mere colour of the flower; and one variety can sometimes be raised from the
seed of the other. From observations which I have made on certain varieties of
hollyhock, I am inclined to suspect that they present analogous facts.
Kolreuter, whose accuracy has been confirmed by every subsequent observer, has
proved the remarkable fact, that one variety of the common tobacco is more
fertile, when crossed with a widely distinct species, than are the other
varieties. He experimentised on five forms, which are commonly reputed to be
varieties, and which he tested by the severest trial, namely, by reciprocal
crosses, and he found their mongrel offspring perfectly fertile. But one of
these five varieties, when used either as father or mother, and crossed with the
Nicotiana glutinosa, always yielded hybrids not so sterile as those which were
produced from the four other varieties when crossed with N. glutinosa. Hence the
reproductive system of this one variety must have been in some manner and in
some degree modified. From these facts; from the great difficulty of
ascertaining the infertility of varieties in a state of nature, for a supposed
variety if infertile in any degree would generally be ranked as species; from
man selecting only external characters in the production of the most distinct
domestic varieties, and from not wishing or being able to produce recondite and
functional differences in the reproductive system; from these several
considerations and facts, I do not think that the very general fertility of
varieties can be proved to be of universal occurrence, or to form a fundamental
distinction between varieties and species. The general fertility of varieties
does not seem to me sufficient to overthrow the view which I have taken with
respect to the very general, but not invariable, sterility of first crosses and
of hybrids, namely, that it is not a special endowment, but is incidental on
slowly acquired modifications, more especially in the reproductive systems of
the forms which are crossed. HYBRIDS AND MONGRELS COMPARED, INDEPENDENTLY OF
THEIR FERTILITY. Independently of the question of fertility, the offspring of
species when crossed and of varieties when crossed may be compared in several
other respects. Gartner, whose strong wish was to draw a marked line of
distinction between species and varieties, could find very few and, as it seems
to me, quite unimportant differences between the so-called hybrid offspring of
species, and the so-called mongrel offspring of varieties. And, on the other
hand, they agree most closely in very many important respects. I shall here
discuss this subject with extreme brevity. The most important distinction is,
that in the first generation mongrels are more variable than hybrids; but
Gartner admits that hybrids from species which have long been cultivated are
often variable in the first generation; and I have myself seen striking
instances of this fact. Gartner further admits that hybrids between very closely
allied species are more variable than those from very distinct species; and this
shows that the difference in the degree of variability graduates away. When
mongrels and the more fertile hybrids are propagated for several generations an
extreme amount of variability in their offspring is notorious; but some few
cases both of hybrids and mongrels long retaining uniformity of character could
be given. The variability, however, in the successive generations of mongrels
is, perhaps, greater than in hybrids. This greater variability of mongrels than
of hybrids does not seem to me at all surprising. For the parents of mongrels
are varieties, and mostly domestic varieties (very few experiments having been
tried on natural varieties), and this implies in most cases that there has been
recent variability; and therefore we might expect that such variability would
often continue and be super-added to that arising from the mere act of crossing.
The slight degree of variability in hybrids from the first cross or in the first
generation, in contrast with their extreme variability in the succeeding
generations, is a curious fact and deserves attention. For it bears on and
corroborates the view which I have taken on the cause of ordinary variability;
namely, that it is due to the reproductive system being eminently sensitive to
any change in the conditions of life, being thus often rendered either impotent
or at least incapable of its proper function of producing offspring identical
with the parent-form. Now hybrids in the first generation are descended from
species (excluding those long cultivated) which have not had their reproductive
systems in any way affected, and they are not variable; but hybrids themselves
have their reproductive systems seriously affected, and their descendants are
highly variable. But to return to our comparison of mongrels and hybrids:
Gartner states that mongrels are more liable than hybrids to revert to either
parent-form; but this, if it be true, is certainly only a difference in degree.
Gartner further insists that when any two species, although most closely allied
to each other, are crossed with a third species, the hybrids are widely
different from each other; whereas if two very distinct varieties of one species
are crossed with another species, the hybrids do not differ much. But this
conclusion, as far as I can make out, is founded on a single experiment; and
seems directly opposed to the results of several experiments made by Kolreuter.
These alone are the unimportant differences, which Gartner is able to point out,
between hybrid and mongrel plants. On the other hand, the resemblance in
mongrels and in hybrids to their respective parents, more especially in hybrids
produced from nearly related species, follows according to Gartner the same
laws. When two species are crossed, one has sometimes a prepotent power of
impressing its likeness on the hybrid; and so I believe it to be with varieties
of plants. With animals one variety certainly often has this prepotent power
over another variety. Hybrid plants produced from a reciprocal cross, generally
resemble each other closely; and so it is with mongrels from a reciprocal cross.
Both hybrids and mongrels can be reduced to either pure parent-form, by repeated
crosses in successive generations with either parent. These several remarks are
apparently applicable to animals; but the subject is here excessively
complicated, partly owing to the existence of secondary sexual characters; but
more especially owing to prepotency in transmitting likeness running more
strongly in one sex than in the other, both when one species is crossed with
another, and when one variety is crossed with another variety. For instance, I
think those authors are right, who maintain that the ass has a prepotent power
over the horse, so that both the mule and the hinny more resemble the ass than
the horse; but that the prepotency runs more strongly in the male-ass than in
the female, so that the mule, which is the offspring of the male-ass and mare,
is more like an ass, than is the hinny, which is the offspring of the female-ass
and stallion. Much stress has been laid by some authors on the supposed fact,
that mongrel animals alone are born closely like one of their parents; but it
can be shown that this does sometimes occur with hybrids; yet I grant much less
frequently with hybrids than with mongrels. Looking to the cases which I have
collected of cross-bred animals closely resembling one parent, the resemblances
seem chiefly confined to characters almost monstrous in their nature, and which
have suddenly appeared--such as albinism, melanism, deficiency of tail or horns,
or additional fingers and toes; and do not relate to characters which have been
slowly acquired by selection. Consequently, sudden reversions to the perfect
character of either parent would be more likely to occur with mongrels, which
are descended from varieties often suddenly produced and semi-monstrous in
character, than with hybrids, which are descended from species slowly and
naturally produced. On the whole I entirely agree with Dr. Prosper Lucas, who,
after arranging an enormous body of facts with respect to animals, comes to the
conclusion, that the laws of resemblance of the child to its parents are the
same, whether the two parents differ much or little from each other, namely in
the union of individuals of the same variety, or of different varieties, or of
distinct species. Laying aside the question of fertility and sterility, in all
other respects there seems to be a general and close similarity in the offspring
of crossed species, and of crossed varieties. If we look at species as having
been specially created, and at varieties as having been produced by secondary
laws, this similarity would be an astonishing fact. But it harmonises perfectly
with the view that there is no essential distinction between species and
varieties. SUMMARY OF CHAPTER. First crosses between forms sufficiently distinct
to be ranked as species, and their hybrids, are very generally, but not
universally, sterile. The sterility is of all degrees, and is often so slight
that the two most careful experimentalists who have ever lived, have come to
diametrically opposite conclusions in ranking forms by this test. The sterility
is innately variable in individuals of the same species, and is eminently
susceptible of favourable and unfavourable conditions. The degree of sterility
does not strictly follow systematic affinity, but is governed by several curious
and complex laws. It is generally different, and sometimes widely different, in
reciprocal crosses between the same two species. It is not always equal in
degree in a first cross and in the hybrid produced from this cross. In the same
manner as in grafting trees, the capacity of one species or variety to take on
another, is incidental on generally unknown differences in their vegetative
systems, so in crossing, the greater or less facility of one species to unite
with another, is incidental on unknown differences in their reproductive
systems. There is no more reason to think that species have been specially
endowed with various degrees of sterility to prevent them crossing and blending
in nature, than to think that trees have been specially endowed with various and
somewhat analogous degrees of difficulty in being grafted together in order to
prevent them becoming inarched in our forests. The sterility of first crosses
between pure species, which have their reproductive systems perfect, seems to
depend on several circumstances; in some cases largely on the early death of the
embryo. The sterility of hybrids, which have their reproductive systems
imperfect, and which have had this system and their whole organisation disturbed
by being compounded of two distinct species, seems closely allied to that
sterility which so frequently affects pure species, when their natural
conditions of life have been disturbed. This view is supported by a parallelism
of another kind;--namely, that the crossing of forms only slightly different is
favourable to the vigour and fertility of their offspring; and that slight
changes in the conditions of life are apparently favourable to the vigour and
fertility of all organic beings. It is not surprising that the degree of
difficulty in uniting two species, and the degree of sterility of their
hybrid-offspring should generally correspond, though due to distinct causes; for
both depend on the amount of difference of some kind between the species which
are crossed. Nor is it surprising that the facility of effecting a first cross,
the fertility of the hybrids produced, and the capacity of being grafted
together--though this latter capacity evidently depends on widely different
circumstances--should all run, to a certain extent, parallel with the systematic
affinity of the forms which are subjected to experiment; for systematic affinity
attempts to express all kinds of resemblance between all species. First crosses
between forms known to be varieties, or sufficiently alike to be considered as
varieties, and their mongrel offspring, are very generally, but not quite
universally, fertile. Nor is this nearly general and perfect fertility
surprising, when we remember how liable we are to argue in a circle with respect
to varieties in a state of nature; and when we remember that the greater number
of varieties have been produced under domestication by the selection of mere
external differences, and not of differences in the reproductive system. In all
other respects, excluding fertility, there is a close general resemblance
between hybrids and mongrels. Finally, then, the facts briefly given in this
chapter do not seem to me opposed to, but even rather to support the view, that
there is no fundamental distinction between species and varieties. CHAPTER 9. ON
THE IMPERFECTION OF THE GEOLOGICAL RECORD. On the absence of intermediate
varieties at the present day. On the nature of extinct intermediate varieties;
on their number. On the vast lapse of time, as inferred from the rate of
deposition and of denudation. On the poorness of our palaeontological
collections. On the intermittence of geological formations. On the absence of
intermediate varieties in any one formation. On the sudden appearance of groups
of species. On their sudden appearance in the lowest known fossiliferous strata.
In the sixth chapter I enumerated the chief objections which might be justly
urged against the views maintained in this volume. Most of them have now been
discussed. One, namely the distinctness of specific forms, and their not being
blended together by innumerable transitional links, is a very obvious
difficulty. I assigned reasons why such links do not commonly occur at the
present day, under the circumstances apparently most favourable for their
presence, namely on an extensive and continuous area with graduated physical
conditions. I endeavoured to show, that the life of each species depends in a
more important manner on the presence of other already defined organic forms,
than on climate; and, therefore, that the really governing conditions of life do
not graduate away quite insensibly like heat or moisture. I endeavoured, also,
to show that intermediate varieties, from existing in lesser numbers than the
forms which they connect, will generally be beaten out and exterminated during
the course of further modification and improvement. The main cause, however, of
innumerable intermediate links not now occurring everywhere throughout nature
depends on the very process of natural selection, through which new varieties
continually take the places of and exterminate their parent-forms. But just in
proportion as this process of extermination has acted on an enormous scale, so
must the number of intermediate varieties, which have formerly existed on the
earth, be truly enormous. Why then is not every geological formation and every
stratum full of such intermediate links? Geology assuredly does not reveal any
such finely graduated organic chain; and this, perhaps, is the most obvious and
gravest objection which can be urged against my theory. The explanation lies, as
I believe, in the extreme imperfection of the geological record. In the first
place it should always be borne in mind what sort of intermediate forms must, on
my theory, have formerly existed. I have found it difficult, when looking at any
two species, to avoid picturing to myself, forms DIRECTLY intermediate between
them. But this is a wholly false view; we should always look for forms
intermediate between each species and a common but unknown progenitor; and the
progenitor will generally have differed in some respects from all its modified
descendants. To give a simple illustration: the fantail and pouter pigeons have
both descended from the rock-pigeon; if we possessed all the intermediate
varieties which have ever existed, we should have an extremely close series
between both and the rock-pigeon; but we should have no varieties directly
intermediate between the fantail and pouter; none, for instance, combining a
tail somewhat expanded with a crop somewhat enlarged, the characteristic
features of these two breeds. These two breeds, moreover, have become so much
modified, that if we had no historical or indirect evidence regarding their
origin, it would not have been possible to have determined from a mere
comparison of their structure with that of the rock-pigeon, whether they had
descended from this species or from some other allied species, such as C. oenas.
So with natural species, if we look to forms very distinct, for instance to the
horse and tapir, we have no reason to suppose that links ever existed directly
intermediate between them, but between each and an unknown common parent. The
common parent will have had in its whole organisation much general resemblance
to the tapir and to the horse; but in some points of structure may have differed
considerably from both, even perhaps more than they differ from each other.
Hence in all such cases, we should be unable to recognise the parent-form of any
two or more species, even if we closely compared the structure of the parent
with that of its modified descendants, unless at the same time we had a nearly
perfect chain of the intermediate links. It is just possible by my theory, that
one of two living forms might have descended from the other; for instance, a
horse from a tapir; and in this case DIRECT intermediate links will have existed
between them. But such a case would imply that one form had remained for a very
long period unaltered, whilst its descendants had undergone a vast amount of
change; and the principle of competition between organism and organism, between
child and parent, will render this a very rare event; for in all cases the new
and improved forms of life will tend to supplant the old and unimproved forms.
By the theory of natural selection all living species have been connected with
the parent-species of each genus, by differences not greater than we see between
the varieties of the same species at the present day; and these parent-species,
now generally extinct, have in their turn been similarly connected with more
ancient species; and so on backwards, always converging to the common ancestor
of each great class. So that the number of intermediate and transitional links,
between all living and extinct species, must have been inconceivably great. But
assuredly, if this theory be true, such have lived upon this earth. ON THE LAPSE
OF TIME. Independently of our not finding fossil remains of such infinitely
numerous connecting links, it may be objected, that time will not have sufficed
for so great an amount of organic change, all changes having been effected very
slowly through natural selection. It is hardly possible for me even to recall to
the reader, who may not be a practical geologist, the facts leading the mind
feebly to comprehend the lapse of time. He who can read Sir Charles Lyell's
grand work on the Principles of Geology, which the future historian will
recognise as having produced a revolution in natural science, yet does not admit
how incomprehensibly vast have been the past periods of time, may at once close
this volume. Not that it suffices to study the Principles of Geology, or to read
special treatises by different observers on separate formations, and to mark how
each author attempts to give an inadequate idea of the duration of each
formation or even each stratum. A man must for years examine for himself great
piles of superimposed strata, and watch the sea at work grinding down old rocks
and making fresh sediment, before he can hope to comprehend anything of the
lapse of time, the monuments of which we see around us. It is good to wander
along lines of sea-coast, when formed of moderately hard rocks, and mark the
process of degradation. The tides in most cases reach the cliffs only for a
short time twice a day, and the waves eat into them only when they are charged
with sand or pebbles; for there is reason to believe that pure water can effect
little or nothing in wearing away rock. At last the base of the cliff is
undermined, huge fragments fall down, and these remaining fixed, have to be worn
away, atom by atom, until reduced in size they can be rolled about by the waves,
and then are more quickly ground into pebbles, sand, or mud. But how often do we
see along the bases of retreating cliffs rounded boulders, all thickly clothed
by marine productions, showing how little they are abraded and how seldom they
are rolled about! Moreover, if we follow for a few miles any line of rocky
cliff, which is undergoing degradation, we find that it is only here and there,
along a short length or round a promontory, that the cliffs are at the present
time suffering. The appearance of the surface and the vegetation show that
elsewhere years have elapsed since the waters washed their base. He who most
closely studies the action of the sea on our shores, will, I believe, be most
deeply impressed with the slowness with which rocky coasts are worn away. The
observations on this head by Hugh Miller, and by that excellent observer Mr.
Smith of Jordan Hill, are most impressive. With the mind thus impressed, let any
one examine beds of conglomerate many thousand feet in thickness, which, though
probably formed at a quicker rate than many other deposits, yet, from being
formed of worn and rounded pebbles, each of which bears the stamp of time, are
good to show how slowly the mass has been accumulated. Let him remember Lyell's
profound remark, that the thickness and extent of sedimentary formations are the
result and measure of the degradation which the earth's crust has elsewhere
suffered. And what an amount of degradation is implied by the sedimentary
deposits of many countries! Professor Ramsay has given me the maximum thickness,
in most cases from actual measurement, in a few cases from estimate, of each
formation in different parts of Great Britain; and this is the result:-- Feet
Palaeozoic strata (not including igneous beds)..57,154. Secondary
strata................................13,190. Tertiary
strata..................................2,240. --making altogether 72,584 feet;
that is, very nearly thirteen and three-quarters British miles. Some of these
formations, which are represented in England by thin beds, are thousands of feet
in thickness on the Continent. Moreover, between each successive formation, we
have, in the opinion of most geologists, enormously long blank periods. So that
the lofty pile of sedimentary rocks in Britain, gives but an inadequate idea of
the time which has elapsed during their accumulation; yet what time this must
have consumed! Good observers have estimated that sediment is deposited by the
great Mississippi river at the rate of only 600 feet in a hundred thousand
years. This estimate may be quite erroneous; yet, considering over what wide
spaces very fine sediment is transported by the currents of the sea, the process
of accumulation in any one area must be extremely slow. But the amount of
denudation which the strata have in many places suffered, independently of the
rate of accumulation of the degraded matter, probably offers the best evidence
of the lapse of time. I remember having been much struck with the evidence of
denudation, when viewing volcanic islands, which have been worn by the waves and
pared all round into perpendicular cliffs of one or two thousand feet in height;
for the gentle slope of the lava-streams, due to their formerly liquid state,
showed at a glance how far the hard, rocky beds had once extended into the open
ocean. The same story is still more plainly told by faults,--those great cracks
along which the strata have been upheaved on one side, or thrown down on the
other, to the height or depth of thousands of feet; for since the crust cracked,
the surface of the land has been so completely planed down by the action of the
sea, that no trace of these vast dislocations is externally visible. The Craven
fault, for instance, extends for upwards of 30 miles, and along this line the
vertical displacement of the strata has varied from 600 to 3000 feet. Professor
Ramsay has published an account of a downthrow in Anglesea of 2300 feet; and he
informs me that he fully believes there is one in Merionethshire of 12,000 feet;
yet in these cases there is nothing on the surface to show such prodigious
movements; the pile of rocks on the one or other side having been smoothly swept
away. The consideration of these facts impresses my mind almost in the same
manner as does the vain endeavour to grapple with the idea of eternity. I am
tempted to give one other case, the well-known one of the denudation of the
Weald. Though it must be admitted that the denudation of the Weald has been a
mere trifle, in comparison with that which has removed masses of our palaeozoic
strata, in parts ten thousand feet in thickness, as shown in Professor Ramsay's
masterly memoir on this subject. Yet it is an admirable lesson to stand on the
North Downs and to look at the distant South Downs; for, remembering that at no
great distance to the west the northern and southern escarpments meet and close,
one can safely picture to oneself the great dome of rocks which must have
covered up the Weald within so limited a period as since the latter part of the
Chalk formation. The distance from the northern to the southern Downs is about
22 miles, and the thickness of the several formations is on an average about
1100 feet, as I am informed by Professor Ramsay. But if, as some geologists
suppose, a range of older rocks underlies the Weald, on the flanks of which the
overlying sedimentary deposits might have accumulated in thinner masses than
elsewhere, the above estimate would be erroneous; but this source of doubt
probably would not greatly affect the estimate as applied to the western
extremity of the district. If, then, we knew the rate at which the sea commonly
wears away a line of cliff of any given height, we could measure the time
requisite to have denuded the Weald. This, of course, cannot be done; but we
may, in order to form some crude notion on the subject, assume that the sea
would eat into cliffs 500 feet in height at the rate of one inch in a century.
This will at first appear much too small an allowance; but it is the same as if
we were to assume a cliff one yard in height to be eaten back along a whole line
of coast at the rate of one yard in nearly every twenty-two years. I doubt
whether any rock, even as soft as chalk, would yield at this rate excepting on
the most exposed coasts; though no doubt the degradation of a lofty cliff would
be more rapid from the breakage of the fallen fragments. On the other hand, I do
not believe that any line of coast, ten or twenty miles in length, ever suffers
degradation at the same time along its whole indented length; and we must
remember that almost all strata contain harder layers or nodules, which from
long resisting attrition form a breakwater at the base. Hence, under ordinary
circumstances, I conclude that for a cliff 500 feet in height, a denudation of
one inch per century for the whole length would be an ample allowance. At this
rate, on the above data, the denudation of the Weald must have required
306,662,400 years; or say three hundred million years. The action of fresh water
on the gently inclined Wealden district, when upraised, could hardly have been
great, but it would somewhat reduce the above estimate. On the other hand,
during oscillations of level, which we know this area has undergone, the surface
may have existed for millions of years as land, and thus have escaped the action
of the sea: when deeply submerged for perhaps equally long periods, it would,
likewise, have escaped the action of the coast-waves. So that in all probability
a far longer period than 300 million years has elapsed since the latter part of
the Secondary period. I have made these few remarks because it is highly
important for us to gain some notion, however imperfect, of the lapse of years.
During each of these years, over the whole world, the land and the water has
been peopled by hosts of living forms. What an infinite number of generations,
which the mind cannot grasp, must have succeeded each other in the long roll of
years! Now turn to our richest geological museums, and what a paltry display we
behold! ON THE POORNESS OF OUR PALAEONTOLOGICAL COLLECTIONS. That our
palaeontological collections are very imperfect, is admitted by every one. The
remark of that admirable palaeontologist, the late Edward Forbes, should not be
forgotten, namely, that numbers of our fossil species are known and named from
single and often broken specimens, or from a few specimens collected on some one
spot. Only a small portion of the surface of the earth has been geologically
explored, and no part with sufficient care, as the important discoveries made
every year in Europe prove. No organism wholly soft can be preserved. Shells and
bones will decay and disappear when left on the bottom of the sea, where
sediment is not accumulating. I believe we are continually taking a most
erroneous view, when we tacitly admit to ourselves that sediment is being
deposited over nearly the whole bed of the sea, at a rate sufficiently quick to
embed and preserve fossil remains. Throughout an enormously large proportion of
the ocean, the bright blue tint of the water bespeaks its purity. The many cases
on record of a formation conformably covered, after an enormous interval of
time, by another and later formation, without the underlying bed having suffered
in the interval any wear and tear, seem explicable only on the view of the
bottom of the sea not rarely lying for ages in an unaltered condition. The
remains which do become embedded, if in sand or gravel, will when the beds are
upraised generally be dissolved by the percolation of rain-water. I suspect that
but few of the very many animals which live on the beach between high and low
watermark are preserved. For instance, the several species of the Chthamalinae
(a sub-family of sessile cirripedes) coat the rocks all over the world in
infinite numbers: they are all strictly littoral, with the exception of a single
Mediterranean species, which inhabits deep water and has been found fossil in
Sicily, whereas not one other species has hitherto been found in any tertiary
formation: yet it is now known that the genus Chthamalus existed during the
chalk period. The molluscan genus Chiton offers a partially analogous case. With
respect to the terrestrial productions which lived during the Secondary and
Palaeozoic periods, it is superfluous to state that our evidence from fossil
remains is fragmentary in an extreme degree. For instance, not a land shell is
known belonging to either of these vast periods, with one exception discovered
by Sir C. Lyell in the carboniferous strata of North America. In regard to
mammiferous remains, a single glance at the historical table published in the
Supplement to Lyell's Manual, will bring home the truth, how accidental and rare
is their preservation, far better than pages of detail. Nor is their rarity
surprising, when we remember how large a proportion of the bones of tertiary
mammals have been discovered either in caves or in lacustrine deposits; and that
not a cave or true lacustrine bed is known belonging to the age of our secondary
or palaeozoic formations. But the imperfection in the geological record mainly
results from another and more important cause than any of the foregoing; namely,
from the several formations being separated from each other by wide intervals of
time. When we see the formations tabulated in written works, or when we follow
them in nature, it is difficult to avoid believing that they are closely
consecutive. But we know, for instance, from Sir R. Murchison's great work on
Russia, what wide gaps there are in that country between the superimposed
formations; so it is in North America, and in many other parts of the world. The
most skilful geologist, if his attention had been exclusively confined to these
large territories, would never have suspected that during the periods which were
blank and barren in his own country, great piles of sediment, charged with new
and peculiar forms of life, had elsewhere been accumulated. And if in each
separate territory, hardly any idea can be formed of the length of time which
has elapsed between the consecutive formations, we may infer that this could
nowhere be ascertained. The frequent and great changes in the mineralogical
composition of consecutive formations, generally implying great changes in the
geography of the surrounding lands, whence the sediment has been derived,
accords with the belief of vast intervals of time having elapsed between each
formation. But we can, I think, see why the geological formations of each region
are almost invariably intermittent; that is, have not followed each other in
close sequence. Scarcely any fact struck me more when examining many hundred
miles of the South American coasts, which have been upraised several hundred
feet within the recent period, than the absence of any recent deposits
sufficiently extensive to last for even a short geological period. Along the
whole west coast, which is inhabited by a peculiar marine fauna, tertiary beds
are so scantily developed, that no record of several successive and peculiar
marine faunas will probably be preserved to a distant age. A little reflection
will explain why along the rising coast of the western side of South America, no
extensive formations with recent or tertiary remains can anywhere be found,
though the supply of sediment must for ages have been great, from the enormous
degradation of the coast-rocks and from muddy streams entering the sea. The
explanation, no doubt, is, that the littoral and sub-littoral deposits are
continually worn away, as soon as they are brought up by the slow and gradual
rising of the land within the grinding action of the coast-waves. We may, I
think, safely conclude that sediment must be accumulated in extremely thick,
solid, or extensive masses, in order to withstand the incessant action of the
waves, when first upraised and during subsequent oscillations of level. Such
thick and extensive accumulations of sediment may be formed in two ways; either,
in profound depths of the sea, in which case, judging from the researches of E.
Forbes, we may conclude that the bottom will be inhabited by extremely few
animals, and the mass when upraised will give a most imperfect record of the
forms of life which then existed; or, sediment may be accumulated to any
thickness and extent over a shallow bottom, if it continue slowly to subside. In
this latter case, as long as the rate of subsidence and supply of sediment
nearly balance each other, the sea will remain shallow and favourable for life,
and thus a fossiliferous formation thick enough, when upraised, to resist any
amount of degradation, may be formed. I am convinced that all our ancient
formations, which are rich in fossils, have thus been formed during subsidence.
Since publishing my views on this subject in 1845, I have watched the progress
of Geology, and have been surprised to note how author after author, in treating
of this or that great formation, has come to the conclusion that it was
accumulated during subsidence. I may add, that the only ancient tertiary
formation on the west coast of South America, which has been bulky enough to
resist such degradation as it has as yet suffered, but which will hardly last to
a distant geological age, was certainly deposited during a downward oscillation
of level, and thus gained considerable thickness. All geological facts tell us
plainly that each area has undergone numerous slow oscillations of level, and
apparently these oscillations have affected wide spaces. Consequently formations
rich in fossils and sufficiently thick and extensive to resist subsequent
degradation, may have been formed over wide spaces during periods of subsidence,
but only where the supply of sediment was sufficient to keep the sea shallow and
to embed and preserve the remains before they had time to decay. On the other
hand, as long as the bed of the sea remained stationary, THICK deposits could
not have been accumulated in the shallow parts, which are the most favourable to
life. Still less could this have happened during the alternate periods of
elevation; or, to speak more accurately, the beds which were then accumulated
will have been destroyed by being upraised and brought within the limits of the
coast-action. Thus the geological record will almost necessarily be rendered
intermittent. I feel much confidence in the truth of these views, for they are
in strict accordance with the general principles inculcated by Sir C. Lyell; and
E. Forbes independently arrived at a similar conclusion. One remark is here
worth a passing notice. During periods of elevation the area of the land and of
the adjoining shoal parts of the sea will be increased, and new stations will
often be formed;--all circumstances most favourable, as previously explained,
for the formation of new varieties and species; but during such periods there
will generally be a blank in the geological record. On the other hand, during
subsidence, the inhabited area and number of inhabitants will decrease
(excepting the productions on the shores of a continent when first broken up
into an archipelago), and consequently during subsidence, though there will be
much extinction, fewer new varieties or species will be formed; and it is during
these very periods of subsidence, that our great deposits rich in fossils have
been accumulated. Nature may almost be said to have guarded against the frequent
discovery of her transitional or linking forms. From the foregoing
considerations it cannot be doubted that the geological record, viewed as a
whole, is extremely imperfect; but if we confine our attention to any one
formation, it becomes more difficult to understand, why we do not therein find
closely graduated varieties between the allied species which lived at its
commencement and at its close. Some cases are on record of the same species
presenting distinct varieties in the upper and lower parts of the same
formation, but, as they are rare, they may be here passed over. Although each
formation has indisputably required a vast number of years for its deposition, I
can see several reasons why each should not include a graduated series of links
between the species which then lived; but I can by no means pretend to assign
due proportional weight to the following considerations. Although each formation
may mark a very long lapse of years, each perhaps is short compared with the
period requisite to change one species into another. I am aware that two
palaeontologists, whose opinions are worthy of much deference, namely Bronn and
Woodward, have concluded that the average duration of each formation is twice or
thrice as long as the average duration of specific forms. But insuperable
difficulties, as it seems to me, prevent us coming to any just conclusion on
this head. When we see a species first appearing in the middle of any formation,
it would be rash in the extreme to infer that it had not elsewhere previously
existed. So again when we find a species disappearing before the uppermost
layers have been deposited, it would be equally rash to suppose that it then
became wholly extinct. We forget how small the area of Europe is compared with
the rest of the world; nor have the several stages of the same formation
throughout Europe been correlated with perfect accuracy. With marine animals of
all kinds, we may safely infer a large amount of migration during climatal and
other changes; and when we see a species first appearing in any formation, the
probability is that it only then first immigrated into that area. It is well
known, for instance, that several species appeared somewhat earlier in the
palaeozoic beds of North America than in those of Europe; time having apparently
been required for their migration from the American to the European seas. In
examining the latest deposits of various quarters of the world, it has
everywhere been noted, that some few still existing species are common in the
deposit, but have become extinct in the immediately surrounding sea; or,
conversely, that some are now abundant in the neighbouring sea, but are rare or
absent in this particular deposit. It is an excellent lesson to reflect on the
ascertained amount of migration of the inhabitants of Europe during the Glacial
period, which forms only a part of one whole geological period; and likewise to
reflect on the great changes of level, on the inordinately great change of
climate, on the prodigious lapse of time, all included within this same glacial
period. Yet it may be doubted whether in any quarter of the world, sedimentary
deposits, INCLUDING FOSSIL REMAINS, have gone on accumulating within the same
area during the whole of this period. It is not, for instance, probable that
sediment was deposited during the whole of the glacial period near the mouth of
the Mississippi, within that limit of depth at which marine animals can
flourish; for we know what vast geographical changes occurred in other parts of
America during this space of time. When such beds as were deposited in shallow
water near the mouth of the Mississippi during some part of the glacial period
shall have been upraised, organic remains will probably first appear and
disappear at different levels, owing to the migration of species and to
geographical changes. And in the distant future, a geologist examining these
beds, might be tempted to conclude that the average duration of life of the
embedded fossils had been less than that of the glacial period, instead of
having been really far greater, that is extending from before the glacial epoch
to the present day. In order to get a perfect gradation between two forms in the
upper and lower parts of the same formation, the deposit must have gone on
accumulating for a very long period, in order to have given sufficient time for
the slow process of variation; hence the deposit will generally have to be a
very thick one; and the species undergoing modification will have had to live on
the same area throughout this whole time. But we have seen that a thick
fossiliferous formation can only be accumulated during a period of subsidence;
and to keep the depth approximately the same, which is necessary in order to
enable the same species to live on the same space, the supply of sediment must
nearly have counterbalanced the amount of subsidence. But this same movement of
subsidence will often tend to sink the area whence the sediment is derived, and
thus diminish the supply whilst the downward movement continues. In fact, this
nearly exact balancing between the supply of sediment and the amount of
subsidence is probably a rare contingency; for it has been observed by more than
one palaeontologist, that very thick deposits are usually barren of organic
remains, except near their upper or lower limits. It would seem that each
separate formation, like the whole pile of formations in any country, has
generally been intermittent in its accumulation. When we see, as is so often the
case, a formation composed of beds of different mineralogical composition, we
may reasonably suspect that the process of deposition has been much interrupted,
as a change in the currents of the sea and a supply of sediment of a different
nature will generally have been due to geographical changes requiring much time.
Nor will the closest inspection of a formation give any idea of the time which
its deposition has consumed. Many instances could be given of beds only a few
feet in thickness, representing formations, elsewhere thousands of feet in
thickness, and which must have required an enormous period for their
accumulation; yet no one ignorant of this fact would have suspected the vast
lapse of time represented by the thinner formation. Many cases could be given of
the lower beds of a formation having been upraised, denuded, submerged, and then
re-covered by the upper beds of the same formation,--facts, showing what wide,
yet easily overlooked, intervals have occurred in its accumulation. In other
cases we have the plainest evidence in great fossilised trees, still standing
upright as they grew, of many long intervals of time and changes of level during
the process of deposition, which would never even have been suspected, had not
the trees chanced to have been preserved: thus, Messrs. Lyell and Dawson found
carboniferous beds 1400 feet thick in Nova Scotia, with ancient root-bearing
strata, one above the other, at no less than sixty-eight different levels.
Hence, when the same species occur at the bottom, middle, and top of a
formation, the probability is that they have not lived on the same spot during
the whole period of deposition, but have disappeared and reappeared, perhaps
many times, during the same geological period. So that if such species were to
undergo a considerable amount of modification during any one geological period,
a section would not probably include all the fine intermediate gradations which
must on my theory have existed between them, but abrupt, though perhaps very
slight, changes of form. It is all-important to remember that naturalists have
no golden rule by which to distinguish species and varieties; they grant some
little variability to each species, but when they meet with a somewhat greater
amount of difference between any two forms, they rank both as species, unless
they are enabled to connect them together by close intermediate gradations. And
this from the reasons just assigned we can seldom hope to effect in any one
geological section. Supposing B and C to be two species, and a third, A, to be
found in an underlying bed; even if A were strictly intermediate between B and
C, it would simply be ranked as a third and distinct species, unless at the same
time it could be most closely connected with either one or both forms by
intermediate varieties. Nor should it be forgotten, as before explained, that A
might be the actual progenitor of B and C, and yet might not at all necessarily
be strictly intermediate between them in all points of structure. So that we
might obtain the parent-species and its several modified descendants from the
lower and upper beds of a formation, and unless we obtained numerous
transitional gradations, we should not recognise their relationship, and should
consequently be compelled to rank them all as distinct species. It is notorious
on what excessively slight differences many palaeontologists have founded their
species; and they do this the more readily if the specimens come from different
sub-stages of the same formation. Some experienced conchologists are now sinking
many of the very fine species of D'Orbigny and others into the rank of
varieties; and on this view we do find the kind of evidence of change which on
my theory we ought to find. Moreover, if we look to rather wider intervals,
namely, to distinct but consecutive stages of the same great formation, we find
that the embedded fossils, though almost universally ranked as specifically
different, yet are far more closely allied to each other than are the species
found in more widely separated formations; but to this subject I shall have to
return in the following chapter. One other consideration is worth notice: with
animals and plants that can propagate rapidly and are not highly locomotive,
there is reason to suspect, as we have formerly seen, that their varieties are
generally at first local; and that such local varieties do not spread widely and
supplant their parent-forms until they have been modified and perfected in some
considerable degree. According to this view, the chance of discovering in a
formation in any one country all the early stages of transition between any two
forms, is small, for the successive changes are supposed to have been local or
confined to some one spot. Most marine animals have a wide range; and we have
seen that with plants it is those which have the widest range, that oftenest
present varieties; so that with shells and other marine animals, it is probably
those which have had the widest range, far exceeding the limits of the known
geological formations of Europe, which have oftenest given rise, first to local
varieties and ultimately to new species; and this again would greatly lessen the
chance of our being able to trace the stages of transition in any one geological
formation. It should not be forgotten, that at the present day, with perfect
specimens for examination, two forms can seldom be connected by intermediate
varieties and thus proved to be the same species, until many specimens have been
collected from many places; and in the case of fossil species this could rarely
be effected by palaeontologists. We shall, perhaps, best perceive the
improbability of our being enabled to connect species by numerous, fine,
intermediate, fossil links, by asking ourselves whether, for instance,
geologists at some future period will be able to prove, that our different
breeds of cattle, sheep, horses, and dogs have descended from a single stock or
from several aboriginal stocks; or, again, whether certain sea-shells inhabiting
the shores of North America, which are ranked by some conchologists as distinct
species from their European representatives, and by other conchologists as only
varieties, are really varieties or are, as it is called, specifically distinct.
This could be effected only by the future geologist discovering in a fossil
state numerous intermediate gradations; and such success seems to me improbable
in the highest degree. Geological research, though it has added numerous species
to existing and extinct genera, and has made the intervals between some few
groups less wide than they otherwise would have been, yet has done scarcely
anything in breaking down the distinction between species, by connecting them
together by numerous, fine, intermediate varieties; and this not having been
effected, is probably the gravest and most obvious of all the many objections
which may be urged against my views. Hence it will be worth while to sum up the
foregoing remarks, under an imaginary illustration. The Malay Archipelago is of
about the size of Europe from the North Cape to the Mediterranean, and from
Britain to Russia; and therefore equals all the geological formations which have
been examined with any accuracy, excepting those of the United States of
America. I fully agree with Mr. Godwin-Austen, that the present condition of the
Malay Archipelago, with its numerous large islands separated by wide and shallow
seas, probably represents the former state of Europe, when most of our
formations were accumulating. The Malay Archipelago is one of the richest
regions of the whole world in organic beings; yet if all the species were to be
collected which have ever lived there, how imperfectly would they represent the
natural history of the world! But we have every reason to believe that the
terrestrial productions of the archipelago would be preserved in an excessively
imperfect manner in the formations which we suppose to be there accumulating. I
suspect that not many of the strictly littoral animals, or of those which lived
on naked submarine rocks, would be embedded; and those embedded in gravel or
sand, would not endure to a distant epoch. Wherever sediment did not accumulate
on the bed of the sea, or where it did not accumulate at a sufficient rate to
protect organic bodies from decay, no remains could be preserved. In our
archipelago, I believe that fossiliferous formations could be formed of
sufficient thickness to last to an age, as distant in futurity as the secondary
formations lie in the past, only during periods of subsidence. These periods of
subsidence would be separated from each other by enormous intervals, during
which the area would be either stationary or rising; whilst rising, each
fossiliferous formation would be destroyed, almost as soon as accumulated, by
the incessant coast-action, as we now see on the shores of South America. During
the periods of subsidence there would probably be much extinction of life;
during the periods of elevation, there would be much variation, but the
geological record would then be least perfect. It may be doubted whether the
duration of any one great period of subsidence over the whole or part of the
archipelago, together with a contemporaneous accumulation of sediment, would
EXCEED the average duration of the same specific forms; and these contingencies
are indispensable for the preservation of all the transitional gradations
between any two or more species. If such gradations were not fully preserved,
transitional varieties would merely appear as so many distinct species. It is,
also, probable that each great period of subsidence would be interrupted by
oscillations of level, and that slight climatal changes would intervene during
such lengthy periods; and in these cases the inhabitants of the archipelago
would have to migrate, and no closely consecutive record of their modifications
could be preserved in any one formation. Very many of the marine inhabitants of
the archipelago now range thousands of miles beyond its confines; and analogy
leads me to believe that it would be chiefly these far-ranging species which
would oftenest produce new varieties; and the varieties would at first generally
be local or confined to one place, but if possessed of any decided advantage, or
when further modified and improved, they would slowly spread and supplant their
parent-forms. When such varieties returned to their ancient homes, as they would
differ from their former state, in a nearly uniform, though perhaps extremely
slight degree, they would, according to the principles followed by many
palaeontologists, be ranked as new and distinct species. If then, there be some
degree of truth in these remarks, we have no right to expect to find in our
geological formations, an infinite number of those fine transitional forms,
which on my theory assuredly have connected all the past and present species of
the same group into one long and branching chain of life. We ought only to look
for a few links, some more closely, some more distantly related to each other;
and these links, let them be ever so close, if found in different stages of the
same formation, would, by most palaeontologists, be ranked as distinct species.
But I do not pretend that I should ever have suspected how poor a record of the
mutations of life, the best preserved geological section presented, had not the
difficulty of our not discovering innumerable transitional links between the
species which appeared at the commencement and close of each formation, pressed
so hardly on my theory. ON THE SUDDEN APPEARANCE OF WHOLE GROUPS OF ALLIED
SPECIES. The abrupt manner in which whole groups of species suddenly appear in
certain formations, has been urged by several palaeontologists, for instance, by
Agassiz, Pictet, and by none more forcibly than by Professor Sedgwick, as a
fatal objection to the belief in the transmutation of species. If numerous
species, belonging to the same genera or families, have really started into life
all at once, the fact would be fatal to the theory of descent with slow
modification through natural selection. For the development of a group of forms,
all of which have descended from some one progenitor, must have been an
extremely slow process; and the progenitors must have lived long ages before
their modified descendants. But we continually over-rate the perfection of the
geological record, and falsely infer, because certain genera or families have
not been found beneath a certain stage, that they did not exist before that
stage. We continually forget how large the world is, compared with the area over
which our geological formations have been carefully examined; we forget that
groups of species may elsewhere have long existed and have slowly multiplied
before they invaded the ancient archipelagoes of Europe and of the United
States. We do not make due allowance for the enormous intervals of time, which
have probably elapsed between our consecutive formations,--longer perhaps in
some cases than the time required for the accumulation of each formation. These
intervals will have given time for the multiplication of species from some one
or some few parent-forms; and in the succeeding formation such species will
appear as if suddenly created. I may here recall a remark formerly made, namely
that it might require a long succession of ages to adapt an organism to some new
and peculiar line of life, for instance to fly through the air; but that when
this had been effected, and a few species had thus acquired a great advantage
over other organisms, a comparatively short time would be necessary to produce
many divergent forms, which would be able to spread rapidly and widely
throughout the world. I will now give a few examples to illustrate these
remarks; and to show how liable we are to error in supposing that whole groups
of species have suddenly been produced. I may recall the well-known fact that in
geological treatises, published not many years ago, the great class of mammals
was always spoken of as having abruptly come in at the commencement of the
tertiary series. And now one of the richest known accumulations of fossil
mammals belongs to the middle of the secondary series; and one true mammal has
been discovered in the new red sandstone at nearly the commencement of this
great series. Cuvier used to urge that no monkey occurred in any tertiary
stratum; but now extinct species have been discovered in India, South America,
and in Europe even as far back as the eocene stage. The most striking case,
however, is that of the Whale family; as these animals have huge bones, are
marine, and range over the world, the fact of not a single bone of a whale
having been discovered in any secondary formation, seemed fully to justify the
belief that this great and distinct order had been suddenly produced in the
interval between the latest secondary and earliest tertiary formation. But now
we may read in the Supplement to Lyell's 'Manual,' published in 1858, clear
evidence of the existence of whales in the upper greensand, some time before the
close of the secondary period. I may give another instance, which from having
passed under my own eyes has much struck me. In a memoir on Fossil Sessile
Cirripedes, I have stated that, from the number of existing and extinct tertiary
species; from the extraordinary abundance of the individuals of many species all
over the world, from the Arctic regions to the equator, inhabiting various zones
of depths from the upper tidal limits to 50 fathoms; from the perfect manner in
which specimens are preserved in the oldest tertiary beds; from the ease with
which even a fragment of a valve can be recognised; from all these
circumstances, I inferred that had sessile cirripedes existed during the
secondary periods, they would certainly have been preserved and discovered; and
as not one species had been discovered in beds of this age, I concluded that
this great group had been suddenly developed at the commencement of the tertiary
series. This was a sore trouble to me, adding as I thought one more instance of
the abrupt appearance of a great group of species. But my work had hardly been
published, when a skilful palaeontologist, M. Bosquet, sent me a drawing of a
perfect specimen of an unmistakeable sessile cirripede, which he had himself
extracted from the chalk of Belgium. And, as if to make the case as striking as
possible, this sessile cirripede was a Chthamalus, a very common, large, and
ubiquitous genus, of which not one specimen has as yet been found even in any
tertiary stratum. Hence we now positively know that sessile cirripedes existed
during the secondary period; and these cirripedes might have been the
progenitors of our many tertiary and existing species. The case most frequently
insisted on by palaeontologists of the apparently sudden appearance of a whole
group of species, is that of the teleostean fishes, low down in the Chalk
period. This group includes the large majority of existing species. Lately,
Professor Pictet has carried their existence one sub-stage further back; and
some palaeontologists believe that certain much older fishes, of which the
affinities are as yet imperfectly known, are really teleostean. Assuming,
however, that the whole of them did appear, as Agassiz believes, at the
commencement of the chalk formation, the fact would certainly be highly
remarkable; but I cannot see that it would be an insuperable difficulty on my
theory, unless it could likewise be shown that the species of this group
appeared suddenly and simultaneously throughout the world at this same period.
It is almost superfluous to remark that hardly any fossil-fish are known from
south of the equator; and by running through Pictet's Palaeontology it will be
seen that very few species are known from several formations in Europe. Some few
families of fish now have a confined range; the teleostean fish might formerly
have had a similarly confined range, and after having been largely developed in
some one sea, might have spread widely. Nor have we any right to suppose that
the seas of the world have always been so freely open from south to north as
they are at present. Even at this day, if the Malay Archipelago were converted
into land, the tropical parts of the Indian Ocean would form a large and
perfectly enclosed basin, in which any great group of marine animals might be
multiplied; and here they would remain confined, until some of the species
became adapted to a cooler climate, and were enabled to double the southern
capes of Africa or Australia, and thus reach other and distant seas. From these
and similar considerations, but chiefly from our ignorance of the geology of
other countries beyond the confines of Europe and the United States; and from
the revolution in our palaeontological ideas on many points, which the
discoveries of even the last dozen years have effected, it seems to me to be
about as rash in us to dogmatize on the succession of organic beings throughout
the world, as it would be for a naturalist to land for five minutes on some one
barren point in Australia, and then to discuss the number and range of its
productions. ON THE SUDDEN APPEARANCE OF GROUPS OF ALLIED SPECIES IN THE LOWEST
KNOWN FOSSILIFEROUS STRATA. There is another and allied difficulty, which is
much graver. I allude to the manner in which numbers of species of the same
group, suddenly appear in the lowest known fossiliferous rocks. Most of the
arguments which have convinced me that all the existing species of the same
group have descended from one progenitor, apply with nearly equal force to the
earliest known species. For instance, I cannot doubt that all the Silurian
trilobites have descended from some one crustacean, which must have lived long
before the Silurian age, and which probably differed greatly from any known
animal. Some of the most ancient Silurian animals, as the Nautilus, Lingula,
etc., do not differ much from living species; and it cannot on my theory be
supposed, that these old species were the progenitors of all the species of the
orders to which they belong, for they do not present characters in any degree
intermediate between them. If, moreover, they had been the progenitors of these
orders, they would almost certainly have been long ago supplanted and
exterminated by their numerous and improved descendants. Consequently, if my
theory be true, it is indisputable that before the lowest Silurian stratum was
deposited, long periods elapsed, as long as, or probably far longer than, the
whole interval from the Silurian age to the present day; and that during these
vast, yet quite unknown, periods of time, the world swarmed with living
creatures. To the question why we do not find records of these vast primordial
periods, I can give no satisfactory answer. Several of the most eminent
geologists, with Sir R. Murchison at their head, are convinced that we see in
the organic remains of the lowest Silurian stratum the dawn of life on this
planet. Other highly competent judges, as Lyell and the late E. Forbes, dispute
this conclusion. We should not forget that only a small portion of the world is
known with accuracy. M. Barrande has lately added another and lower stage to the
Silurian system, abounding with new and peculiar species. Traces of life have
been detected in the Longmynd beds beneath Barrande's so-called primordial zone.
The presence of phosphatic nodules and bituminous matter in some of the lowest
azoic rocks, probably indicates the former existence of life at these periods.
But the difficulty of understanding the absence of vast piles of fossiliferous
strata, which on my theory no doubt were somewhere accumulated before the
Silurian epoch, is very great. If these most ancient beds had been wholly worn
away by denudation, or obliterated by metamorphic action, we ought to find only
small remnants of the formations next succeeding them in age, and these ought to
be very generally in a metamorphosed condition. But the descriptions which we
now possess of the Silurian deposits over immense territories in Russia and in
North America, do not support the view, that the older a formation is, the more
it has suffered the extremity of denudation and metamorphism. The case at
present must remain inexplicable; and may be truly urged as a valid argument
against the views here entertained. To show that it may hereafter receive some
explanation, I will give the following hypothesis. From the nature of the
organic remains, which do not appear to have inhabited profound depths, in the
several formations of Europe and of the United States; and from the amount of
sediment, miles in thickness, of which the formations are composed, we may infer
that from first to last large islands or tracts of land, whence the sediment was
derived, occurred in the neighbourhood of the existing continents of Europe and
North America. But we do not know what was the state of things in the intervals
between the successive formations; whether Europe and the United States during
these intervals existed as dry land, or as a submarine surface near land, on
which sediment was not deposited, or again as the bed of an open and
unfathomable sea. Looking to the existing oceans, which are thrice as extensive
as the land, we see them studded with many islands; but not one oceanic island
is as yet known to afford even a remnant of any palaeozoic or secondary
formation. Hence we may perhaps infer, that during the palaeozoic and secondary
periods, neither continents nor continental islands existed where our oceans now
extend; for had they existed there, palaeozoic and secondary formations would in
all probability have been accumulated from sediment derived from their wear and
tear; and would have been at least partially upheaved by the oscillations of
level, which we may fairly conclude must have intervened during these enormously
long periods. If then we may infer anything from these facts, we may infer that
where our oceans now extend, oceans have extended from the remotest period of
which we have any record; and on the other hand, that where continents now
exist, large tracts of land have existed, subjected no doubt to great
oscillations of level, since the earliest silurian period. The coloured map
appended to my volume on Coral Reefs, led me to conclude that the great oceans
are still mainly areas of subsidence, the great archipelagoes still areas of
oscillations of level, and the continents areas of elevation. But have we any
right to assume that things have thus remained from eternity? Our continents
seem to have been formed by a preponderance, during many oscillations of level,
of the force of elevation; but may not the areas of preponderant movement have
changed in the lapse of ages? At a period immeasurably antecedent to the
silurian epoch, continents may have existed where oceans are now spread out; and
clear and open oceans may have existed where our continents now stand. Nor
should we be justified in assuming that if, for instance, the bed of the Pacific
Ocean were now converted into a continent, we should there find formations older
than the silurian strata, supposing such to have been formerly deposited; for it
might well happen that strata which had subsided some miles nearer to the centre
of the earth, and which had been pressed on by an enormous weight of
superincumbent water, might have undergone far more metamorphic action than
strata which have always remained nearer to the surface. The immense areas in
some parts of the world, for instance in South America, of bare metamorphic
rocks, which must have been heated under great pressure, have always seemed to
me to require some special explanation; and we may perhaps believe that we see
in these large areas, the many formations long anterior to the silurian epoch in
a completely metamorphosed condition. The several difficulties here discussed,
namely our not finding in the successive formations infinitely numerous
transitional links between the many species which now exist or have existed; the
sudden manner in which whole groups of species appear in our European
formations; the almost entire absence, as at present known, of fossiliferous
formations beneath the Silurian strata, are all undoubtedly of the gravest
nature. We see this in the plainest manner by the fact that all the most eminent
palaeontologists, namely Cuvier, Owen, Agassiz, Barrande, Falconer, E. Forbes,
etc., and all our greatest geologists, as Lyell, Murchison, Sedgwick, etc., have
unanimously, often vehemently, maintained the immutability of species. But I
have reason to believe that one great authority, Sir Charles Lyell, from further
reflexion entertains grave doubts on this subject. I feel how rash it is to
differ from these great authorities, to whom, with others, we owe all our
knowledge. Those who think the natural geological record in any degree perfect,
and who do not attach much weight to the facts and arguments of other kinds
given in this volume, will undoubtedly at once reject my theory. For my part,
following out Lyell's metaphor, I look at the natural geological record, as a
history of the world imperfectly kept, and written in a changing dialect; of
this history we possess the last volume alone, relating only to two or three
countries. Of this volume, only here and there a short chapter has been
preserved; and of each page, only here and there a few lines. Each word of the
slowly-changing language, in which the history is supposed to be written, being
more or less different in the interrupted succession of chapters, may represent
the apparently abruptly changed forms of life, entombed in our consecutive, but
widely separated formations. On this view, the difficulties above discussed are
greatly diminished, or even disappear. CHAPTER 10. ON THE GEOLOGICAL SUCCESSION
OF ORGANIC BEINGS. On the slow and successive appearance of new species. On
their different rates of change. Species once lost do not reappear. Groups of
species follow the same general rules in their appearance and disappearance as
do single species. On Extinction. On simultaneous changes in the forms of life
throughout the world. On the affinities of extinct species to each other and to
living species. On the state of development of ancient forms. On the succession
of the same types within the same areas. Summary of preceding and present
chapters. Let us now see whether the several facts and rules relating to the
geological succession of organic beings, better accord with the common view of
the immutability of species, or with that of their slow and gradual
modification, through descent and natural selection. New species have appeared
very slowly, one after another, both on the land and in the waters. Lyell has
shown that it is hardly possible to resist the evidence on this head in the case
of the several tertiary stages; and every year tends to fill up the blanks
between them, and to make the percentage system of lost and new forms more
gradual. In some of the most recent beds, though undoubtedly of high antiquity
if measured by years, only one or two species are lost forms, and only one or
two are new forms, having here appeared for the first time, either locally, or,
as far as we know, on the face of the earth. If we may trust the observations of
Philippi in Sicily, the successive changes in the marine inhabitants of that
island have been many and most gradual. The secondary formations are more
broken; but, as Bronn has remarked, neither the appearance nor disappearance of
their many now extinct species has been simultaneous in each separate formation.
Species of different genera and classes have not changed at the same rate, or in
the same degree. In the oldest tertiary beds a few living shells may still be
found in the midst of a multitude of extinct forms. Falconer has given a
striking instance of a similar fact, in an existing crocodile associated with
many strange and lost mammals and reptiles in the sub-Himalayan deposits. The
Silurian Lingula differs but little from the living species of this genus;
whereas most of the other Silurian Molluscs and all the Crustaceans have changed
greatly. The productions of the land seem to change at a quicker rate than those
of the sea, of which a striking instance has lately been observed in
Switzerland. There is some reason to believe that organisms, considered high in
the scale of nature, change more quickly than those that are low: though there
are exceptions to this rule. The amount of organic change, as Pictet has
remarked, does not strictly correspond with the succession of our geological
formations; so that between each two consecutive formations, the forms of life
have seldom changed in exactly the same degree. Yet if we compare any but the
most closely related formations, all the species will be found to have undergone
some change. When a species has once disappeared from the face of the earth, we
have reason to believe that the same identical form never reappears. The
strongest apparent exception to this latter rule, is that of the so-called
"colonies" of M. Barrande, which intrude for a period in the midst of an older
formation, and then allow the pre-existing fauna to reappear; but Lyell's
explanation, namely, that it is a case of temporary migration from a distinct
geographical province, seems to me satisfactory. These several facts accord well
with my theory. I believe in no fixed law of development, causing all the
inhabitants of a country to change abruptly, or simultaneously, or to an equal
degree. The process of modification must be extremely slow. The variability of
each species is quite independent of that of all others. Whether such
variability be taken advantage of by natural selection, and whether the
variations be accumulated to a greater or lesser amount, thus causing a greater
or lesser amount of modification in the varying species, depends on many complex
contingencies,--on the variability being of a beneficial nature, on the power of
intercrossing, on the rate of breeding, on the slowly changing physical
conditions of the country, and more especially on the nature of the other
inhabitants with which the varying species comes into competition. Hence it is
by no means surprising that one species should retain the same identical form
much longer than others; or, if changing, that it should change less. We see the
same fact in geographical distribution; for instance, in the land-shells and
coleopterous insects of Madeira having come to differ considerably from their
nearest allies on the continent of Europe, whereas the marine shells and birds
have remained unaltered. We can perhaps understand the apparently quicker rate
of change in terrestrial and in more highly organised productions compared with
marine and lower productions, by the more complex relations of the higher beings
to their organic and inorganic conditions of life, as explained in a former
chapter. When many of the inhabitants of a country have become modified and
improved, we can understand, on the principle of competition, and on that of the
many all-important relations of organism to organism, that any form which does
not become in some degree modified and improved, will be liable to be
exterminated. Hence we can see why all the species in the same region do at
last, if we look to wide enough intervals of time, become modified; for those
which do not change will become extinct. In members of the same class the
average amount of change, during long and equal periods of time, may, perhaps,
be nearly the same; but as the accumulation of long-enduring fossiliferous
formations depends on great masses of sediment having been deposited on areas
whilst subsiding, our formations have been almost necessarily accumulated at
wide and irregularly intermittent intervals; consequently the amount of organic
change exhibited by the fossils embedded in consecutive formations is not equal.
Each formation, on this view, does not mark a new and complete act of creation,
but only an occasional scene, taken almost at hazard, in a slowly changing
drama. We can clearly understand why a species when once lost should never
reappear, even if the very same conditions of life, organic and inorganic,
should recur. For though the offspring of one species might be adapted (and no
doubt this has occurred in innumerable instances) to fill the exact place of
another species in the economy of nature, and thus supplant it; yet the two
forms--the old and the new--would not be identically the same; for both would
almost certainly inherit different characters from their distinct progenitors.
For instance, it is just possible, if our fantail-pigeons were all destroyed,
that fanciers, by striving during long ages for the same object, might make a
new breed hardly distinguishable from our present fantail; but if the parent
rock-pigeon were also destroyed, and in nature we have every reason to believe
that the parent-form will generally be supplanted and exterminated by its
improved offspring, it is quite incredible that a fantail, identical with the
existing breed, could be raised from any other species of pigeon, or even from
the other well-established races of the domestic pigeon, for the newly-formed
fantail would be almost sure to inherit from its new progenitor some slight
characteristic differences. Groups of species, that is, genera and families,
follow the same general rules in their appearance and disappearance as do single
species, changing more or less quickly, and in a greater or lesser degree. A
group does not reappear after it has once disappeared; or its existence, as long
as it lasts, is continuous. I am aware that there are some apparent exceptions
to this rule, but the exceptions are surprisingly few, so few, that E. Forbes,
Pictet, and Woodward (though all strongly opposed to such views as I maintain)
admit its truth; and the rule strictly accords with my theory. For as all the
species of the same group have descended from some one species, it is clear that
as long as any species of the group have appeared in the long succession of
ages, so long must its members have continuously existed, in order to have
generated either new and modified or the same old and unmodified forms. Species
of the genus Lingula, for instance, must have continuously existed by an
unbroken succession of generations, from the lowest Silurian stratum to the
present day. We have seen in the last chapter that the species of a group
sometimes falsely appear to have come in abruptly; and I have attempted to give
an explanation of this fact, which if true would have been fatal to my views.
But such cases are certainly exceptional; the general rule being a gradual
increase in number, till the group reaches its maximum, and then, sooner or
later, it gradually decreases. If the number of the species of a genus, or the
number of the genera of a family, be represented by a vertical line of varying
thickness, crossing the successive geological formations in which the species
are found, the line will sometimes falsely appear to begin at its lower end, not
in a sharp point, but abruptly; it then gradually thickens upwards, sometimes
keeping for a space of equal thickness, and ultimately thins out in the upper
beds, marking the decrease and final extinction of the species. This gradual
increase in number of the species of a group is strictly conformable with my
theory; as the species of the same genus, and the genera of the same family, can
increase only slowly and progressively; for the process of modification and the
production of a number of allied forms must be slow and gradual,--one species
giving rise first to two or three varieties, these being slowly converted into
species, which in their turn produce by equally slow steps other species, and so
on, like the branching of a great tree from a single stem, till the group
becomes large. ON EXTINCTION. We have as yet spoken only incidentally of the
disappearance of species and of groups of species. On the theory of natural
selection the extinction of old forms and the production of new and improved
forms are intimately connected together. The old notion of all the inhabitants
of the earth having been swept away at successive periods by catastrophes, is
very generally given up, even by those geologists, as Elie de Beaumont,
Murchison, Barrande, etc., whose general views would naturally lead them to this
conclusion. On the contrary, we have every reason to believe, from the study of
the tertiary formations, that species and groups of species gradually disappear,
one after another, first from one spot, then from another, and finally from the
world. Both single species and whole groups of species last for very unequal
periods; some groups, as we have seen, having endured from the earliest known
dawn of life to the present day; some having disappeared before the close of the
palaeozoic period. No fixed law seems to determine the length of time during
which any single species or any single genus endures. There is reason to believe
that the complete extinction of the species of a group is generally a slower
process than their production: if the appearance and disappearance of a group of
species be represented, as before, by a vertical line of varying thickness, the
line is found to taper more gradually at its upper end, which marks the progress
of extermination, than at its lower end, which marks the first appearance and
increase in numbers of the species. In some cases, however, the extermination of
whole groups of beings, as of ammonites towards the close of the secondary
period, has been wonderfully sudden. The whole subject of the extinction of
species has been involved in the most gratuitous mystery. Some authors have even
supposed that as the individual has a definite length of life, so have species a
definite duration. No one I think can have marvelled more at the extinction of
species, than I have done. When I found in La Plata the tooth of a horse
embedded with the remains of Mastodon, Megatherium, Toxodon, and other extinct
monsters, which all co-existed with still living shells at a very late
geological period, I was filled with astonishment; for seeing that the horse,
since its introduction by the Spaniards into South America, has run wild over
the whole country and has increased in numbers at an unparalleled rate, I asked
myself what could so recently have exterminated the former horse under
conditions of life apparently so favourable. But how utterly groundless was my
astonishment! Professor Owen soon perceived that the tooth, though so like that
of the existing horse, belonged to an extinct species. Had this horse been still
living, but in some degree rare, no naturalist would have felt the least
surprise at its rarity; for rarity is the attribute of a vast number of species
of all classes, in all countries. If we ask ourselves why this or that species
is rare, we answer that something is unfavourable in its conditions of life; but
what that something is, we can hardly ever tell. On the supposition of the
fossil horse still existing as a rare species, we might have felt certain from
the analogy of all other mammals, even of the slow-breeding elephant, and from
the history of the naturalisation of the domestic horse in South America, that
under more favourable conditions it would in a very few years have stocked the
whole continent. But we could not have told what the unfavourable conditions
were which checked its increase, whether some one or several contingencies, and
at what period of the horse's life, and in what degree, they severally acted. If
the conditions had gone on, however slowly, becoming less and less favourable,
we assuredly should not have perceived the fact, yet the fossil horse would
certainly have become rarer and rarer, and finally extinct;--its place being
seized on by some more successful competitor. It is most difficult always to
remember that the increase of every living being is constantly being checked by
unperceived injurious agencies; and that these same unperceived agencies are
amply sufficient to cause rarity, and finally extinction. We see in many cases
in the more recent tertiary formations, that rarity precedes extinction; and we
know that this has been the progress of events with those animals which have
been exterminated, either locally or wholly, through man's agency. I may repeat
what I published in 1845, namely, that to admit that species generally become
rare before they become extinct--to feel no surprise at the rarity of a species,
and yet to marvel greatly when it ceases to exist, is much the same as to admit
that sickness in the individual is the forerunner of death--to feel no surprise
at sickness, but when the sick man dies, to wonder and to suspect that he died
by some unknown deed of violence. The theory of natural selection is grounded on
the belief that each new variety, and ultimately each new species, is produced
and maintained by having some advantage over those with which it comes into
competition; and the consequent extinction of less-favoured forms almost
inevitably follows. It is the same with our domestic productions: when a new and
slightly improved variety has been raised, it at first supplants the less
improved varieties in the same neighbourhood; when much improved it is
transported far and near, like our short-horn cattle, and takes the place of
other breeds in other countries. Thus the appearance of new forms and the
disappearance of old forms, both natural and artificial, are bound together. In
certain flourishing groups, the number of new specific forms which have been
produced within a given time is probably greater than that of the old forms
which have been exterminated; but we know that the number of species has not
gone on indefinitely increasing, at least during the later geological periods,
so that looking to later times we may believe that the production of new forms
has caused the extinction of about the same number of old forms. The competition
will generally be most severe, as formerly explained and illustrated by
examples, between the forms which are most like each other in all respects.
Hence the improved and modified descendants of a species will generally cause
the extermination of the parent-species; and if many new forms have been
developed from any one species, the nearest allies of that species, i.e. the
species of the same genus, will be the most liable to extermination. Thus, as I
believe, a number of new species descended from one species, that is a new
genus, comes to supplant an old genus, belonging to the same family. But it must
often have happened that a new species belonging to some one group will have
seized on the place occupied by a species belonging to a distinct group, and
thus caused its extermination; and if many allied forms be developed from the
successful intruder, many will have to yield their places; and it will generally
be allied forms, which will suffer from some inherited inferiority in common.
But whether it be species belonging to the same or to a distinct class, which
yield their places to other species which have been modified and improved, a few
of the sufferers may often long be preserved, from being fitted to some peculiar
line of life, or from inhabiting some distant and isolated station, where they
have escaped severe competition. For instance, a single species of Trigonia, a
great genus of shells in the secondary formations, survives in the Australian
seas; and a few members of the great and almost extinct group of Ganoid fishes
still inhabit our fresh waters. Therefore the utter extinction of a group is
generally, as we have seen, a slower process than its production. With respect
to the apparently sudden extermination of whole families or orders, as of
Trilobites at the close of the palaeozoic period and of Ammonites at the close
of the secondary period, we must remember what has been already said on the
probable wide intervals of time between our consecutive formations; and in these
intervals there may have been much slow extermination. Moreover, when by sudden
immigration or by unusually rapid development, many species of a new group have
taken possession of a new area, they will have exterminated in a correspondingly
rapid manner many of the old inhabitants; and the forms which thus yield their
places will commonly be allied, for they will partake of some inferiority in
common. Thus, as it seems to me, the manner in which single species and whole
groups of species become extinct, accords well with the theory of natural
selection. We need not marvel at extinction; if we must marvel, let it be at our
presumption in imagining for a moment that we understand the many complex
contingencies, on which the existence of each species depends. If we forget for
an instant, that each species tends to increase inordinately, and that some
check is always in action, yet seldom perceived by us, the whole economy of
nature will be utterly obscured. Whenever we can precisely say why this species
is more abundant in individuals than that; why this species and not another can
be naturalised in a given country; then, and not till then, we may justly feel
surprise why we cannot account for the extinction of this particular species or
group of species. ON THE FORMS OF LIFE CHANGING ALMOST SIMULTANEOUSLY THROUGHOUT
THE WORLD. Scarcely any palaeontological discovery is more striking than the
fact, that the forms of life change almost simultaneously throughout the world.
Thus our European Chalk formation can be recognised in many distant parts of the
world, under the most different climates, where not a fragment of the mineral
chalk itself can be found; namely, in North America, in equatorial South
America, in Tierra del Fuego, at the Cape of Good Hope, and in the peninsula of
India. For at these distant points, the organic remains in certain beds present
an unmistakeable degree of resemblance to those of the Chalk. It is not that the
same species are met with; for in some cases not one species is identically the
same, but they belong to the same families, genera, and sections of genera, and
sometimes are similarly characterised in such trifling points as mere
superficial sculpture. Moreover other forms, which are not found in the Chalk of
Europe, but which occur in the formations either above or below, are similarly
absent at these distant points of the world. In the several successive
palaeozoic formations of Russia, Western Europe and North America, a similar
parallelism in the forms of life has been observed by several authors: so it is,
according to Lyell, with the several European and North American tertiary
deposits. Even if the few fossil species which are common to the Old and New
Worlds be kept wholly out of view, the general parallelism in the successive
forms of life, in the stages of the widely separated palaeozoic and tertiary
periods, would still be manifest, and the several formations could be easily
correlated. These observations, however, relate to the marine inhabitants of
distant parts of the world: we have not sufficient data to judge whether the
productions of the land and of fresh water change at distant points in the same
parallel manner. We may doubt whether they have thus changed: if the
Megatherium, Mylodon, Macrauchenia, and Toxodon had been brought to Europe from
La Plata, without any information in regard to their geological position, no one
would have suspected that they had coexisted with still living sea-shells; but
as these anomalous monsters coexisted with the Mastodon and Horse, it might at
least have been inferred that they had lived during one of the latter tertiary
stages. When the marine forms of life are spoken of as having changed
simultaneously throughout the world, it must not be supposed that this
expression relates to the same thousandth or hundred-thousandth year, or even
that it has a very strict geological sense; for if all the marine animals which
live at the present day in Europe, and all those that lived in Europe during the
pleistocene period (an enormously remote period as measured by years, including
the whole glacial epoch), were to be compared with those now living in South
America or in Australia, the most skilful naturalist would hardly be able to say
whether the existing or the pleistocene inhabitants of Europe resembled most
closely those of the southern hemisphere. So, again, several highly competent
observers believe that the existing productions of the United States are more
closely related to those which lived in Europe during certain later tertiary
stages, than to those which now live here; and if this be so, it is evident that
fossiliferous beds deposited at the present day on the shores of North America
would hereafter be liable to be classed with somewhat older European beds.
Nevertheless, looking to a remotely future epoch, there can, I think, be little
doubt that all the more modern MARINE formations, namely, the upper pliocene,
the pleistocene and strictly modern beds, of Europe, North and South America,
and Australia, from containing fossil remains in some degree allied, and from
not including those forms which are only found in the older underlying deposits,
would be correctly ranked as simultaneous in a geological sense. The fact of the
forms of life changing simultaneously, in the above large sense, at distant
parts of the world, has greatly struck those admirable observers, MM. de
Verneuil and d'Archiac. After referring to the parallelism of the palaeozoic
forms of life in various parts of Europe, they add, "If struck by this strange
sequence, we turn our attention to North America, and there discover a series of
analogous phenomena, it will appear certain that all these modifications of
species, their extinction, and the introduction of new ones, cannot be owing to
mere changes in marine currents or other causes more or less local and
temporary, but depend on general laws which govern the whole animal kingdom." M.
Barrande has made forcible remarks to precisely the same effect. It is, indeed,
quite futile to look to changes of currents, climate, or other physical
conditions, as the cause of these great mutations in the forms of life
throughout the world, under the most different climates. We must, as Barrande
has remarked, look to some special law. We shall see this more clearly when we
treat of the present distribution of organic beings, and find how slight is the
relation between the physical conditions of various countries, and the nature of
their inhabitants. This great fact of the parallel succession of the forms of
life throughout the world, is explicable on the theory of natural selection. New
species are formed by new varieties arising, which have some advantage over
older forms; and those forms, which are already dominant, or have some advantage
over the other forms in their own country, would naturally oftenest give rise to
new varieties or incipient species; for these latter must be victorious in a
still higher degree in order to be preserved and to survive. We have distinct
evidence on this head, in the plants which are dominant, that is, which are
commonest in their own homes, and are most widely diffused, having produced the
greatest number of new varieties. It is also natural that the dominant, varying,
and far-spreading species, which already have invaded to a certain extent the
territories of other species, should be those which would have the best chance
of spreading still further, and of giving rise in new countries to new varieties
and species. The process of diffusion may often be very slow, being dependent on
climatal and geographical changes, or on strange accidents, but in the long run
the dominant forms will generally succeed in spreading. The diffusion would, it
is probable, be slower with the terrestrial inhabitants of distinct continents
than with the marine inhabitants of the continuous sea. We might therefore
expect to find, as we apparently do find, a less strict degree of parallel
succession in the productions of the land than of the sea. Dominant species
spreading from any region might encounter still more dominant species, and then
their triumphant course, or even their existence, would cease. We know not at
all precisely what are all the conditions most favourable for the multiplication
of new and dominant species; but we can, I think, clearly see that a number of
individuals, from giving a better chance of the appearance of favourable
variations, and that severe competition with many already existing forms, would
be highly favourable, as would be the power of spreading into new territories. A
certain amount of isolation, recurring at long intervals of time, would probably
be also favourable, as before explained. One quarter of the world may have been
most favourable for the production of new and dominant species on the land, and
another for those in the waters of the sea. If two great regions had been for a
long period favourably circumstanced in an equal degree, whenever their
inhabitants met, the battle would be prolonged and severe; and some from one
birthplace and some from the other might be victorious. But in the course of
time, the forms dominant in the highest degree, wherever produced, would tend
everywhere to prevail. As they prevailed, they would cause the extinction of
other and inferior forms; and as these inferior forms would be allied in groups
by inheritance, whole groups would tend slowly to disappear; though here and
there a single member might long be enabled to survive. Thus, as it seems to me,
the parallel, and, taken in a large sense, simultaneous, succession of the same
forms of life throughout the world, accords well with the principle of new
species having been formed by dominant species spreading widely and varying; the
new species thus produced being themselves dominant owing to inheritance, and to
having already had some advantage over their parents or over other species;
these again spreading, varying, and producing new species. The forms which are
beaten and which yield their places to the new and victorious forms, will
generally be allied in groups, from inheriting some inferiority in common; and
therefore as new and improved groups spread throughout the world, old groups
will disappear from the world; and the succession of forms in both ways will
everywhere tend to correspond. There is one other remark connected with this
subject worth making. I have given my reasons for believing that all our greater
fossiliferous formations were deposited during periods of subsidence; and that
blank intervals of vast duration occurred during the periods when the bed of the
sea was either stationary or rising, and likewise when sediment was not thrown
down quickly enough to embed and preserve organic remains. During these long and
blank intervals I suppose that the inhabitants of each region underwent a
considerable amount of modification and extinction, and that there was much
migration from other parts of the world. As we have reason to believe that large
areas are affected by the same movement, it is probable that strictly
contemporaneous formations have often been accumulated over very wide spaces in
the same quarter of the world; but we are far from having any right to conclude
that this has invariably been the case, and that large areas have invariably
been affected by the same movements. When two formations have been deposited in
two regions during nearly, but not exactly the same period, we should find in
both, from the causes explained in the foregoing paragraphs, the same general
succession in the forms of life; but the species would not exactly correspond;
for there will have been a little more time in the one region than in the other
for modification, extinction, and immigration. I suspect that cases of this
nature have occurred in Europe. Mr. Prestwich, in his admirable Memoirs on the
eocene deposits of England and France, is able to draw a close general
parallelism between the successive stages in the two countries; but when he
compares certain stages in England with those in France, although he finds in
both a curious accordance in the numbers of the species belonging to the same
genera, yet the species themselves differ in a manner very difficult to account
for, considering the proximity of the two areas,--unless, indeed, it be assumed
that an isthmus separated two seas inhabited by distinct, but contemporaneous,
faunas. Lyell has made similar observations on some of the later tertiary
formations. Barrande, also, shows that there is a striking general parallelism
in the successive Silurian deposits of Bohemia and Scandinavia; nevertheless he
finds a surprising amount of difference in the species. If the several
formations in these regions have not been deposited during the same exact
periods,--a formation in one region often corresponding with a blank interval in
the other,--and if in both regions the species have gone on slowly changing
during the accumulation of the several formations and during the long intervals
of time between them; in this case, the several formations in the two regions
could be arranged in the same order, in accordance with the general succession
of the form of life, and the order would falsely appear to be strictly parallel;
nevertheless the species would not all be the same in the apparently
corresponding stages in the two regions. $ ON THE AFFINITIES OF EXTINCT SPECIES
TO EACH OTHER, AND TO LIVING FORMS. Let us now look to the mutual affinities of
extinct and living species. They all fall into one grand natural system; and
this fact is at once explained on the principle of descent. The more ancient any
form is, the more, as a general rule, it differs from living forms. But, as
Buckland long ago remarked, all fossils can be classed either in still existing
groups, or between them. That the extinct forms of life help to fill up the wide
intervals between existing genera, families, and orders, cannot be disputed. For
if we confine our attention either to the living or to the extinct alone, the
series is far less perfect than if we combine both into one general system. With
respect to the Vertebrata, whole pages could be filled with striking
illustrations from our great palaeontologist, Owen, showing how extinct animals
fall in between existing groups. Cuvier ranked the Ruminants and Pachyderms, as
the two most distinct orders of mammals; but Owen has discovered so many fossil
links, that he has had to alter the whole classification of these two orders;
and has placed certain pachyderms in the same sub-order with ruminants: for
example, he dissolves by fine gradations the apparently wide difference between
the pig and the camel. In regard to the Invertebrata, Barrande, and a higher
authority could not be named, asserts that he is every day taught that
palaeozoic animals, though belonging to the same orders, families, or genera
with those living at the present day, were not at this early epoch limited in
such distinct groups as they now are. Some writers have objected to any extinct
species or group of species being considered as intermediate between living
species or groups. If by this term it is meant that an extinct form is directly
intermediate in all its characters between two living forms, the objection is
probably valid. But I apprehend that in a perfectly natural classification many
fossil species would have to stand between living species, and some extinct
genera between living genera, even between genera belonging to distinct
families. The most common case, especially with respect to very distinct groups,
such as fish and reptiles, seems to be, that supposing them to be distinguished
at the present day from each other by a dozen characters, the ancient members of
the same two groups would be distinguished by a somewhat lesser number of
characters, so that the two groups, though formerly quite distinct, at that
period made some small approach to each other. It is a common belief that the
more ancient a form is, by so much the more it tends to connect by some of its
characters groups now widely separated from each other. This remark no doubt
must be restricted to those groups which have undergone much change in the
course of geological ages; and it would be difficult to prove the truth of the
proposition, for every now and then even a living animal, as the Lepidosiren, is
discovered having affinities directed towards very distinct groups. Yet if we
compare the older Reptiles and Batrachians, the older Fish, the older
Cephalopods, and the eocene Mammals, with the more recent members of the same
classes, we must admit that there is some truth in the remark. Let us see how
far these several facts and inferences accord with the theory of descent with
modification. As the subject is somewhat complex, I must request the reader to
turn to the diagram in the fourth chapter. We may suppose that the numbered
letters represent genera, and the dotted lines diverging from them the species
in each genus. The diagram is much too simple, too few genera and too few
species being given, but this is unimportant for us. The horizontal lines may
represent successive geological formations, and all the forms beneath the
uppermost line may be considered as extinct. The three existing genera, a14,
q14, p14, will form a small family; b14 and f14 a closely allied family or
sub-family; and o14, e14, m14, a third family. These three families, together
with the many extinct genera on the several lines of descent diverging from the
parent-form A, will form an order; for all will have inherited something in
common from their ancient and common progenitor. On the principle of the
continued tendency to divergence of character, which was formerly illustrated by
this diagram, the more recent any form is, the more it will generally differ
from its ancient progenitor. Hence we can understand the rule that the most
ancient fossils differ most from existing forms. We must not, however, assume
that divergence of character is a necessary contingency; it depends solely on
the descendants from a species being thus enabled to seize on many and different
places in the economy of nature. Therefore it is quite possible, as we have seen
in the case of some Silurian forms, that a species might go on being slightly
modified in relation to its slightly altered conditions of life, and yet retain
throughout a vast period the same general characteristics. This is represented
in the diagram by the letter F14. All the many forms, extinct and recent,
descended from A, make, as before remarked, one order; and this order, from the
continued effects of extinction and divergence of character, has become divided
into several sub-families and families, some of which are supposed to have
perished at different periods, and some to have endured to the present day. By
looking at the diagram we can see that if many of the extinct forms, supposed to
be embedded in the successive formations, were discovered at several points low
down in the series, the three existing families on the uppermost line would be
rendered less distinct from each other. If, for instance, the genera a1, a5,
a10, f8, m3, m6, m9 were disinterred, these three families would be so closely
linked together that they probably would have to be united into one great
family, in nearly the same manner as has occurred with ruminants and pachyderms.
Yet he who objected to call the extinct genera, which thus linked the living
genera of three families together, intermediate in character, would be
justified, as they are intermediate, not directly, but only by a long and
circuitous course through many widely different forms. If many extinct forms
were to be discovered above one of the middle horizontal lines or geological
formations--for instance, above Number VI.--but none from beneath this line,
then only the two families on the left hand (namely, a14, etc., and b14, etc.)
would have to be united into one family; and the two other families (namely, a14
to f14 now including five genera, and o14 to m14) would yet remain distinct.
These two families, however, would be less distinct from each other than they
were before the discovery of the fossils. If, for instance, we suppose the
existing genera of the two families to differ from each other by a dozen
characters, in this case the genera, at the early period marked VI., would
differ by a lesser number of characters; for at this early stage of descent they
have not diverged in character from the common progenitor of the order, nearly
so much as they subsequently diverged. Thus it comes that ancient and extinct
genera are often in some slight degree intermediate in character between their
modified descendants, or between their collateral relations. In nature the case
will be far more complicated than is represented in the diagram; for the groups
will have been more numerous, they will have endured for extremely unequal
lengths of time, and will have been modified in various degrees. As we possess
only the last volume of the geological record, and that in a very broken
condition, we have no right to expect, except in very rare cases, to fill up
wide intervals in the natural system, and thus unite distinct families or
orders. All that we have a right to expect, is that those groups, which have
within known geological periods undergone much modification, should in the older
formations make some slight approach to each other; so that the older members
should differ less from each other in some of their characters than do the
existing members of the same groups; and this by the concurrent evidence of our
best palaeontologists seems frequently to be the case. Thus, on the theory of
descent with modification, the main facts with respect to the mutual affinities
of the extinct forms of life to each other and to living forms, seem to me
explained in a satisfactory manner. And they are wholly inexplicable on any
other view. On this same theory, it is evident that the fauna of any great
period in the earth's history will be intermediate in general character between
that which preceded and that which succeeded it. Thus, the species which lived
at the sixth great stage of descent in the diagram are the modified offspring of
those which lived at the fifth stage, and are the parents of those which became
still more modified at the seventh stage; hence they could hardly fail to be
nearly intermediate in character between the forms of life above and below. We
must, however, allow for the entire extinction of some preceding forms, and for
the coming in of quite new forms by immigration, and for a large amount of
modification, during the long and blank intervals between the successive
formations. Subject to these allowances, the fauna of each geological period
undoubtedly is intermediate in character, between the preceding and succeeding
faunas. I need give only one instance, namely, the manner in which the fossils
of the Devonian system, when this system was first discovered, were at once
recognised by palaeontologists as intermediate in character between those of the
overlying carboniferous, and underlying Silurian system. But each fauna is not
necessarily exactly intermediate, as unequal intervals of time have elapsed
between consecutive formations. It is no real objection to the truth of the
statement, that the fauna of each period as a whole is nearly intermediate in
character between the preceding and succeeding faunas, that certain genera offer
exceptions to the rule. For instance, mastodons and elephants, when arranged by
Dr. Falconer in two series, first according to their mutual affinities and then
according to their periods of existence, do not accord in arrangement. The
species extreme in character are not the oldest, or the most recent; nor are
those which are intermediate in character, intermediate in age. But supposing
for an instant, in this and other such cases, that the record of the first
appearance and disappearance of the species was perfect, we have no reason to
believe that forms successively produced necessarily endure for corresponding
lengths of time: a very ancient form might occasionally last much longer than a
form elsewhere subsequently produced, especially in the case of terrestrial
productions inhabiting separated districts. To compare small things with great:
if the principal living and extinct races of the domestic pigeon were arranged
as well as they could be in serial affinity, this arrangement would not closely
accord with the order in time of their production, and still less with the order
of their disappearance; for the parent rock-pigeon now lives; and many varieties
between the rock-pigeon and the carrier have become extinct; and carriers which
are extreme in the important character of length of beak originated earlier than
short-beaked tumblers, which are at the opposite end of the series in this same
respect. Closely connected with the statement, that the organic remains from an
intermediate formation are in some degree intermediate in character, is the
fact, insisted on by all palaeontologists, that fossils from two consecutive
formations are far more closely related to each other, than are the fossils from
two remote formations. Pictet gives as a well-known instance, the general
resemblance of the organic remains from the several stages of the chalk
formation, though the species are distinct in each stage. This fact alone, from
its generality, seems to have shaken Professor Pictet in his firm belief in the
immutability of species. He who is acquainted with the distribution of existing
species over the globe, will not attempt to account for the close resemblance of
the distinct species in closely consecutive formations, by the physical
conditions of the ancient areas having remained nearly the same. Let it be
remembered that the forms of life, at least those inhabiting the sea, have
changed almost simultaneously throughout the world, and therefore under the most
different climates and conditions. Consider the prodigious vicissitudes of
climate during the pleistocene period, which includes the whole glacial period,
and note how little the specific forms of the inhabitants of the sea have been
affected. On the theory of descent, the full meaning of the fact of fossil
remains from closely consecutive formations, though ranked as distinct species,
being closely related, is obvious. As the accumulation of each formation has
often been interrupted, and as long blank intervals have intervened between
successive formations, we ought not to expect to find, as I attempted to show in
the last chapter, in any one or two formations all the intermediate varieties
between the species which appeared at the commencement and close of these
periods; but we ought to find after intervals, very long as measured by years,
but only moderately long as measured geologically, closely allied forms, or, as
they have been called by some authors, representative species; and these we
assuredly do find. We find, in short, such evidence of the slow and scarcely
sensible mutation of specific forms, as we have a just right to expect to find.
ON THE STATE OF DEVELOPMENT OF ANCIENT FORMS. There has been much discussion
whether recent forms are more highly developed than ancient. I will not here
enter on this subject, for naturalists have not as yet defined to each other's
satisfaction what is meant by high and low forms. But in one particular sense
the more recent forms must, on my theory, be higher than the more ancient; for
each new species is formed by having had some advantage in the struggle for life
over other and preceding forms. If under a nearly similar climate, the eocene
inhabitants of one quarter of the world were put into competition with the
existing inhabitants of the same or some other quarter, the eocene fauna or
flora would certainly be beaten and exterminated; as would a secondary fauna by
an eocene, and a palaeozoic fauna by a secondary fauna. I do not doubt that this
process of improvement has affected in a marked and sensible manner the
organisation of the more recent and victorious forms of life, in comparison with
the ancient and beaten forms; but I can see no way of testing this sort of
progress. Crustaceans, for instance, not the highest in their own class, may
have beaten the highest molluscs. From the extraordinary manner in which
European productions have recently spread over New Zealand, and have seized on
places which must have been previously occupied, we may believe, if all the
animals and plants of Great Britain were set free in New Zealand, that in the
course of time a multitude of British forms would become thoroughly naturalized
there, and would exterminate many of the natives. On the other hand, from what
we see now occurring in New Zealand, and from hardly a single inhabitant of the
southern hemisphere having become wild in any part of Europe, we may doubt, if
all the productions of New Zealand were set free in Great Britain, whether any
considerable number would be enabled to seize on places now occupied by our
native plants and animals. Under this point of view, the productions of Great
Britain may be said to be higher than those of New Zealand. Yet the most skilful
naturalist from an examination of the species of the two countries could not
have foreseen this result. Agassiz insists that ancient animals resemble to a
certain extent the embryos of recent animals of the same classes; or that the
geological succession of extinct forms is in some degree parallel to the
embryological development of recent forms. I must follow Pictet and Huxley in
thinking that the truth of this doctrine is very far from proved. Yet I fully
expect to see it hereafter confirmed, at least in regard to subordinate groups,
which have branched off from each other within comparatively recent times. For
this doctrine of Agassiz accords well with the theory of natural selection. In a
future chapter I shall attempt to show that the adult differs from its embryo,
owing to variations supervening at a not early age, and being inherited at a
corresponding age. This process, whilst it leaves the embryo almost unaltered,
continually adds, in the course of successive generations, more and more
difference to the adult. Thus the embryo comes to be left as a sort of picture,
preserved by nature, of the ancient and less modified condition of each animal.
This view may be true, and yet it may never be capable of full proof. Seeing,
for instance, that the oldest known mammals, reptiles, and fish strictly belong
to their own proper classes, though some of these old forms are in a slight
degree less distinct from each other than are the typical members of the same
groups at the present day, it would be vain to look for animals having the
common embryological character of the Vertebrata, until beds far beneath the
lowest Silurian strata are discovered--a discovery of which the chance is very
small. ON THE SUCCESSION OF THE SAME TYPES WITHIN THE SAME AREAS, DURING THE
LATER TERTIARY PERIODS. Mr. Clift many years ago showed that the fossil mammals
from the Australian caves were closely allied to the living marsupials of that
continent. In South America, a similar relationship is manifest, even to an
uneducated eye, in the gigantic pieces of armour like those of the armadillo,
found in several parts of La Plata; and Professor Owen has shown in the most
striking manner that most of the fossil mammals, buried there in such numbers,
are related to South American types. This relationship is even more clearly seen
in the wonderful collection of fossil bones made by MM. Lund and Clausen in the
caves of Brazil. I was so much impressed with these facts that I strongly
insisted, in 1839 and 1845, on this "law of the succession of types,"--on "this
wonderful relationship in the same continent between the dead and the living."
Professor Owen has subsequently extended the same generalisation to the mammals
of the Old World. We see the same law in this author's restorations of the
extinct and gigantic birds of New Zealand. We see it also in the birds of the
caves of Brazil. Mr. Woodward has shown that the same law holds good with
sea-shells, but from the wide distribution of most genera of molluscs, it is not
well displayed by them. Other cases could be added, as the relation between the
extinct and living land-shells of Madeira; and between the extinct and living
brackish-water shells of the Aralo-Caspian Sea. Now what does this remarkable
law of the succession of the same types within the same areas mean? He would be
a bold man, who after comparing the present climate of Australia and of parts of
South America under the same latitude, would attempt to account, on the one
hand, by dissimilar physical conditions for the dissimilarity of the inhabitants
of these two continents, and, on the other hand, by similarity of conditions,
for the uniformity of the same types in each during the later tertiary periods.
Nor can it be pretended that it is an immutable law that marsupials should have
been chiefly or solely produced in Australia; or that Edentata and other
American types should have been solely produced in South America. For we know
that Europe in ancient times was peopled by numerous marsupials; and I have
shown in the publications above alluded to, that in America the law of
distribution of terrestrial mammals was formerly different from what it now is.
North America formerly partook strongly of the present character of the southern
half of the continent; and the southern half was formerly more closely allied,
than it is at present, to the northern half. In a similar manner we know from
Falconer and Cautley's discoveries, that northern India was formerly more
closely related in its mammals to Africa than it is at the present time.
Analogous facts could be given in relation to the distribution of marine
animals. On the theory of descent with modification, the great law of the long
enduring, but not immutable, succession of the same types within the same areas,
is at once explained; for the inhabitants of each quarter of the world will
obviously tend to leave in that quarter, during the next succeeding period of
time, closely allied though in some degree modified descendants. If the
inhabitants of one continent formerly differed greatly from those of another
continent, so will their modified descendants still differ in nearly the same
manner and degree. But after very long intervals of time and after great
geographical changes, permitting much inter-migration, the feebler will yield to
the more dominant forms, and there will be nothing immutable in the laws of past
and present distribution. It may be asked in ridicule, whether I suppose that
the megatherium and other allied huge monsters have left behind them in South
America the sloth, armadillo, and anteater, as their degenerate descendants.
This cannot for an instant be admitted. These huge animals have become wholly
extinct, and have left no progeny. But in the caves of Brazil, there are many
extinct species which are closely allied in size and in other characters to the
species still living in South America; and some of these fossils may be the
actual progenitors of living species. It must not be forgotten that, on my
theory, all the species of the same genus have descended from some one species;
so that if six genera, each having eight species, be found in one geological
formation, and in the next succeeding formation there be six other allied or
representative genera with the same number of species, then we may conclude that
only one species of each of the six older genera has left modified descendants,
constituting the six new genera. The other seven species of the old genera have
all died out and have left no progeny. Or, which would probably be a far
commoner case, two or three species of two or three alone of the six older
genera will have been the parents of the six new genera; the other old species
and the other whole genera having become utterly extinct. In failing orders,
with the genera and species decreasing in numbers, as apparently is the case of
the Edentata of South America, still fewer genera and species will have left
modified blood-descendants. SUMMARY OF THE PRECEDING AND PRESENT CHAPTERS. I
have attempted to show that the geological record is extremely imperfect; that
only a small portion of the globe has been geologically explored with care; that
only certain classes of organic beings have been largely preserved in a fossil
state; that the number both of specimens and of species, preserved in our
museums, is absolutely as nothing compared with the incalculable number of
generations which must have passed away even during a single formation; that,
owing to subsidence being necessary for the accumulation of fossiliferous
deposits thick enough to resist future degradation, enormous intervals of time
have elapsed between the successive formations; that there has probably been
more extinction during the periods of subsidence, and more variation during the
periods of elevation, and during the latter the record will have been least
perfectly kept; that each single formation has not been continuously deposited;
that the duration of each formation is, perhaps, short compared with the average
duration of specific forms; that migration has played an important part in the
first appearance of new forms in any one area and formation; that widely ranging
species are those which have varied most, and have oftenest given rise to new
species; and that varieties have at first often been local. All these causes
taken conjointly, must have tended to make the geological record extremely
imperfect, and will to a large extent explain why we do not find interminable
varieties, connecting together all the extinct and existing forms of life by the
finest graduated steps. He who rejects these views on the nature of the
geological record, will rightly reject my whole theory. For he may ask in vain
where are the numberless transitional links which must formerly have connected
the closely allied or representative species, found in the several stages of the
same great formation. He may disbelieve in the enormous intervals of time which
have elapsed between our consecutive formations; he may overlook how important a
part migration must have played, when the formations of any one great region
alone, as that of Europe, are considered; he may urge the apparent, but often
falsely apparent, sudden coming in of whole groups of species. He may ask where
are the remains of those infinitely numerous organisms which must have existed
long before the first bed of the Silurian system was deposited: I can answer
this latter question only hypothetically, by saying that as far as we can see,
where our oceans now extend they have for an enormous period extended, and where
our oscillating continents now stand they have stood ever since the Silurian
epoch; but that long before that period, the world may have presented a wholly
different aspect; and that the older continents, formed of formations older than
any known to us, may now all be in a metamorphosed condition, or may lie buried
under the ocean. Passing from these difficulties, all the other great leading
facts in palaeontology seem to me simply to follow on the theory of descent with
modification through natural selection. We can thus understand how it is that
new species come in slowly and successively; how species of different classes do
not necessarily change together, or at the same rate, or in the same degree; yet
in the long run that all undergo modification to some extent. The extinction of
old forms is the almost inevitable consequence of the production of new forms.
We can understand why when a species has once disappeared it never reappears.
Groups of species increase in numbers slowly, and endure for unequal periods of
time; for the process of modification is necessarily slow, and depends on many
complex contingencies. The dominant species of the larger dominant groups tend
to leave many modified descendants, and thus new sub-groups and groups are
formed. As these are formed, the species of the less vigorous groups, from their
inferiority inherited from a common progenitor, tend to become extinct together,
and to leave no modified offspring on the face of the earth. But the utter
extinction of a whole group of species may often be a very slow process, from
the survival of a few descendants, lingering in protected and isolated
situations. When a group has once wholly disappeared, it does not reappear; for
the link of generation has been broken. We can understand how the spreading of
the dominant forms of life, which are those that oftenest vary, will in the long
run tend to people the world with allied, but modified, descendants; and these
will generally succeed in taking the places of those groups of species which are
their inferiors in the struggle for existence. Hence, after long intervals of
time, the productions of the world will appear to have changed simultaneously.
We can understand how it is that all the forms of life, ancient and recent, make
together one grand system; for all are connected by generation. We can
understand, from the continued tendency to divergence of character, why the more
ancient a form is, the more it generally differs from those now living. Why
ancient and extinct forms often tend to fill up gaps between existing forms,
sometimes blending two groups previously classed as distinct into one; but more
commonly only bringing them a little closer together. The more ancient a form
is, the more often, apparently, it displays characters in some degree
intermediate between groups now distinct; for the more ancient a form is, the
more nearly it will be related to, and consequently resemble, the common
progenitor of groups, since become widely divergent. Extinct forms are seldom
directly intermediate between existing forms; but are intermediate only by a
long and circuitous course through many extinct and very different forms. We can
clearly see why the organic remains of closely consecutive formations are more
closely allied to each other, than are those of remote formations; for the forms
are more closely linked together by generation: we can clearly see why the
remains of an intermediate formation are intermediate in character. The
inhabitants of each successive period in the world's history have beaten their
predecessors in the race for life, and are, in so far, higher in the scale of
nature; and this may account for that vague yet ill-defined sentiment, felt by
many palaeontologists, that organisation on the whole has progressed. If it
should hereafter be proved that ancient animals resemble to a certain extent the
embryos of more recent animals of the same class, the fact will be intelligible.
The succession of the same types of structure within the same areas during the
later geological periods ceases to be mysterious, and is simply explained by
inheritance. If then the geological record be as imperfect as I believe it to
be, and it may at least be asserted that the record cannot be proved to be much
more perfect, the main objections to the theory of natural selection are greatly
diminished or disappear. On the other hand, all the chief laws of palaeontology
plainly proclaim, as it seems to me, that species have been produced by ordinary
generation: old forms having been supplanted by new and improved forms of life,
produced by the laws of variation still acting round us, and preserved by
Natural Selection. CHAPTER 11. GEOGRAPHICAL DISTRIBUTION. Present distribution
cannot be accounted for by differences in physical conditions. Importance of
barriers. Affinity of the productions of the same continent. Centres of
creation. Means of dispersal, by changes of climate and of the level of the
land, and by occasional means. Dispersal during the Glacial period co-extensive
with the world. In considering the distribution of organic beings over the face
of the globe, the first great fact which strikes us is, that neither the
similarity nor the dissimilarity of the inhabitants of various regions can be
accounted for by their climatal and other physical conditions. Of late, almost
every author who has studied the subject has come to this conclusion. The case
of America alone would almost suffice to prove its truth: for if we exclude the
northern parts where the circumpolar land is almost continuous, all authors
agree that one of the most fundamental divisions in geographical distribution is
that between the New and Old Worlds; yet if we travel over the vast American
continent, from the central parts of the United States to its extreme southern
point, we meet with the most diversified conditions; the most humid districts,
arid deserts, lofty mountains, grassy plains, forests, marshes, lakes, and great
rivers, under almost every temperature. There is hardly a climate or condition
in the Old World which cannot be paralleled in the New--at least as closely as
the same species generally require; for it is a most rare case to find a group
of organisms confined to any small spot, having conditions peculiar in only a
slight degree; for instance, small areas in the Old World could be pointed out
hotter than any in the New World, yet these are not inhabited by a peculiar
fauna or flora. Notwithstanding this parallelism in the conditions of the Old
and New Worlds, how widely different are their living productions! In the
southern hemisphere, if we compare large tracts of land in Australia, South
Africa, and western South America, between latitudes 25 deg and 35 deg, we shall
find parts extremely similar in all their conditions, yet it would not be
possible to point out three faunas and floras more utterly dissimilar. Or again
we may compare the productions of South America south of lat. 35 deg with those
north of 25 deg, which consequently inhabit a considerably different climate,
and they will be found incomparably more closely related to each other, than
they are to the productions of Australia or Africa under nearly the same
climate. Analogous facts could be given with respect to the inhabitants of the
sea. A second great fact which strikes us in our general review is, that
barriers of any kind, or obstacles to free migration, are related in a close and
important manner to the differences between the productions of various regions.
We see this in the great difference of nearly all the terrestrial productions of
the New and Old Worlds, excepting in the northern parts, where the land almost
joins, and where, under a slightly different climate, there might have been free
migration for the northern temperate forms, as there now is for the strictly
arctic productions. We see the same fact in the great difference between the
inhabitants of Australia, Africa, and South America under the same latitude: for
these countries are almost as much isolated from each other as is possible. On
each continent, also, we see the same fact; for on the opposite sides of lofty
and continuous mountain-ranges, and of great deserts, and sometimes even of
large rivers, we find different productions; though as mountain chains, deserts,
etc., are not as impassable, or likely to have endured so long as the oceans
separating continents, the differences are very inferior in degree to those
characteristic of distinct continents. Turning to the sea, we find the same law.
No two marine faunas are more distinct, with hardly a fish, shell, or crab in
common, than those of the eastern and western shores of South and Central
America; yet these great faunas are separated only by the narrow, but
impassable, isthmus of Panama. Westward of the shores of America, a wide space
of open ocean extends, with not an island as a halting-place for emigrants; here
we have a barrier of another kind, and as soon as this is passed we meet in the
eastern islands of the Pacific, with another and totally distinct fauna. So that
here three marine faunas range far northward and southward, in parallel lines
not far from each other, under corresponding climates; but from being separated
from each other by impassable barriers, either of land or open sea, they are
wholly distinct. On the other hand, proceeding still further westward from the
eastern islands of the tropical parts of the Pacific, we encounter no impassable
barriers, and we have innumerable islands as halting-places, until after
travelling over a hemisphere we come to the shores of Africa; and over this vast
space we meet with no well-defined and distinct marine faunas. Although hardly
one shell, crab or fish is common to the above-named three approximate faunas of
Eastern and Western America and the eastern Pacific islands, yet many fish range
from the Pacific into the Indian Ocean, and many shells are common to the
eastern islands of the Pacific and the eastern shores of Africa, on almost
exactly opposite meridians of longitude. A third great fact, partly included in
the foregoing statements, is the affinity of the productions of the same
continent or sea, though the species themselves are distinct at different points
and stations. It is a law of the widest generality, and every continent offers
innumerable instances. Nevertheless the naturalist in travelling, for instance,
from north to south never fails to be struck by the manner in which successive
groups of beings, specifically distinct, yet clearly related, replace each
other. He hears from closely allied, yet distinct kinds of birds, notes nearly
similar, and sees their nests similarly constructed, but not quite alike, with
eggs coloured in nearly the same manner. The plains near the Straits of Magellan
are inhabited by one species of Rhea (American ostrich), and northward the
plains of La Plata by another species of the same genus; and not by a true
ostrich or emeu, like those found in Africa and Australia under the same
latitude. On these same plains of La Plata, we see the agouti and bizcacha,
animals having nearly the same habits as our hares and rabbits and belonging to
the same order of Rodents, but they plainly display an American type of
structure. We ascend the lofty peaks of the Cordillera and we find an alpine
species of bizcacha; we look to the waters, and we do not find the beaver or
musk-rat, but the coypu and capybara, rodents of the American type. Innumerable
other instances could be given. If we look to the islands off the American
shore, however much they may differ in geological structure, the inhabitants,
though they may be all peculiar species, are essentially American. We may look
back to past ages, as shown in the last chapter, and we find American types then
prevalent on the American continent and in the American seas. We see in these
facts some deep organic bond, prevailing throughout space and time, over the
same areas of land and water, and independent of their physical conditions. The
naturalist must feel little curiosity, who is not led to inquire what this bond
is. This bond, on my theory, is simply inheritance, that cause which alone, as
far as we positively know, produces organisms quite like, or, as we see in the
case of varieties nearly like each other. The dissimilarity of the inhabitants
of different regions may be attributed to modification through natural
selection, and in a quite subordinate degree to the direct influence of
different physical conditions. The degree of dissimilarity will depend on the
migration of the more dominant forms of life from one region into another having
been effected with more or less ease, at periods more or less remote;--on the
nature and number of the former immigrants;--and on their action and reaction,
in their mutual struggles for life;--the relation of organism to organism being,
as I have already often remarked, the most important of all relations. Thus the
high importance of barriers comes into play by checking migration; as does time
for the slow process of modification through natural selection. Widely-ranging
species, abounding in individuals, which have already triumphed over many
competitors in their own widely-extended homes will have the best chance of
seizing on new places, when they spread into new countries. In their new homes
they will be exposed to new conditions, and will frequently undergo further
modification and improvement; and thus they will become still further
victorious, and will produce groups of modified descendants. On this principle
of inheritance with modification, we can understand how it is that sections of
genera, whole genera, and even families are confined to the same areas, as is so
commonly and notoriously the case. I believe, as was remarked in the last
chapter, in no law of necessary development. As the variability of each species
is an independent property, and will be taken advantage of by natural selection,
only so far as it profits the individual in its complex struggle for life, so
the degree of modification in different species will be no uniform quantity. If,
for instance, a number of species, which stand in direct competition with each
other, migrate in a body into a new and afterwards isolated country, they will
be little liable to modification; for neither migration nor isolation in
themselves can do anything. These principles come into play only by bringing
organisms into new relations with each other, and in a lesser degree with the
surrounding physical conditions. As we have seen in the last chapter that some
forms have retained nearly the same character from an enormously remote
geological period, so certain species have migrated over vast spaces, and have
not become greatly modified. On these views, it is obvious, that the several
species of the same genus, though inhabiting the most distant quarters of the
world, must originally have proceeded from the same source, as they have
descended from the same progenitor. In the case of those species, which have
undergone during whole geological periods but little modification, there is not
much difficulty in believing that they may have migrated from the same region;
for during the vast geographical and climatal changes which will have supervened
since ancient times, almost any amount of migration is possible. But in many
other cases, in which we have reason to believe that the species of a genus have
been produced within comparatively recent times, there is great difficulty on
this head. It is also obvious that the individuals of the same species, though
now inhabiting distant and isolated regions, must have proceeded from one spot,
where their parents were first produced: for, as explained in the last chapter,
it is incredible that individuals identically the same should ever have been
produced through natural selection from parents specifically distinct. We are
thus brought to the question which has been largely discussed by naturalists,
namely, whether species have been created at one or more points of the earth's
surface. Undoubtedly there are very many cases of extreme difficulty, in
understanding how the same species could possibly have migrated from some one
point to the several distant and isolated points, where now found. Nevertheless
the simplicity of the view that each species was first produced within a single
region captivates the mind. He who rejects it, rejects the vera causa of
ordinary generation with subsequent migration, and calls in the agency of a
miracle. It is universally admitted, that in most cases the area inhabited by a
species is continuous; and when a plant or animal inhabits two points so distant
from each other, or with an interval of such a nature, that the space could not
be easily passed over by migration, the fact is given as something remarkable
and exceptional. The capacity of migrating across the sea is more distinctly
limited in terrestrial mammals, than perhaps in any other organic beings; and,
accordingly, we find no inexplicable cases of the same mammal inhabiting distant
points of the world. No geologist will feel any difficulty in such cases as
Great Britain having been formerly united to Europe, and consequently possessing
the same quadrupeds. But if the same species can be produced at two separate
points, why do we not find a single mammal common to Europe and Australia or
South America? The conditions of life are nearly the same, so that a multitude
of European animals and plants have become naturalised in America and Australia;
and some of the aboriginal plants are identically the same at these distant
points of the northern and southern hemispheres? The answer, as I believe, is,
that mammals have not been able to migrate, whereas some plants, from their
varied means of dispersal, have migrated across the vast and broken interspace.
The great and striking influence which barriers of every kind have had on
distribution, is intelligible only on the view that the great majority of
species have been produced on one side alone, and have not been able to migrate
to the other side. Some few families, many sub-families, very many genera, and a
still greater number of sections of genera are confined to a single region; and
it has been observed by several naturalists, that the most natural genera, or
those genera in which the species are most closely related to each other, are
generally local, or confined to one area. What a strange anomaly it would be,
if, when coming one step lower in the series, to the individuals of the same
species, a directly opposite rule prevailed; and species were not local, but had
been produced in two or more distinct areas! Hence it seems to me, as it has to
many other naturalists, that the view of each species having been produced in
one area alone, and having subsequently migrated from that area as far as its
powers of migration and subsistence under past and present conditions permitted,
is the most probable. Undoubtedly many cases occur, in which we cannot explain
how the same species could have passed from one point to the other. But the
geographical and climatal changes, which have certainly occurred within recent
geological times, must have interrupted or rendered discontinuous the formerly
continuous range of many species. So that we are reduced to consider whether the
exceptions to continuity of range are so numerous and of so grave a nature, that
we ought to give up the belief, rendered probable by general considerations,
that each species has been produced within one area, and has migrated thence as
far as it could. It would be hopelessly tedious to discuss all the exceptional
cases of the same species, now living at distant and separated points; nor do I
for a moment pretend that any explanation could be offered of many such cases.
But after some preliminary remarks, I will discuss a few of the most striking
classes of facts; namely, the existence of the same species on the summits of
distant mountain-ranges, and at distant points in the arctic and antarctic
regions; and secondly (in the following chapter), the wide distribution of
freshwater productions; and thirdly, the occurrence of the same terrestrial
species on islands and on the mainland, though separated by hundreds of miles of
open sea. If the existence of the same species at distant and isolated points of
the earth's surface, can in many instances be explained on the view of each
species having migrated from a single birthplace; then, considering our
ignorance with respect to former climatal and geographical changes and various
occasional means of transport, the belief that this has been the universal law,
seems to me incomparably the safest. In discussing this subject, we shall be
enabled at the same time to consider a point equally important for us, namely,
whether the several distinct species of a genus, which on my theory have all
descended from a common progenitor, can have migrated (undergoing modification
during some part of their migration) from the area inhabited by their
progenitor. If it can be shown to be almost invariably the case, that a region,
of which most of its inhabitants are closely related to, or belong to the same
genera with the species of a second region, has probably received at some former
period immigrants from this other region, my theory will be strengthened; for we
can clearly understand, on the principle of modification, why the inhabitants of
a region should be related to those of another region, whence it has been
stocked. A volcanic island, for instance, upheaved and formed at the distance of
a few hundreds of miles from a continent, would probably receive from it in the
course of time a few colonists, and their descendants, though modified, would
still be plainly related by inheritance to the inhabitants of the continent.
Cases of this nature are common, and are, as we shall hereafter more fully see,
inexplicable on the theory of independent creation. This view of the relation of
species in one region to those in another, does not differ much (by substituting
the word variety for species) from that lately advanced in an ingenious paper by
Mr. Wallace, in which he concludes, that "every species has come into existence
coincident both in space and time with a pre-existing closely allied species."
And I now know from correspondence, that this coincidence he attributes to
generation with modification. The previous remarks on "single and multiple
centres of creation" do not directly bear on another allied question,--namely
whether all the individuals of the same species have descended from a single
pair, or single hermaphrodite, or whether, as some authors suppose, from many
individuals simultaneously created. With those organic beings which never
intercross (if such exist), the species, on my theory, must have descended from
a succession of improved varieties, which will never have blended with other
individuals or varieties, but will have supplanted each other; so that, at each
successive stage of modification and improvement, all the individuals of each
variety will have descended from a single parent. But in the majority of cases,
namely, with all organisms which habitually unite for each birth, or which often
intercross, I believe that during the slow process of modification the
individuals of the species will have been kept nearly uniform by intercrossing;
so that many individuals will have gone on simultaneously changing, and the
whole amount of modification will not have been due, at each stage, to descent
from a single parent. To illustrate what I mean: our English racehorses differ
slightly from the horses of every other breed; but they do not owe their
difference and superiority to descent from any single pair, but to continued
care in selecting and training many individuals during many generations. Before
discussing the three classes of facts, which I have selected as presenting the
greatest amount of difficulty on the theory of "single centres of creation," I
must say a few words on the means of dispersal. MEANS OF DISPERSAL. Sir C. Lyell
and other authors have ably treated this subject. I can give here only the
briefest abstract of the more important facts. Change of climate must have had a
powerful influence on migration: a region when its climate was different may
have been a high road for migration, but now be impassable; I shall, however,
presently have to discuss this branch of the subject in some detail. Changes of
level in the land must also have been highly influential: a narrow isthmus now
separates two marine faunas; submerge it, or let it formerly have been
submerged, and the two faunas will now blend or may formerly have blended: where
the sea now extends, land may at a former period have connected islands or
possibly even continents together, and thus have allowed terrestrial productions
to pass from one to the other. No geologist will dispute that great mutations of
level have occurred within the period of existing organisms. Edward Forbes
insisted that all the islands in the Atlantic must recently have been connected
with Europe or Africa, and Europe likewise with America. Other authors have thus
hypothetically bridged over every ocean, and have united almost every island to
some mainland. If indeed the arguments used by Forbes are to be trusted, it must
be admitted that scarcely a single island exists which has not recently been
united to some continent. This view cuts the Gordian knot of the dispersal of
the same species to the most distant points, and removes many a difficulty: but
to the best of my judgment we are not authorized in admitting such enormous
geographical changes within the period of existing species. It seems to me that
we have abundant evidence of great oscillations of level in our continents; but
not of such vast changes in their position and extension, as to have united them
within the recent period to each other and to the several intervening oceanic
islands. I freely admit the former existence of many islands, now buried beneath
the sea, which may have served as halting places for plants and for many animals
during their migration. In the coral-producing oceans such sunken islands are
now marked, as I believe, by rings of coral or atolls standing over them.
Whenever it is fully admitted, as I believe it will some day be, that each
species has proceeded from a single birthplace, and when in the course of time
we know something definite about the means of distribution, we shall be enabled
to speculate with security on the former extension of the land. But I do not
believe that it will ever be proved that within the recent period continents
which are now quite separate, have been continuously, or almost continuously,
united with each other, and with the many existing oceanic islands. Several
facts in distribution,--such as the great difference in the marine faunas on the
opposite sides of almost every continent,--the close relation of the tertiary
inhabitants of several lands and even seas to their present inhabitants,--a
certain degree of relation (as we shall hereafter see) between the distribution
of mammals and the depth of the sea,--these and other such facts seem to me
opposed to the admission of such prodigious geographical revolutions within the
recent period, as are necessitated on the view advanced by Forbes and admitted
by his many followers. The nature and relative proportions of the inhabitants of
oceanic islands likewise seem to me opposed to the belief of their former
continuity with continents. Nor does their almost universally volcanic
composition favour the admission that they are the wrecks of sunken
continents;--if they had originally existed as mountain-ranges on the land, some
at least of the islands would have been formed, like other mountain-summits, of
granite, metamorphic schists, old fossiliferous or other such rocks, instead of
consisting of mere piles of volcanic matter. I must now say a few words on what
are called accidental means, but which more properly might be called occasional
means of distribution. I shall here confine myself to plants. In botanical
works, this or that plant is stated to be ill adapted for wide dissemination;
but for transport across the sea, the greater or less facilities may be said to
be almost wholly unknown. Until I tried, with Mr. Berkeley's aid, a few
experiments, it was not even known how far seeds could resist the injurious
action of sea-water. To my surprise I found that out of 87 kinds, 64 germinated
after an immersion of 28 days, and a few survived an immersion of 137 days. For
convenience sake I chiefly tried small seeds, without the capsule or fruit; and
as all of these sank in a few days, they could not be floated across wide spaces
of the sea, whether or not they were injured by the salt-water. Afterwards I
tried some larger fruits, capsules, etc., and some of these floated for a long
time. It is well known what a difference there is in the buoyancy of green and
seasoned timber; and it occurred to me that floods might wash down plants or
branches, and that these might be dried on the banks, and then by a fresh rise
in the stream be washed into the sea. Hence I was led to dry stems and branches
of 94 plants with ripe fruit, and to place them on sea water. The majority sank
quickly, but some which whilst green floated for a very short time, when dried
floated much longer; for instance, ripe hazel-nuts sank immediately, but when
dried, they floated for 90 days and afterwards when planted they germinated; an
asparagus plant with ripe berries floated for 23 days, when dried it floated for
85 days, and the seeds afterwards germinated: the ripe seeds of Helosciadium
sank in two days, when dried they floated for above 90 days, and afterwards
germinated. Altogether out of the 94 dried plants, 18 floated for above 28 days,
and some of the 18 floated for a very much longer period. So that as 64/87 seeds
germinated after an immersion of 28 days; and as 18/94 plants with ripe fruit
(but not all the same species as in the foregoing experiment) floated, after
being dried, for above 28 days, as far as we may infer anything from these
scanty facts, we may conclude that the seeds of 14/100 plants of any country
might be floated by sea-currents during 28 days, and would retain their power of
germination. In Johnston's Physical Atlas, the average rate of the several
Atlantic currents is 33 miles per diem (some currents running at the rate of 60
miles per diem); on this average, the seeds of 14/100 plants belonging to one
country might be floated across 924 miles of sea to another country; and when
stranded, if blown to a favourable spot by an inland gale, they would germinate.
Subsequently to my experiments, M. Martens tried similar ones, but in a much
better manner, for he placed the seeds in a box in the actual sea, so that they
were alternately wet and exposed to the air like really floating plants. He
tried 98 seeds, mostly different from mine; but he chose many large fruits and
likewise seeds from plants which live near the sea; and this would have favoured
the average length of their flotation and of their resistance to the injurious
action of the salt-water. On the other hand he did not previously dry the plants
or branches with the fruit; and this, as we have seen, would have caused some of
them to have floated much longer. The result was that 18/98 of his seeds floated
for 42 days, and were then capable of germination. But I do not doubt that
plants exposed to the waves would float for a less time than those protected
from violent movement as in our experiments. Therefore it would perhaps be safer
to assume that the seeds of about 10/100 plants of a flora, after having been
dried, could be floated across a space of sea 900 miles in width, and would then
germinate. The fact of the larger fruits often floating longer than the small,
is interesting; as plants with large seeds or fruit could hardly be transported
by any other means; and Alph. de Candolle has shown that such plants generally
have restricted ranges. But seeds may be occasionally transported in another
manner. Drift timber is thrown up on most islands, even on those in the midst of
the widest oceans; and the natives of the coral-islands in the Pacific, procure
stones for their tools, solely from the roots of drifted trees, these stones
being a valuable royal tax. I find on examination, that when irregularly shaped
stones are embedded in the roots of trees, small parcels of earth are very
frequently enclosed in their interstices and behind them,--so perfectly that not
a particle could be washed away in the longest transport: out of one small
portion of earth thus COMPLETELY enclosed by wood in an oak about 50 years old,
three dicotyledonous plants germinated: I am certain of the accuracy of this
observation. Again, I can show that the carcasses of birds, when floating on the
sea, sometimes escape being immediately devoured; and seeds of many kinds in the
crops of floating birds long retain their vitality: peas and vetches, for
instance, are killed by even a few days' immersion in sea-water; but some taken
out of the crop of a pigeon, which had floated on artificial salt-water for 30
days, to my surprise nearly all germinated. Living birds can hardly fail to be
highly effective agents in the transportation of seeds. I could give many facts
showing how frequently birds of many kinds are blown by gales to vast distances
across the ocean. We may I think safely assume that under such circumstances
their rate of flight would often be 35 miles an hour; and some authors have
given a far higher estimate. I have never seen an instance of nutritious seeds
passing through the intestines of a bird; but hard seeds of fruit will pass
uninjured through even the digestive organs of a turkey. In the course of two
months, I picked up in my garden 12 kinds of seeds, out of the excrement of
small birds, and these seemed perfect, and some of them, which I tried,
germinated. But the following fact is more important: the crops of birds do not
secrete gastric juice, and do not in the least injure, as I know by trial, the
germination of seeds; now after a bird has found and devoured a large supply of
food, it is positively asserted that all the grains do not pass into the gizzard
for 12 or even 18 hours. A bird in this interval might easily be blown to the
distance of 500 miles, and hawks are known to look out for tired birds, and the
contents of their torn crops might thus readily get scattered. Mr. Brent informs
me that a friend of his had to give up flying carrier-pigeons from France to
England, as the hawks on the English coast destroyed so many on their arrival.
Some hawks and owls bolt their prey whole, and after an interval of from twelve
to twenty hours, disgorge pellets, which, as I know from experiments made in the
Zoological Gardens, include seeds capable of germination. Some seeds of the oat,
wheat, millet, canary, hemp, clover, and beet germinated after having been from
twelve to twenty-one hours in the stomachs of different birds of prey; and two
seeds of beet grew after having been thus retained for two days and fourteen
hours. Freshwater fish, I find, eat seeds of many land and water plants: fish
are frequently devoured by birds, and thus the seeds might be transported from
place to place. I forced many kinds of seeds into the stomachs of dead fish, and
then gave their bodies to fishing-eagles, storks, and pelicans; these birds
after an interval of many hours, either rejected the seeds in pellets or passed
them in their excrement; and several of these seeds retained their power of
germination. Certain seeds, however, were always killed by this process.
Although the beaks and feet of birds are generally quite clean, I can show that
earth sometimes adheres to them: in one instance I removed twenty-two grains of
dry argillaceous earth from one foot of a partridge, and in this earth there was
a pebble quite as large as the seed of a vetch. Thus seeds might occasionally be
transported to great distances; for many facts could be given showing that soil
almost everywhere is charged with seeds. Reflect for a moment on the millions of
quails which annually cross the Mediterranean; and can we doubt that the earth
adhering to their feet would sometimes include a few minute seeds? But I shall
presently have to recur to this subject. As icebergs are known to be sometimes
loaded with earth and stones, and have even carried brushwood, bones, and the
nest of a land-bird, I can hardly doubt that they must occasionally have
transported seeds from one part to another of the arctic and antarctic regions,
as suggested by Lyell; and during the Glacial period from one part of the now
temperate regions to another. In the Azores, from the large number of the
species of plants common to Europe, in comparison with the plants of other
oceanic islands nearer to the mainland, and (as remarked by Mr. H. C. Watson)
from the somewhat northern character of the flora in comparison with the
latitude, I suspected that these islands had been partly stocked by ice-borne
seeds, during the Glacial epoch. At my request Sir C. Lyell wrote to M. Hartung
to inquire whether he had observed erratic boulders on these islands, and he
answered that he had found large fragments of granite and other rocks, which do
not occur in the archipelago. Hence we may safely infer that icebergs formerly
landed their rocky burthens on the shores of these mid-ocean islands, and it is
at least possible that they may have brought thither the seeds of northern
plants. Considering that the several above means of transport, and that several
other means, which without doubt remain to be discovered, have been in action
year after year, for centuries and tens of thousands of years, it would I think
be a marvellous fact if many plants had not thus become widely transported.
These means of transport are sometimes called accidental, but this is not
strictly correct: the currents of the sea are not accidental, nor is the
direction of prevalent gales of wind. It should be observed that scarcely any
means of transport would carry seeds for very great distances; for seeds do not
retain their vitality when exposed for a great length of time to the action of
seawater; nor could they be long carried in the crops or intestines of birds.
These means, however, would suffice for occasional transport across tracts of
sea some hundred miles in breadth, or from island to island, or from a continent
to a neighbouring island, but not from one distant continent to another. The
floras of distant continents would not by such means become mingled in any great
degree; but would remain as distinct as we now see them to be. The currents,
from their course, would never bring seeds from North America to Britain, though
they might and do bring seeds from the West Indies to our western shores, where,
if not killed by so long an immersion in salt-water, they could not endure our
climate. Almost every year, one or two land-birds are blown across the whole
Atlantic Ocean, from North America to the western shores of Ireland and England;
but seeds could be transported by these wanderers only by one means, namely, in
dirt sticking to their feet, which is in itself a rare accident. Even in this
case, how small would the chance be of a seed falling on favourable soil, and
coming to maturity! But it would be a great error to argue that because a
well-stocked island, like Great Britain, has not, as far as is known (and it
would be very difficult to prove this), received within the last few centuries,
through occasional means of transport, immigrants from Europe or any other
continent, that a poorly-stocked island, though standing more remote from the
mainland, would not receive colonists by similar means. I do not doubt that out
of twenty seeds or animals transported to an island, even if far less
well-stocked than Britain, scarcely more than one would be so well fitted to its
new home, as to become naturalised. But this, as it seems to me, is no valid
argument against what would be effected by occasional means of transport, during
the long lapse of geological time, whilst an island was being upheaved and
formed, and before it had become fully stocked with inhabitants. On almost bare
land, with few or no destructive insects or birds living there, nearly every
seed, which chanced to arrive, would be sure to germinate and survive. DISPERSAL
DURING THE GLACIAL PERIOD. The identity of many plants and animals, on
mountain-summits, separated from each other by hundreds of miles of lowlands,
where the Alpine species could not possibly exist, is one of the most striking
cases known of the same species living at distant points, without the apparent
possibility of their having migrated from one to the other. It is indeed a
remarkable fact to see so many of the same plants living on the snowy regions of
the Alps or Pyrenees, and in the extreme northern parts of Europe; but it is far
more remarkable, that the plants on the White Mountains, in the United States of
America, are all the same with those of Labrador, and nearly all the same, as we
hear from Asa Gray, with those on the loftiest mountains of Europe. Even as long
ago as 1747, such facts led Gmelin to conclude that the same species must have
been independently created at several distinct points; and we might have
remained in this same belief, had not Agassiz and others called vivid attention
to the Glacial period, which, as we shall immediately see, affords a simple
explanation of these facts. We have evidence of almost every conceivable kind,
organic and inorganic, that within a very recent geological period, central
Europe and North America suffered under an Arctic climate. The ruins of a house
burnt by fire do not tell their tale more plainly, than do the mountains of
Scotland and Wales, with their scored flanks, polished surfaces, and perched
boulders, of the icy streams with which their valleys were lately filled. So
greatly has the climate of Europe changed, that in Northern Italy, gigantic
moraines, left by old glaciers, are now clothed by the vine and maize.
Throughout a large part of the United States, erratic boulders, and rocks scored
by drifted icebergs and coast-ice, plainly reveal a former cold period. The
former influence of the glacial climate on the distribution of the inhabitants
of Europe, as explained with remarkable clearness by Edward Forbes, is
substantially as follows. But we shall follow the changes more readily, by
supposing a new glacial period to come slowly on, and then pass away, as
formerly occurred. As the cold came on, and as each more southern zone became
fitted for arctic beings and ill-fitted for their former more temperate
inhabitants, the latter would be supplanted and arctic productions would take
their places. The inhabitants of the more temperate regions would at the same
time travel southward, unless they were stopped by barriers, in which case they
would perish. The mountains would become covered with snow and ice, and their
former Alpine inhabitants would descend to the plains. By the time that the cold
had reached its maximum, we should have a uniform arctic fauna and flora,
covering the central parts of Europe, as far south as the Alps and Pyrenees, and
even stretching into Spain. The now temperate regions of the United States would
likewise be covered by arctic plants and animals, and these would be nearly the
same with those of Europe; for the present circumpolar inhabitants, which we
suppose to have everywhere travelled southward, are remarkably uniform round the
world. We may suppose that the Glacial period came on a little earlier or later
in North America than in Europe, so will the southern migration there have been
a little earlier or later; but this will make no difference in the final result.
As the warmth returned, the arctic forms would retreat northward, closely
followed up in their retreat by the productions of the more temperate regions.
And as the snow melted from the bases of the mountains, the arctic forms would
seize on the cleared and thawed ground, always ascending higher and higher, as
the warmth increased, whilst their brethren were pursuing their northern
journey. Hence, when the warmth had fully returned, the same arctic species,
which had lately lived in a body together on the lowlands of the Old and New
Worlds, would be left isolated on distant mountain-summits (having been
exterminated on all lesser heights) and in the arctic regions of both
hemispheres. Thus we can understand the identity of many plants at points so
immensely remote as on the mountains of the United States and of Europe. We can
thus also understand the fact that the Alpine plants of each mountain-range are
more especially related to the arctic forms living due north or nearly due north
of them: for the migration as the cold came on, and the re-migration on the
returning warmth, will generally have been due south and north. The Alpine
plants, for example, of Scotland, as remarked by Mr. H. C. Watson, and those of
the Pyrenees, as remarked by Ramond, are more especially allied to the plants of
northern Scandinavia; those of the United States to Labrador; those of the
mountains of Siberia to the arctic regions of that country. These views,
grounded as they are on the perfectly well-ascertained occurrence of a former
Glacial period, seem to me to explain in so satisfactory a manner the present
distribution of the Alpine and Arctic productions of Europe and America, that
when in other regions we find the same species on distant mountain-summits, we
may almost conclude without other evidence, that a colder climate permitted
their former migration across the low intervening tracts, since become too warm
for their existence. If the climate, since the Glacial period, has ever been in
any degree warmer than at present (as some geologists in the United States
believe to have been the case, chiefly from the distribution of the fossil
Gnathodon), then the arctic and temperate productions will at a very late period
have marched a little further north, and subsequently have retreated to their
present homes; but I have met with no satisfactory evidence with respect to this
intercalated slightly warmer period, since the Glacial period. The arctic forms,
during their long southern migration and re-migration northward, will have been
exposed to nearly the same climate, and, as is especially to be noticed, they
will have kept in a body together; consequently their mutual relations will not
have been much disturbed, and, in accordance with the principles inculcated in
this volume, they will not have been liable to much modification. But with our
Alpine productions, left isolated from the moment of the returning warmth, first
at the bases and ultimately on the summits of the mountains, the case will have
been somewhat different; for it is not likely that all the same arctic species
will have been left on mountain ranges distant from each other, and have
survived there ever since; they will, also, in all probability have become
mingled with ancient Alpine species, which must have existed on the mountains
before the commencement of the Glacial epoch, and which during its coldest
period will have been temporarily driven down to the plains; they will, also,
have been exposed to somewhat different climatal influences. Their mutual
relations will thus have been in some degree disturbed; consequently they will
have been liable to modification; and this we find has been the case; for if we
compare the present Alpine plants and animals of the several great European
mountain-ranges, though very many of the species are identically the same, some
present varieties, some are ranked as doubtful forms, and some few are distinct
yet closely allied or representative species. In illustrating what, as I
believe, actually took place during the Glacial period, I assumed that at its
commencement the arctic productions were as uniform round the polar regions as
they are at the present day. But the foregoing remarks on distribution apply not
only to strictly arctic forms, but also to many sub-arctic and to some few
northern temperate forms, for some of these are the same on the lower mountains
and on the plains of North America and Europe; and it may be reasonably asked
how I account for the necessary degree of uniformity of the sub-arctic and
northern temperate forms round the world, at the commencement of the Glacial
period. At the present day, the sub-arctic and northern temperate productions of
the Old and New Worlds are separated from each other by the Atlantic Ocean and
by the extreme northern part of the Pacific. During the Glacial period, when the
inhabitants of the Old and New Worlds lived further southwards than at present,
they must have been still more completely separated by wider spaces of ocean. I
believe the above difficulty may be surmounted by looking to still earlier
changes of climate of an opposite nature. We have good reason to believe that
during the newer Pliocene period, before the Glacial epoch, and whilst the
majority of the inhabitants of the world were specifically the same as now, the
climate was warmer than at the present day. Hence we may suppose that the
organisms now living under the climate of latitude 60 deg, during the Pliocene
period lived further north under the Polar Circle, in latitude 66 deg-67 deg;
and that the strictly arctic productions then lived on the broken land still
nearer to the pole. Now if we look at a globe, we shall see that under the Polar
Circle there is almost continuous land from western Europe, through Siberia, to
eastern America. And to this continuity of the circumpolar land, and to the
consequent freedom for intermigration under a more favourable climate, I
attribute the necessary amount of uniformity in the sub-arctic and northern
temperate productions of the Old and New Worlds, at a period anterior to the
Glacial epoch. Believing, from reasons before alluded to, that our continents
have long remained in nearly the same relative position, though subjected to
large, but partial oscillations of level, I am strongly inclined to extend the
above view, and to infer that during some earlier and still warmer period, such
as the older Pliocene period, a large number of the same plants and animals
inhabited the almost continuous circumpolar land; and that these plants and
animals, both in the Old and New Worlds, began slowly to migrate southwards as
the climate became less warm, long before the commencement of the Glacial
period. We now see, as I believe, their descendants, mostly in a modified
condition, in the central parts of Europe and the United States. On this view we
can understand the relationship, with very little identity, between the
productions of North America and Europe,--a relationship which is most
remarkable, considering the distance of the two areas, and their separation by
the Atlantic Ocean. We can further understand the singular fact remarked on by
several observers, that the productions of Europe and America during the later
tertiary stages were more closely related to each other than they are at the
present time; for during these warmer periods the northern parts of the Old and
New Worlds will have been almost continuously united by land, serving as a
bridge, since rendered impassable by cold, for the inter-migration of their
inhabitants. During the slowly decreasing warmth of the Pliocene period, as soon
as the species in common, which inhabited the New and Old Worlds, migrated south
of the Polar Circle, they must have been completely cut off from each other.
This separation, as far as the more temperate productions are concerned, took
place long ages ago. And as the plants and animals migrated southward, they will
have become mingled in the one great region with the native American
productions, and have had to compete with them; and in the other great region,
with those of the Old World. Consequently we have here everything favourable for
much modification,--for far more modification than with the Alpine productions,
left isolated, within a much more recent period, on the several mountain-ranges
and on the arctic lands of the two Worlds. Hence it has come, that when we
compare the now living productions of the temperate regions of the New and Old
Worlds, we find very few identical species (though Asa Gray has lately shown
that more plants are identical than was formerly supposed), but we find in every
great class many forms, which some naturalists rank as geographical races, and
others as distinct species; and a host of closely allied or representative forms
which are ranked by all naturalists as specifically distinct. As on the land, so
in the waters of the sea, a slow southern migration of a marine fauna, which
during the Pliocene or even a somewhat earlier period, was nearly uniform along
the continuous shores of the Polar Circle, will account, on the theory of
modification, for many closely allied forms now living in areas completely
sundered. Thus, I think, we can understand the presence of many existing and
tertiary representative forms on the eastern and western shores of temperate
North America; and the still more striking case of many closely allied
crustaceans (as described in Dana's admirable work), of some fish and other
marine animals, in the Mediterranean and in the seas of Japan,--areas now
separated by a continent and by nearly a hemisphere of equatorial ocean. These
cases of relationship, without identity, of the inhabitants of seas now
disjoined, and likewise of the past and present inhabitants of the temperate
lands of North America and Europe, are inexplicable on the theory of creation.
We cannot say that they have been created alike, in correspondence with the
nearly similar physical conditions of the areas; for if we compare, for
instance, certain parts of South America with the southern continents of the Old
World, we see countries closely corresponding in all their physical conditions,
but with their inhabitants utterly dissimilar. But we must return to our more
immediate subject, the Glacial period. I am convinced that Forbes's view may be
largely extended. In Europe we have the plainest evidence of the cold period,
from the western shores of Britain to the Oural range, and southward to the
Pyrenees. We may infer, from the frozen mammals and nature of the mountain
vegetation, that Siberia was similarly affected. Along the Himalaya, at points
900 miles apart, glaciers have left the marks of their former low descent; and
in Sikkim, Dr. Hooker saw maize growing on gigantic ancient moraines. South of
the equator, we have some direct evidence of former glacial action in New
Zealand; and the same plants, found on widely separated mountains in this
island, tell the same story. If one account which has been published can be
trusted, we have direct evidence of glacial action in the south-eastern corner
of Australia. Looking to America; in the northern half, ice-borne fragments of
rock have been observed on the eastern side as far south as lat. 36 deg-37 deg,
and on the shores of the Pacific, where the climate is now so different, as far
south as lat. 46 deg; erratic boulders have, also, been noticed on the Rocky
Mountains. In the Cordillera of Equatorial South America, glaciers once extended
far below their present level. In central Chile I was astonished at the
structure of a vast mound of detritus, about 800 feet in height, crossing a
valley of the Andes; and this I now feel convinced was a gigantic moraine, left
far below any existing glacier. Further south on both sides of the continent,
from lat. 41 deg to the southernmost extremity, we have the clearest evidence of
former glacial action, in huge boulders transported far from their parent
source. We do not know that the Glacial epoch was strictly simultaneous at these
several far distant points on opposite sides of the world. But we have good
evidence in almost every case, that the epoch was included within the latest
geological period. We have, also, excellent evidence, that it endured for an
enormous time, as measured by years, at each point. The cold may have come on,
or have ceased, earlier at one point of the globe than at another, but seeing
that it endured for long at each, and that it was contemporaneous in a
geological sense, it seems to me probable that it was, during a part at least of
the period, actually simultaneous throughout the world. Without some distinct
evidence to the contrary, we may at least admit as probable that the glacial
action was simultaneous on the eastern and western sides of North America, in
the Cordillera under the equator and under the warmer temperate zones, and on
both sides of the southern extremity of the continent. If this be admitted, it
is difficult to avoid believing that the temperature of the whole world was at
this period simultaneously cooler. But it would suffice for my purpose, if the
temperature was at the same time lower along certain broad belts of longitude.
On this view of the whole world, or at least of broad longitudinal belts, having
been simultaneously colder from pole to pole, much light can be thrown on the
present distribution of identical and allied species. In America, Dr. Hooker has
shown that between forty and fifty of the flowering plants of Tierra del Fuego,
forming no inconsiderable part of its scanty flora, are common to Europe,
enormously remote as these two points are; and there are many closely allied
species. On the lofty mountains of equatorial America a host of peculiar species
belonging to European genera occur. On the highest mountains of Brazil, some few
European genera were found by Gardner, which do not exist in the wide
intervening hot countries. So on the Silla of Caraccas the illustrious Humboldt
long ago found species belonging to genera characteristic of the Cordillera. On
the mountains of Abyssinia, several European forms and some few representatives
of the peculiar flora of the Cape of Good Hope occur. At the Cape of Good Hope a
very few European species, believed not to have been introduced by man, and on
the mountains, some few representative European forms are found, which have not
been discovered in the intertropical parts of Africa. On the Himalaya, and on
the isolated mountain-ranges of the peninsula of India, on the heights of
Ceylon, and on the volcanic cones of Java, many plants occur, either identically
the same or representing each other, and at the same time representing plants of
Europe, not found in the intervening hot lowlands. A list of the genera
collected on the loftier peaks of Java raises a picture of a collection made on
a hill in Europe! Still more striking is the fact that southern Australian forms
are clearly represented by plants growing on the summits of the mountains of
Borneo. Some of these Australian forms, as I hear from Dr. Hooker, extend along
the heights of the peninsula of Malacca, and are thinly scattered, on the one
hand over India and on the other as far north as Japan. On the southern
mountains of Australia, Dr. F. Muller has discovered several European species;
other species, not introduced by man, occur on the lowlands; and a long list can
be given, as I am informed by Dr. Hooker, of European genera, found in
Australia, but not in the intermediate torrid regions. In the admirable
'Introduction to the Flora of New Zealand,' by Dr. Hooker, analogous and
striking facts are given in regard to the plants of that large island. Hence we
see that throughout the world, the plants growing on the more lofty mountains,
and on the temperate lowlands of the northern and southern hemispheres, are
sometimes identically the same; but they are much oftener specifically distinct,
though related to each other in a most remarkable manner. This brief abstract
applies to plants alone: some strictly analogous facts could be given on the
distribution of terrestrial animals. In marine productions, similar cases occur;
as an example, I may quote a remark by the highest authority, Professor Dana,
that "it is certainly a wonderful fact that New Zealand should have a closer
resemblance in its crustacea to Great Britain, its antipode, than to any other
part of the world." Sir J. Richardson, also, speaks of the reappearance on the
shores of New Zealand, Tasmania, etc., of northern forms of fish. Dr. Hooker
informs me that twenty-five species of Algae are common to New Zealand and to
Europe, but have not been found in the intermediate tropical seas. It should be
observed that the northern species and forms found in the southern parts of the
southern hemisphere, and on the mountain-ranges of the intertropical regions,
are not arctic, but belong to the northern temperate zones. As Mr. H. C. Watson
has recently remarked, "In receding from polar towards equatorial latitudes, the
Alpine or mountain floras really become less and less arctic." Many of the forms
living on the mountains of the warmer regions of the earth and in the southern
hemisphere are of doubtful value, being ranked by some naturalists as
specifically distinct, by others as varieties; but some are certainly identical,
and many, though closely related to northern forms, must be ranked as distinct
species. Now let us see what light can be thrown on the foregoing facts, on the
belief, supported as it is by a large body of geological evidence, that the
whole world, or a large part of it, was during the Glacial period simultaneously
much colder than at present. The Glacial period, as measured by years, must have
been very long; and when we remember over what vast spaces some naturalised
plants and animals have spread within a few centuries, this period will have
been ample for any amount of migration. As the cold came slowly on, all the
tropical plants and other productions will have retreated from both sides
towards the equator, followed in the rear by the temperate productions, and
these by the arctic; but with the latter we are not now concerned. The tropical
plants probably suffered much extinction; how much no one can say; perhaps
formerly the tropics supported as many species as we see at the present day
crowded together at the Cape of Good Hope, and in parts of temperate Australia.
As we know that many tropical plants and animals can withstand a considerable
amount of cold, many might have escaped extermination during a moderate fall of
temperature, more especially by escaping into the warmest spots. But the great
fact to bear in mind is, that all tropical productions will have suffered to a
certain extent. On the other hand, the temperate productions, after migrating
nearer to the equator, though they will have been placed under somewhat new
conditions, will have suffered less. And it is certain that many temperate
plants, if protected from the inroads of competitors, can withstand a much
warmer climate than their own. Hence, it seems to me possible, bearing in mind
that the tropical productions were in a suffering state and could not have
presented a firm front against intruders, that a certain number of the more
vigorous and dominant temperate forms might have penetrated the native ranks and
have reached or even crossed the equator. The invasion would, of course, have
been greatly favoured by high land, and perhaps by a dry climate; for Dr.
Falconer informs me that it is the damp with the heat of the tropics which is so
destructive to perennial plants from a temperate climate. On the other hand, the
most humid and hottest districts will have afforded an asylum to the tropical
natives. The mountain-ranges north-west of the Himalaya, and the long line of
the Cordillera, seem to have afforded two great lines of invasion: and it is a
striking fact, lately communicated to me by Dr. Hooker, that all the flowering
plants, about forty-six in number, common to Tierra del Fuego and to Europe
still exist in North America, which must have lain on the line of march. But I
do not doubt that some temperate productions entered and crossed even the
LOWLANDS of the tropics at the period when the cold was most intense,--when
arctic forms had migrated some twenty-five degrees of latitude from their native
country and covered the land at the foot of the Pyrenees. At this period of
extreme cold, I believe that the climate under the equator at the level of the
sea was about the same with that now felt there at the height of six or seven
thousand feet. During this the coldest period, I suppose that large spaces of
the tropical lowlands were clothed with a mingled tropical and temperate
vegetation, like that now growing with strange luxuriance at the base of the
Himalaya, as graphically described by Hooker. Thus, as I believe, a considerable
number of plants, a few terrestrial animals, and some marine productions,
migrated during the Glacial period from the northern and southern temperate
zones into the intertropical regions, and some even crossed the equator. As the
warmth returned, these temperate forms would naturally ascend the higher
mountains, being exterminated on the lowlands; those which had not reached the
equator, would re-migrate northward or southward towards their former homes; but
the forms, chiefly northern, which had crossed the equator, would travel still
further from their homes into the more temperate latitudes of the opposite
hemisphere. Although we have reason to believe from geological evidence that the
whole body of arctic shells underwent scarcely any modification during their
long southern migration and re-migration northward, the case may have been
wholly different with those intruding forms which settled themselves on the
intertropical mountains, and in the southern hemisphere. These being surrounded
by strangers will have had to compete with many new forms of life; and it is
probable that selected modifications in their structure, habits, and
constitutions will have profited them. Thus many of these wanderers, though
still plainly related by inheritance to their brethren of the northern or
southern hemispheres, now exist in their new homes as well-marked varieties or
as distinct species. It is a remarkable fact, strongly insisted on by Hooker in
regard to America, and by Alph. de Candolle in regard to Australia, that many
more identical plants and allied forms have apparently migrated from the north
to the south, than in a reversed direction. We see, however, a few southern
vegetable forms on the mountains of Borneo and Abyssinia. I suspect that this
preponderant migration from north to south is due to the greater extent of land
in the north, and to the northern forms having existed in their own homes in
greater numbers, and having consequently been advanced through natural selection
and competition to a higher stage of perfection or dominating power, than the
southern forms. And thus, when they became commingled during the Glacial period,
the northern forms were enabled to beat the less powerful southern forms. Just
in the same manner as we see at the present day, that very many European
productions cover the ground in La Plata, and in a lesser degree in Australia,
and have to a certain extent beaten the natives; whereas extremely few southern
forms have become naturalised in any part of Europe, though hides, wool, and
other objects likely to carry seeds have been largely imported into Europe
during the last two or three centuries from La Plata, and during the last thirty
or forty years from Australia. Something of the same kind must have occurred on
the intertropical mountains: no doubt before the Glacial period they were
stocked with endemic Alpine forms; but these have almost everywhere largely
yielded to the more dominant forms, generated in the larger areas and more
efficient workshops of the north. In many islands the native productions are
nearly equalled or even outnumbered by the naturalised; and if the natives have
not been actually exterminated, their numbers have been greatly reduced, and
this is the first stage towards extinction. A mountain is an island on the land;
and the intertropical mountains before the Glacial period must have been
completely isolated; and I believe that the productions of these islands on the
land yielded to those produced within the larger areas of the north, just in the
same way as the productions of real islands have everywhere lately yielded to
continental forms, naturalised by man's agency. I am far from supposing that all
difficulties are removed on the view here given in regard to the range and
affinities of the allied species which live in the northern and southern
temperate zones and on the mountains of the intertropical regions. Very many
difficulties remain to be solved. I do not pretend to indicate the exact lines
and means of migration, or the reason why certain species and not others have
migrated; why certain species have been modified and have given rise to new
groups of forms, and others have remained unaltered. We cannot hope to explain
such facts, until we can say why one species and not another becomes naturalised
by man's agency in a foreign land; why one ranges twice or thrice as far, and is
twice or thrice as common, as another species within their own homes. I have
said that many difficulties remain to be solved: some of the most remarkable are
stated with admirable clearness by Dr. Hooker in his botanical works on the
antarctic regions. These cannot be here discussed. I will only say that as far
as regards the occurrence of identical species at points so enormously remote as
Kerguelen Land, New Zealand, and Fuegia, I believe that towards the close of the
Glacial period, icebergs, as suggested by Lyell, have been largely concerned in
their dispersal. But the existence of several quite distinct species, belonging
to genera exclusively confined to the south, at these and other distant points
of the southern hemisphere, is, on my theory of descent with modification, a far
more remarkable case of difficulty. For some of these species are so distinct,
that we cannot suppose that there has been time since the commencement of the
Glacial period for their migration, and for their subsequent modification to the
necessary degree. The facts seem to me to indicate that peculiar and very
distinct species have migrated in radiating lines from some common centre; and I
am inclined to look in the southern, as in the northern hemisphere, to a former
and warmer period, before the commencement of the Glacial period, when the
antarctic lands, now covered with ice, supported a highly peculiar and isolated
flora. I suspect that before this flora was exterminated by the Glacial epoch, a
few forms were widely dispersed to various points of the southern hemisphere by
occasional means of transport, and by the aid, as halting-places, of existing
and now sunken islands, and perhaps at the commencement of the Glacial period,
by icebergs. By these means, as I believe, the southern shores of America,
Australia, New Zealand have become slightly tinted by the same peculiar forms of
vegetable life. Sir C. Lyell in a striking passage has speculated, in language
almost identical with mine, on the effects of great alternations of climate on
geographical distribution. I believe that the world has recently felt one of his
great cycles of change; and that on this view, combined with modification
through natural selection, a multitude of facts in the present distribution both
of the same and of allied forms of life can be explained. The living waters may
be said to have flowed during one short period from the north and from the
south, and to have crossed at the equator; but to have flowed with greater force
from the north so as to have freely inundated the south. As the tide leaves its
drift in horizontal lines, though rising higher on the shores where the tide
rises highest, so have the living waters left their living drift on our
mountain-summits, in a line gently rising from the arctic lowlands to a great
height under the equator. The various beings thus left stranded may be compared
with savage races of man, driven up and surviving in the mountain-fastnesses of
almost every land, which serve as a record, full of interest to us, of the
former inhabitants of the surrounding lowlands. CHAPTER 12. GEOGRAPHICAL
DISTRIBUTION--continued. Distribution of fresh-water productions. On the
inhabitants of oceanic islands. Absence of Batrachians and of terrestrial
Mammals. On the relation of the inhabitants of islands to those of the nearest
mainland. On colonisation from the nearest source with subsequent modification.
Summary of the last and present chapters. As lakes and river-systems are
separated from each other by barriers of land, it might have been thought that
fresh-water productions would not have ranged widely within the same country,
and as the sea is apparently a still more impassable barrier, that they never
would have extended to distant countries. But the case is exactly the reverse.
Not only have many fresh-water species, belonging to quite different classes, an
enormous range, but allied species prevail in a remarkable manner throughout the
world. I well remember, when first collecting in the fresh waters of Brazil,
feeling much surprise at the similarity of the fresh-water insects, shells,
etc., and at the dissimilarity of the surrounding terrestrial beings, compared
with those of Britain. But this power in fresh-water productions of ranging
widely, though so unexpected, can, I think, in most cases be explained by their
having become fitted, in a manner highly useful to them, for short and frequent
migrations from pond to pond, or from stream to stream; and liability to wide
dispersal would follow from this capacity as an almost necessary consequence. We
can here consider only a few cases. In regard to fish, I believe that the same
species never occur in the fresh waters of distant continents. But on the same
continent the species often range widely and almost capriciously; for two
river-systems will have some fish in common and some different. A few facts seem
to favour the possibility of their occasional transport by accidental means;
like that of the live fish not rarely dropped by whirlwinds in India, and the
vitality of their ova when removed from the water. But I am inclined to
attribute the dispersal of fresh-water fish mainly to slight changes within the
recent period in the level of the land, having caused rivers to flow into each
other. Instances, also, could be given of this having occurred during floods,
without any change of level. We have evidence in the loess of the Rhine of
considerable changes of level in the land within a very recent geological
period, and when the surface was peopled by existing land and fresh-water
shells. The wide difference of the fish on opposite sides of continuous
mountain-ranges, which from an early period must have parted river-systems and
completely prevented their inosculation, seems to lead to this same conclusion.
With respect to allied fresh-water fish occurring at very distant points of the
world, no doubt there are many cases which cannot at present be explained: but
some fresh-water fish belong to very ancient forms, and in such cases there will
have been ample time for great geographical changes, and consequently time and
means for much migration. In the second place, salt-water fish can with care be
slowly accustomed to live in fresh water; and, according to Valenciennes, there
is hardly a single group of fishes confined exclusively to fresh water, so that
we may imagine that a marine member of a fresh-water group might travel far
along the shores of the sea, and subsequently become modified and adapted to the
fresh waters of a distant land. Some species of fresh-water shells have a very
wide range, and allied species, which, on my theory, are descended from a common
parent and must have proceeded from a single source, prevail throughout the
world. Their distribution at first perplexed me much, as their ova are not
likely to be transported by birds, and they are immediately killed by sea water,
as are the adults. I could not even understand how some naturalised species have
rapidly spread throughout the same country. But two facts, which I have
observed--and no doubt many others remain to be observed--throw some light on
this subject. When a duck suddenly emerges from a pond covered with duck-weed, I
have twice seen these little plants adhering to its back; and it has happened to
me, in removing a little duck-weed from one aquarium to another, that I have
quite unintentionally stocked the one with fresh-water shells from the other.
But another agency is perhaps more effectual: I suspended a duck's feet, which
might represent those of a bird sleeping in a natural pond, in an aquarium,
where many ova of fresh-water shells were hatching; and I found that numbers of
the extremely minute and just hatched shells crawled on the feet, and clung to
them so firmly that when taken out of the water they could not be jarred off,
though at a somewhat more advanced age they would voluntarily drop off. These
just hatched molluscs, though aquatic in their nature, survived on the duck's
feet, in damp air, from twelve to twenty hours; and in this length of time a
duck or heron might fly at least six or seven hundred miles, and would be sure
to alight on a pool or rivulet, if blown across sea to an oceanic island or to
any other distant point. Sir Charles Lyell also informs me that a Dyticus has
been caught with an Ancylus (a fresh-water shell like a limpet) firmly adhering
to it; and a water-beetle of the same family, a Colymbetes, once flew on board
the 'Beagle,' when forty-five miles distant from the nearest land: how much
farther it might have flown with a favouring gale no one can tell. With respect
to plants, it has long been known what enormous ranges many fresh-water and even
marsh-species have, both over continents and to the most remote oceanic islands.
This is strikingly shown, as remarked by Alph. de Candolle, in large groups of
terrestrial plants, which have only a very few aquatic members; for these latter
seem immediately to acquire, as if in consequence, a very wide range. I think
favourable means of dispersal explain this fact. I have before mentioned that
earth occasionally, though rarely, adheres in some quantity to the feet and
beaks of birds. Wading birds, which frequent the muddy edges of ponds, if
suddenly flushed, would be the most likely to have muddy feet. Birds of this
order I can show are the greatest wanderers, and are occasionally found on the
most remote and barren islands in the open ocean; they would not be likely to
alight on the surface of the sea, so that the dirt would not be washed off their
feet; when making land, they would be sure to fly to their natural fresh-water
haunts. I do not believe that botanists are aware how charged the mud of ponds
is with seeds: I have tried several little experiments, but will here give only
the most striking case: I took in February three table-spoonfuls of mud from
three different points, beneath water, on the edge of a little pond; this mud
when dry weighed only 6 3/4 ounces; I kept it covered up in my study for six
months, pulling up and counting each plant as it grew; the plants were of many
kinds, and were altogether 537 in number; and yet the viscid mud was all
contained in a breakfast cup! Considering these facts, I think it would be an
inexplicable circumstance if water-birds did not transport the seeds of
fresh-water plants to vast distances, and if consequently the range of these
plants was not very great. The same agency may have come into play with the eggs
of some of the smaller fresh-water animals. Other and unknown agencies probably
have also played a part. I have stated that fresh-water fish eat some kinds of
seeds, though they reject many other kinds after having swallowed them; even
small fish swallow seeds of moderate size, as of the yellow water-lily and
Potamogeton. Herons and other birds, century after century, have gone on daily
devouring fish; they then take flight and go to other waters, or are blown
across the sea; and we have seen that seeds retain their power of germination,
when rejected in pellets or in excrement, many hours afterwards. When I saw the
great size of the seeds of that fine water-lily, the Nelumbium, and remembered
Alph. de Candolle's remarks on this plant, I thought that its distribution must
remain quite inexplicable; but Audubon states that he found the seeds of the
great southern water-lily (probably, according to Dr. Hooker, the Nelumbium
luteum) in a heron's stomach; although I do not know the fact, yet analogy makes
me believe that a heron flying to another pond and getting a hearty meal of
fish, would probably reject from its stomach a pellet containing the seeds of
the Nelumbium undigested; or the seeds might be dropped by the bird whilst
feeding its young, in the same way as fish are known sometimes to be dropped. In
considering these several means of distribution, it should be remembered that
when a pond or stream is first formed, for instance, on a rising islet, it will
be unoccupied; and a single seed or egg will have a good chance of succeeding.
Although there will always be a struggle for life between the individuals of the
species, however few, already occupying any pond, yet as the number of kinds is
small, compared with those on the land, the competition will probably be less
severe between aquatic than between terrestrial species; consequently an
intruder from the waters of a foreign country, would have a better chance of
seizing on a place, than in the case of terrestrial colonists. We should, also,
remember that some, perhaps many, fresh-water productions are low in the scale
of nature, and that we have reason to believe that such low beings change or
become modified less quickly than the high; and this will give longer time than
the average for the migration of the same aquatic species. We should not forget
the probability of many species having formerly ranged as continuously as
fresh-water productions ever can range, over immense areas, and having
subsequently become extinct in intermediate regions. But the wide distribution
of fresh-water plants and of the lower animals, whether retaining the same
identical form or in some degree modified, I believe mainly depends on the wide
dispersal of their seeds and eggs by animals, more especially by fresh-water
birds, which have large powers of flight, and naturally travel from one to
another and often distant piece of water. Nature, like a careful gardener, thus
takes her seeds from a bed of a particular nature, and drops them in another
equally well fitted for them. ON THE INHABITANTS OF OCEANIC ISLANDS. We now come
to the last of the three classes of facts, which I have selected as presenting
the greatest amount of difficulty, on the view that all the individuals both of
the same and of allied species have descended from a single parent; and
therefore have all proceeded from a common birthplace, notwithstanding that in
the course of time they have come to inhabit distant points of the globe. I have
already stated that I cannot honestly admit Forbes's view on continental
extensions, which, if legitimately followed out, would lead to the belief that
within the recent period all existing islands have been nearly or quite joined
to some continent. This view would remove many difficulties, but it would not, I
think, explain all the facts in regard to insular productions. In the following
remarks I shall not confine myself to the mere question of dispersal; but shall
consider some other facts, which bear on the truth of the two theories of
independent creation and of descent with modification. The species of all kinds
which inhabit oceanic islands are few in number compared with those on equal
continental areas: Alph. de Candolle admits this for plants, and Wollaston for
insects. If we look to the large size and varied stations of New Zealand,
extending over 780 miles of latitude, and compare its flowering plants, only 750
in number, with those on an equal area at the Cape of Good Hope or in Australia,
we must, I think, admit that something quite independently of any difference in
physical conditions has caused so great a difference in number. Even the uniform
county of Cambridge has 847 plants, and the little island of Anglesea 764, but a
few ferns and a few introduced plants are included in these numbers, and the
comparison in some other respects is not quite fair. We have evidence that the
barren island of Ascension aboriginally possessed under half-a-dozen flowering
plants; yet many have become naturalised on it, as they have on New Zealand and
on every other oceanic island which can be named. In St. Helena there is reason
to believe that the naturalised plants and animals have nearly or quite
exterminated many native productions. He who admits the doctrine of the creation
of each separate species, will have to admit, that a sufficient number of the
best adapted plants and animals have not been created on oceanic islands; for
man has unintentionally stocked them from various sources far more fully and
perfectly than has nature. Although in oceanic islands the number of kinds of
inhabitants is scanty, the proportion of endemic species (i.e. those found
nowhere else in the world) is often extremely large. If we compare, for
instance, the number of the endemic land-shells in Madeira, or of the endemic
birds in the Galapagos Archipelago, with the number found on any continent, and
then compare the area of the islands with that of the continent, we shall see
that this is true. This fact might have been expected on my theory, for, as
already explained, species occasionally arriving after long intervals in a new
and isolated district, and having to compete with new associates, will be
eminently liable to modification, and will often produce groups of modified
descendants. But it by no means follows, that, because in an island nearly all
the species of one class are peculiar, those of another class, or of another
section of the same class, are peculiar; and this difference seems to depend on
the species which do not become modified having immigrated with facility and in
a body, so that their mutual relations have not been much disturbed. Thus in the
Galapagos Islands nearly every land-bird, but only two out of the eleven marine
birds, are peculiar; and it is obvious that marine birds could arrive at these
islands more easily than land-birds. Bermuda, on the other hand, which lies at
about the same distance from North America as the Galapagos Islands do from
South America, and which has a very peculiar soil, does not possess one endemic
land bird; and we know from Mr. J. M. Jones's admirable account of Bermuda, that
very many North American birds, during their great annual migrations, visit
either periodically or occasionally this island. Madeira does not possess one
peculiar bird, and many European and African birds are almost every year blown
there, as I am informed by Mr. E. V. Harcourt. So that these two islands of
Bermuda and Madeira have been stocked by birds, which for long ages have
struggled together in their former homes, and have become mutually adapted to
each other; and when settled in their new homes, each kind will have been kept
by the others to their proper places and habits, and will consequently have been
little liable to modification. Madeira, again, is inhabited by a wonderful
number of peculiar land-shells, whereas not one species of sea-shell is confined
to its shores: now, though we do not know how seashells are dispersed, yet we
can see that their eggs or larvae, perhaps attached to seaweed or floating
timber, or to the feet of wading-birds, might be transported far more easily
than land-shells, across three or four hundred miles of open sea. The different
orders of insects in Madeira apparently present analogous facts. Oceanic islands
are sometimes deficient in certain classes, and their places are apparently
occupied by the other inhabitants; in the Galapagos Islands reptiles, and in New
Zealand gigantic wingless birds, take the place of mammals. In the plants of the
Galapagos Islands, Dr. Hooker has shown that the proportional numbers of the
different orders are very different from what they are elsewhere. Such cases are
generally accounted for by the physical conditions of the islands; but this
explanation seems to me not a little doubtful. Facility of immigration, I
believe, has been at least as important as the nature of the conditions. Many
remarkable little facts could be given with respect to the inhabitants of remote
islands. For instance, in certain islands not tenanted by mammals, some of the
endemic plants have beautifully hooked seeds; yet few relations are more
striking than the adaptation of hooked seeds for transportal by the wool and fur
of quadrupeds. This case presents no difficulty on my view, for a hooked seed
might be transported to an island by some other means; and the plant then
becoming slightly modified, but still retaining its hooked seeds, would form an
endemic species, having as useless an appendage as any rudimentary organ,--for
instance, as the shrivelled wings under the soldered elytra of many insular
beetles. Again, islands often possess trees or bushes belonging to orders which
elsewhere include only herbaceous species; now trees, as Alph. de Candolle has
shown, generally have, whatever the cause may be, confined ranges. Hence trees
would be little likely to reach distant oceanic islands; and an herbaceous
plant, though it would have no chance of successfully competing in stature with
a fully developed tree, when established on an island and having to compete with
herbaceous plants alone, might readily gain an advantage by growing taller and
taller and overtopping the other plants. If so, natural selection would often
tend to add to the stature of herbaceous plants when growing on an island, to
whatever order they belonged, and thus convert them first into bushes and
ultimately into trees. With respect to the absence of whole orders on oceanic
islands, Bory St. Vincent long ago remarked that Batrachians (frogs, toads,
newts) have never been found on any of the many islands with which the great
oceans are studded. I have taken pains to verify this assertion, and I have
found it strictly true. I have, however, been assured that a frog exists on the
mountains of the great island of New Zealand; but I suspect that this exception
(if the information be correct) may be explained through glacial agency. This
general absence of frogs, toads, and newts on so many oceanic islands cannot be
accounted for by their physical conditions; indeed it seems that islands are
peculiarly well fitted for these animals; for frogs have been introduced into
Madeira, the Azores, and Mauritius, and have multiplied so as to become a
nuisance. But as these animals and their spawn are known to be immediately
killed by sea-water, on my view we can see that there would be great difficulty
in their transportal across the sea, and therefore why they do not exist on any
oceanic island. But why, on the theory of creation, they should not have been
created there, it would be very difficult to explain. Mammals offer another and
similar case. I have carefully searched the oldest voyages, but have not
finished my search; as yet I have not found a single instance, free from doubt,
of a terrestrial mammal (excluding domesticated animals kept by the natives)
inhabiting an island situated above 300 miles from a continent or great
continental island; and many islands situated at a much less distance are
equally barren. The Falkland Islands, which are inhabited by a wolf-like fox,
come nearest to an exception; but this group cannot be considered as oceanic, as
it lies on a bank connected with the mainland; moreover, icebergs formerly
brought boulders to its western shores, and they may have formerly transported
foxes, as so frequently now happens in the arctic regions. Yet it cannot be said
that small islands will not support small mammals, for they occur in many parts
of the world on very small islands, if close to a continent; and hardly an
island can be named on which our smaller quadrupeds have not become naturalised
and greatly multiplied. It cannot be said, on the ordinary view of creation,
that there has not been time for the creation of mammals; many volcanic islands
are sufficiently ancient, as shown by the stupendous degradation which they have
suffered and by their tertiary strata: there has also been time for the
production of endemic species belonging to other classes; and on continents it
is thought that mammals appear and disappear at a quicker rate than other and
lower animals. Though terrestrial mammals do not occur on oceanic islands,
aerial mammals do occur on almost every island. New Zealand possesses two bats
found nowhere else in the world: Norfolk Island, the Viti Archipelago, the Bonin
Islands, the Caroline and Marianne Archipelagoes, and Mauritius, all possess
their peculiar bats. Why, it may be asked, has the supposed creative force
produced bats and no other mammals on remote islands? On my view this question
can easily be answered; for no terrestrial mammal can be transported across a
wide space of sea, but bats can fly across. Bats have been seen wandering by day
far over the Atlantic Ocean; and two North American species either regularly or
occasionally visit Bermuda, at the distance of 600 miles from the mainland. I
hear from Mr. Tomes, who has specially studied this family, that many of the
same species have enormous ranges, and are found on continents and on far
distant islands. Hence we have only to suppose that such wandering species have
been modified through natural selection in their new homes in relation to their
new position, and we can understand the presence of endemic bats on islands,
with the absence of all terrestrial mammals. Besides the absence of terrestrial
mammals in relation to the remoteness of islands from continents, there is also
a relation, to a certain extent independent of distance, between the depth of
the sea separating an island from the neighbouring mainland, and the presence in
both of the same mammiferous species or of allied species in a more or less
modified condition. Mr. Windsor Earl has made some striking observations on this
head in regard to the great Malay Archipelago, which is traversed near Celebes
by a space of deep ocean; and this space separates two widely distinct mammalian
faunas. On either side the islands are situated on moderately deep submarine
banks, and they are inhabited by closely allied or identical quadrupeds. No
doubt some few anomalies occur in this great archipelago, and there is much
difficulty in forming a judgment in some cases owing to the probable
naturalisation of certain mammals through man's agency; but we shall soon have
much light thrown on the natural history of this archipelago by the admirable
zeal and researches of Mr. Wallace. I have not as yet had time to follow up this
subject in all other quarters of the world; but as far as I have gone, the
relation generally holds good. We see Britain separated by a shallow channel
from Europe, and the mammals are the same on both sides; we meet with analogous
facts on many islands separated by similar channels from Australia. The West
Indian Islands stand on a deeply submerged bank, nearly 1000 fathoms in depth,
and here we find American forms, but the species and even the genera are
distinct. As the amount of modification in all cases depends to a certain degree
on the lapse of time, and as during changes of level it is obvious that islands
separated by shallow channels are more likely to have been continuously united
within a recent period to the mainland than islands separated by deeper
channels, we can understand the frequent relation between the depth of the sea
and the degree of affinity of the mammalian inhabitants of islands with those of
a neighbouring continent,--an inexplicable relation on the view of independent
acts of creation. All the foregoing remarks on the inhabitants of oceanic
islands,--namely, the scarcity of kinds--the richness in endemic forms in
particular classes or sections of classes,--the absence of whole groups, as of
batrachians, and of terrestrial mammals notwithstanding the presence of aerial
bats,--the singular proportions of certain orders of plants,--herbaceous forms
having been developed into trees, etc.,--seem to me to accord better with the
view of occasional means of transport having been largely efficient in the long
course of time, than with the view of all our oceanic islands having been
formerly connected by continuous land with the nearest continent; for on this
latter view the migration would probably have been more complete; and if
modification be admitted, all the forms of life would have been more equally
modified, in accordance with the paramount importance of the relation of
organism to organism. I do not deny that there are many and grave difficulties
in understanding how several of the inhabitants of the more remote islands,
whether still retaining the same specific form or modified since their arrival,
could have reached their present homes. But the probability of many islands
having existed as halting-places, of which not a wreck now remains, must not be
overlooked. I will here give a single instance of one of the cases of
difficulty. Almost all oceanic islands, even the most isolated and smallest, are
inhabited by land-shells, generally by endemic species, but sometimes by species
found elsewhere. Dr. Aug. A. Gould has given several interesting cases in regard
to the land-shells of the islands of the Pacific. Now it is notorious that
land-shells are very easily killed by salt; their eggs, at least such as I have
tried, sink in sea-water and are killed by it. Yet there must be, on my view,
some unknown, but highly efficient means for their transportal. Would the
just-hatched young occasionally crawl on and adhere to the feet of birds
roosting on the ground, and thus get transported? It occurred to me that
land-shells, when hybernating and having a membranous diaphragm over the mouth
of the shell, might be floated in chinks of drifted timber across moderately
wide arms of the sea. And I found that several species did in this state
withstand uninjured an immersion in sea-water during seven days: one of these
shells was the Helix pomatia, and after it had again hybernated I put it in
sea-water for twenty days, and it perfectly recovered. As this species has a
thick calcareous operculum, I removed it, and when it had formed a new
membranous one, I immersed it for fourteen days in sea-water, and it recovered
and crawled away: but more experiments are wanted on this head. The most
striking and important fact for us in regard to the inhabitants of islands, is
their affinity to those of the nearest mainland, without being actually the same
species. Numerous instances could be given of this fact. I will give only one,
that of the Galapagos Archipelago, situated under the equator, between 500 and
600 miles from the shores of South America. Here almost every product of the
land and water bears the unmistakeable stamp of the American continent. There
are twenty-six land birds, and twenty-five of these are ranked by Mr. Gould as
distinct species, supposed to have been created here; yet the close affinity of
most of these birds to American species in every character, in their habits,
gestures, and tones of voice, was manifest. So it is with the other animals, and
with nearly all the plants, as shown by Dr. Hooker in his admirable memoir on
the Flora of this archipelago. The naturalist, looking at the inhabitants of
these volcanic islands in the Pacific, distant several hundred miles from the
continent, yet feels that he is standing on American land. Why should this be
so? why should the species which are supposed to have been created in the
Galapagos Archipelago, and nowhere else, bear so plain a stamp of affinity to
those created in America? There is nothing in the conditions of life, in the
geological nature of the islands, in their height or climate, or in the
proportions in which the several classes are associated together, which
resembles closely the conditions of the South American coast: in fact there is a
considerable dissimilarity in all these respects. On the other hand, there is a
considerable degree of resemblance in the volcanic nature of the soil, in
climate, height, and size of the islands, between the Galapagos and Cape de
Verde Archipelagos: but what an entire and absolute difference in their
inhabitants! The inhabitants of the Cape de Verde Islands are related to those
of Africa, like those of the Galapagos to America. I believe this grand fact can
receive no sort of explanation on the ordinary view of independent creation;
whereas on the view here maintained, it is obvious that the Galapagos Islands
would be likely to receive colonists, whether by occasional means of transport
or by formerly continuous land, from America; and the Cape de Verde Islands from
Africa; and that such colonists would be liable to modification;--the principle
of inheritance still betraying their original birthplace. Many analogous facts
could be given: indeed it is an almost universal rule that the endemic
productions of islands are related to those of the nearest continent, or of
other near islands. The exceptions are few, and most of them can be explained.
Thus the plants of Kerguelen Land, though standing nearer to Africa than to
America, are related, and that very closely, as we know from Dr. Hooker's
account, to those of America: but on the view that this island has been mainly
stocked by seeds brought with earth and stones on icebergs, drifted by the
prevailing currents, this anomaly disappears. New Zealand in its endemic plants
is much more closely related to Australia, the nearest mainland, than to any
other region: and this is what might have been expected; but it is also plainly
related to South America, which, although the next nearest continent, is so
enormously remote, that the fact becomes an anomaly. But this difficulty almost
disappears on the view that both New Zealand, South America, and other southern
lands were long ago partially stocked from a nearly intermediate though distant
point, namely from the antarctic islands, when they were clothed with
vegetation, before the commencement of the Glacial period. The affinity, which,
though feeble, I am assured by Dr. Hooker is real, between the flora of the
south-western corner of Australia and of the Cape of Good Hope, is a far more
remarkable case, and is at present inexplicable: but this affinity is confined
to the plants, and will, I do not doubt, be some day explained. The law which
causes the inhabitants of an archipelago, though specifically distinct, to be
closely allied to those of the nearest continent, we sometimes see displayed on
a small scale, yet in a most interesting manner, within the limits of the same
archipelago. Thus the several islands of the Galapagos Archipelago are tenanted,
as I have elsewhere shown, in a quite marvellous manner, by very closely related
species; so that the inhabitants of each separate island, though mostly
distinct, are related in an incomparably closer degree to each other than to the
inhabitants of any other part of the world. And this is just what might have
been expected on my view, for the islands are situated so near each other that
they would almost certainly receive immigrants from the same original source, or
from each other. But this dissimilarity between the endemic inhabitants of the
islands may be used as an argument against my views; for it may be asked, how
has it happened in the several islands situated within sight of each other,
having the same geological nature, the same height, climate, etc., that many of
the immigrants should have been differently modified, though only in a small
degree. This long appeared to me a great difficulty: but it arises in chief part
from the deeply-seated error of considering the physical conditions of a country
as the most important for its inhabitants; whereas it cannot, I think, be
disputed that the nature of the other inhabitants, with which each has to
compete, is at least as important, and generally a far more important element of
success. Now if we look to those inhabitants of the Galapagos Archipelago which
are found in other parts of the world (laying on one side for the moment the
endemic species, which cannot be here fairly included, as we are considering how
they have come to be modified since their arrival), we find a considerable
amount of difference in the several islands. This difference might indeed have
been expected on the view of the islands having been stocked by occasional means
of transport--a seed, for instance, of one plant having been brought to one
island, and that of another plant to another island. Hence when in former times
an immigrant settled on any one or more of the islands, or when it subsequently
spread from one island to another, it would undoubtedly be exposed to different
conditions of life in the different islands, for it would have to compete with
different sets of organisms: a plant, for instance, would find the best-fitted
ground more perfectly occupied by distinct plants in one island than in another,
and it would be exposed to the attacks of somewhat different enemies. If then it
varied, natural selection would probably favour different varieties in the
different islands. Some species, however, might spread and yet retain the same
character throughout the group, just as we see on continents some species
spreading widely and remaining the same. The really surprising fact in this case
of the Galapagos Archipelago, and in a lesser degree in some analogous
instances, is that the new species formed in the separate islands have not
quickly spread to the other islands. But the islands, though in sight of each
other, are separated by deep arms of the sea, in most cases wider than the
British Channel, and there is no reason to suppose that they have at any former
period been continuously united. The currents of the sea are rapid and sweep
across the archipelago, and gales of wind are extraordinarily rare; so that the
islands are far more effectually separated from each other than they appear to
be on a map. Nevertheless a good many species, both those found in other parts
of the world and those confined to the archipelago, are common to the several
islands, and we may infer from certain facts that these have probably spread
from some one island to the others. But we often take, I think, an erroneous
view of the probability of closely allied species invading each other's
territory, when put into free intercommunication. Undoubtedly if one species has
any advantage whatever over another, it will in a very brief time wholly or in
part supplant it; but if both are equally well fitted for their own places in
nature, both probably will hold their own places and keep separate for almost
any length of time. Being familiar with the fact that many species, naturalised
through man's agency, have spread with astonishing rapidity over new countries,
we are apt to infer that most species would thus spread; but we should remember
that the forms which become naturalised in new countries are not generally
closely allied to the aboriginal inhabitants, but are very distinct species,
belonging in a large proportion of cases, as shown by Alph. de Candolle, to
distinct genera. In the Galapagos Archipelago, many even of the birds, though so
well adapted for flying from island to island, are distinct on each; thus there
are three closely-allied species of mocking-thrush, each confined to its own
island. Now let us suppose the mocking-thrush of Chatham Island to be blown to
Charles Island, which has its own mocking-thrush: why should it succeed in
establishing itself there? We may safely infer that Charles Island is well
stocked with its own species, for annually more eggs are laid there than can
possibly be reared; and we may infer that the mocking-thrush peculiar to Charles
Island is at least as well fitted for its home as is the species peculiar to
Chatham Island. Sir C. Lyell and Mr. Wollaston have communicated to me a
remarkable fact bearing on this subject; namely, that Madeira and the adjoining
islet of Porto Santo possess many distinct but representative land-shells, some
of which live in crevices of stone; and although large quantities of stone are
annually transported from Porto Santo to Madeira, yet this latter island has not
become colonised by the Porto Santo species: nevertheless both islands have been
colonised by some European land-shells, which no doubt had some advantage over
the indigenous species. From these considerations I think we need not greatly
marvel at the endemic and representative species, which inhabit the several
islands of the Galapagos Archipelago, not having universally spread from island
to island. In many other instances, as in the several districts of the same
continent, pre-occupation has probably played an important part in checking the
commingling of species under the same conditions of life. Thus, the south-east
and south-west corners of Australia have nearly the same physical conditions,
and are united by continuous land, yet they are inhabited by a vast number of
distinct mammals, birds, and plants. The principle which determines the general
character of the fauna and flora of oceanic islands, namely, that the
inhabitants, when not identically the same, yet are plainly related to the
inhabitants of that region whence colonists could most readily have been
derived,--the colonists having been subsequently modified and better fitted to
their new homes,--is of the widest application throughout nature. We see this on
every mountain, in every lake and marsh. For Alpine species, excepting in so far
as the same forms, chiefly of plants, have spread widely throughout the world
during the recent Glacial epoch, are related to those of the surrounding
lowlands;--thus we have in South America, Alpine humming-birds, Alpine rodents,
Alpine plants, etc., all of strictly American forms, and it is obvious that a
mountain, as it became slowly upheaved, would naturally be colonised from the
surrounding lowlands. So it is with the inhabitants of lakes and marshes,
excepting in so far as great facility of transport has given the same general
forms to the whole world. We see this same principle in the blind animals
inhabiting the caves of America and of Europe. Other analogous facts could be
given. And it will, I believe, be universally found to be true, that wherever in
two regions, let them be ever so distant, many closely allied or representative
species occur, there will likewise be found some identical species, showing, in
accordance with the foregoing view, that at some former period there has been
intercommunication or migration between the two regions. And wherever many
closely-allied species occur, there will be found many forms which some
naturalists rank as distinct species, and some as varieties; these doubtful
forms showing us the steps in the process of modification. This relation between
the power and extent of migration of a species, either at the present time or at
some former period under different physical conditions, and the existence at
remote points of the world of other species allied to it, is shown in another
and more general way. Mr. Gould remarked to me long ago, that in those genera of
birds which range over the world, many of the species have very wide ranges. I
can hardly doubt that this rule is generally true, though it would be difficult
to prove it. Amongst mammals, we see it strikingly displayed in Bats, and in a
lesser degree in the Felidae and Canidae. We see it, if we compare the
distribution of butterflies and beetles. So it is with most fresh-water
productions, in which so many genera range over the world, and many individual
species have enormous ranges. It is not meant that in world-ranging genera all
the species have a wide range, or even that they have on an AVERAGE a wide
range; but only that some of the species range very widely; for the facility
with which widely-ranging species vary and give rise to new forms will largely
determine their average range. For instance, two varieties of the same species
inhabit America and Europe, and the species thus has an immense range; but, if
the variation had been a little greater, the two varieties would have been
ranked as distinct species, and the common range would have been greatly
reduced. Still less is it meant, that a species which apparently has the
capacity of crossing barriers and ranging widely, as in the case of certain
powerfully-winged birds, will necessarily range widely; for we should never
forget that to range widely implies not only the power of crossing barriers, but
the more important power of being victorious in distant lands in the struggle
for life with foreign associates. But on the view of all the species of a genus
having descended from a single parent, though now distributed to the most remote
points of the world, we ought to find, and I believe as a general rule we do
find, that some at least of the species range very widely; for it is necessary
that the unmodified parent should range widely, undergoing modification during
its diffusion, and should place itself under diverse conditions favourable for
the conversion of its offspring, firstly into new varieties and ultimately into
new species. In considering the wide distribution of certain genera, we should
bear in mind that some are extremely ancient, and must have branched off from a
common parent at a remote epoch; so that in such cases there will have been
ample time for great climatal and geographical changes and for accidents of
transport; and consequently for the migration of some of the species into all
quarters of the world, where they may have become slightly modified in relation
to their new conditions. There is, also, some reason to believe from geological
evidence that organisms low in the scale within each great class, generally
change at a slower rate than the higher forms; and consequently the lower forms
will have had a better chance of ranging widely and of still retaining the same
specific character. This fact, together with the seeds and eggs of many low
forms being very minute and better fitted for distant transportation, probably
accounts for a law which has long been observed, and which has lately been
admirably discussed by Alph. de Candolle in regard to plants, namely, that the
lower any group of organisms is, the more widely it is apt to range. The
relations just discussed,--namely, low and slowly-changing organisms ranging
more widely than the high,--some of the species of widely-ranging genera
themselves ranging widely,--such facts, as alpine, lacustrine, and marsh
productions being related (with the exceptions before specified) to those on the
surrounding low lands and dry lands, though these stations are so different--the
very close relation of the distinct species which inhabit the islets of the same
archipelago,--and especially the striking relation of the inhabitants of each
whole archipelago or island to those of the nearest mainland,--are, I think,
utterly inexplicable on the ordinary view of the independent creation of each
species, but are explicable on the view of colonisation from the nearest and
readiest source, together with the subsequent modification and better adaptation
of the colonists to their new homes. SUMMARY OF LAST AND PRESENT CHAPTERS. In
these chapters I have endeavoured to show, that if we make due allowance for our
ignorance of the full effects of all the changes of climate and of the level of
the land, which have certainly occurred within the recent period, and of other
similar changes which may have occurred within the same period; if we remember
how profoundly ignorant we are with respect to the many and curious means of
occasional transport,--a subject which has hardly ever been properly
experimentised on; if we bear in mind how often a species may have ranged
continuously over a wide area, and then have become extinct in the intermediate
tracts, I think the difficulties in believing that all the individuals of the
same species, wherever located, have descended from the same parents, are not
insuperable. And we are led to this conclusion, which has been arrived at by
many naturalists under the designation of single centres of creation, by some
general considerations, more especially from the importance of barriers and from
the analogical distribution of sub-genera, genera, and families. With respect to
the distinct species of the same genus, which on my theory must have spread from
one parent-source; if we make the same allowances as before for our ignorance,
and remember that some forms of life change most slowly, enormous periods of
time being thus granted for their migration, I do not think that the
difficulties are insuperable; though they often are in this case, and in that of
the individuals of the same species, extremely grave. As exemplifying the
effects of climatal changes on distribution, I have attempted to show how
important has been the influence of the modern Glacial period, which I am fully
convinced simultaneously affected the whole world, or at least great meridional
belts. As showing how diversified are the means of occasional transport, I have
discussed at some little length the means of dispersal of fresh-water
productions. If the difficulties be not insuperable in admitting that in the
long course of time the individuals of the same species, and likewise of allied
species, have proceeded from some one source; then I think all the grand leading
facts of geographical distribution are explicable on the theory of migration
(generally of the more dominant forms of life), together with subsequent
modification and the multiplication of new forms. We can thus understand the
high importance of barriers, whether of land or water, which separate our
several zoological and botanical provinces. We can thus understand the
localisation of sub-genera, genera, and families; and how it is that under
different latitudes, for instance in South America, the inhabitants of the
plains and mountains, of the forests, marshes, and deserts, are in so mysterious
a manner linked together by affinity, and are likewise linked to the extinct
beings which formerly inhabited the same continent. Bearing in mind that the
mutual relations of organism to organism are of the highest importance, we can
see why two areas having nearly the same physical conditions should often be
inhabited by very different forms of life; for according to the length of time
which has elapsed since new inhabitants entered one region; according to the
nature of the communication which allowed certain forms and not others to enter,
either in greater or lesser numbers; according or not, as those which entered
happened to come in more or less direct competition with each other and with the
aborigines; and according as the immigrants were capable of varying more or less
rapidly, there would ensue in different regions, independently of their physical
conditions, infinitely diversified conditions of life,--there would be an almost
endless amount of organic action and reaction,--and we should find, as we do
find, some groups of beings greatly, and some only slightly modified,--some
developed in great force, some existing in scanty numbers--in the different
great geographical provinces of the world. On these same principles, we can
understand, as I have endeavoured to show, why oceanic islands should have few
inhabitants, but of these a great number should be endemic or peculiar; and why,
in relation to the means of migration, one group of beings, even within the same
class, should have all its species endemic, and another group should have all
its species common to other quarters of the world. We can see why whole groups
of organisms, as batrachians and terrestrial mammals, should be absent from
oceanic islands, whilst the most isolated islands possess their own peculiar
species of aerial mammals or bats. We can see why there should be some relation
between the presence of mammals, in a more or less modified condition, and the
depth of the sea between an island and the mainland. We can clearly see why all
the inhabitants of an archipelago, though specifically distinct on the several
islets, should be closely related to each other, and likewise be related, but
less closely, to those of the nearest continent or other source whence
immigrants were probably derived. We can see why in two areas, however distant
from each other, there should be a correlation, in the presence of identical
species, of varieties, of doubtful species, and of distinct but representative
species. As the late Edward Forbes often insisted, there is a striking
parallelism in the laws of life throughout time and space: the laws governing
the succession of forms in past times being nearly the same with those governing
at the present time the differences in different areas. We see this in many
facts. The endurance of each species and group of species is continuous in time;
for the exceptions to the rule are so few, that they may fairly be attributed to
our not having as yet discovered in an intermediate deposit the forms which are
therein absent, but which occur above and below: so in space, it certainly is
the general rule that the area inhabited by a single species, or by a group of
species, is continuous; and the exceptions, which are not rare, may, as I have
attempted to show, be accounted for by migration at some former period under
different conditions or by occasional means of transport, and by the species
having become extinct in the intermediate tracts. Both in time and space,
species and groups of species have their points of maximum development. Groups
of species, belonging either to a certain period of time, or to a certain area,
are often characterised by trifling characters in common, as of sculpture or
colour. In looking to the long succession of ages, as in now looking to distant
provinces throughout the world, we find that some organisms differ little,
whilst others belonging to a different class, or to a different order, or even
only to a different family of the same order, differ greatly. In both time and
space the lower members of each class generally change less than the higher; but
there are in both cases marked exceptions to the rule. On my theory these
several relations throughout time and space are intelligible; for whether we
look to the forms of life which have changed during successive ages within the
same quarter of the world, or to those which have changed after having migrated
into distant quarters, in both cases the forms within each class have been
connected by the same bond of ordinary generation; and the more nearly any two
forms are related in blood, the nearer they will generally stand to each other
in time and space; in both cases the laws of variation have been the same, and
modifications have been accumulated by the same power of natural selection.
CHAPTER 13. MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY: EMBRYOLOGY:
RUDIMENTARY ORGANS. CLASSIFICATION, groups subordinate to groups. Natural
system. Rules and difficulties in classification, explained on the theory of
descent with modification. Classification of varieties. Descent always used in
classification. Analogical or adaptive characters. Affinities, general, complex
and radiating. Extinction separates and defines groups. MORPHOLOGY, between
members of the same class, between parts of the same individual. EMBRYOLOGY,
laws of, explained by variations not supervening at an early age, and being
inherited at a corresponding age. RUDIMENTARY ORGANS; their origin explained.
Summary. From the first dawn of life, all organic beings are found to resemble
each other in descending degrees, so that they can be classed in groups under
groups. This classification is evidently not arbitrary like the grouping of the
stars in constellations. The existence of groups would have been of simple
signification, if one group had been exclusively fitted to inhabit the land, and
another the water; one to feed on flesh, another on vegetable matter, and so on;
but the case is widely different in nature; for it is notorious how commonly
members of even the same subgroup have different habits. In our second and
fourth chapters, on Variation and on Natural Selection, I have attempted to show
that it is the widely ranging, the much diffused and common, that is the
dominant species belonging to the larger genera, which vary most. The varieties,
or incipient species, thus produced ultimately become converted, as I believe,
into new and distinct species; and these, on the principle of inheritance, tend
to produce other new and dominant species. Consequently the groups which are now
large, and which generally include many dominant species, tend to go on
increasing indefinitely in size. I further attempted to show that from the
varying descendants of each species trying to occupy as many and as different
places as possible in the economy of nature, there is a constant tendency in
their characters to diverge. This conclusion was supported by looking at the
great diversity of the forms of life which, in any small area, come into the
closest competition, and by looking to certain facts in naturalisation. I
attempted also to show that there is a constant tendency in the forms which are
increasing in number and diverging in character, to supplant and exterminate the
less divergent, the less improved, and preceding forms. I request the reader to
turn to the diagram illustrating the action, as formerly explained, of these
several principles; and he will see that the inevitable result is that the
modified descendants proceeding from one progenitor become broken up into groups
subordinate to groups. In the diagram each letter on the uppermost line may
represent a genus including several species; and all the genera on this line
form together one class, for all have descended from one ancient but unseen
parent, and, consequently, have inherited something in common. But the three
genera on the left hand have, on this same principle, much in common, and form a
sub-family, distinct from that including the next two genera on the right hand,
which diverged from a common parent at the fifth stage of descent. These five
genera have also much, though less, in common; and they form a family distinct
from that including the three genera still further to the right hand, which
diverged at a still earlier period. And all these genera, descended from (A),
form an order distinct from the genera descended from (I). So that we here have
many species descended from a single progenitor grouped into genera; and the
genera are included in, or subordinate to, sub-families, families, and orders,
all united into one class. Thus, the grand fact in natural history of the
subordination of group under group, which, from its familiarity, does not always
sufficiently strike us, is in my judgment fully explained. Naturalists try to
arrange the species, genera, and families in each class, on what is called the
Natural System. But what is meant by this system? Some authors look at it merely
as a scheme for arranging together those living objects which are most alike,
and for separating those which are most unlike; or as an artificial means for
enunciating, as briefly as possible, general propositions,--that is, by one
sentence to give the characters common, for instance, to all mammals, by another
those common to all carnivora, by another those common to the dog-genus, and
then by adding a single sentence, a full description is given of each kind of
dog. The ingenuity and utility of this system are indisputable. But many
naturalists think that something more is meant by the Natural System; they
believe that it reveals the plan of the Creator; but unless it be specified
whether order in time or space, or what else is meant by the plan of the
Creator, it seems to me that nothing is thus added to our knowledge. Such
expressions as that famous one of Linnaeus, and which we often meet with in a
more or less concealed form, that the characters do not make the genus, but that
the genus gives the characters, seem to imply that something more is included in
our classification, than mere resemblance. I believe that something more is
included; and that propinquity of descent,--the only known cause of the
similarity of organic beings,--is the bond, hidden as it is by various degrees
of modification, which is partially revealed to us by our classifications. Let
us now consider the rules followed in classification, and the difficulties which
are encountered on the view that classification either gives some unknown plan
of creation, or is simply a scheme for enunciating general propositions and of
placing together the forms most like each other. It might have been thought (and
was in ancient times thought) that those parts of the structure which determined
the habits of life, and the general place of each being in the economy of
nature, would be of very high importance in classification. Nothing can be more
false. No one regards the external similarity of a mouse to a shrew, of a dugong
to a whale, of a whale to a fish, as of any importance. These resemblances,
though so intimately connected with the whole life of the being, are ranked as
merely "adaptive or analogical characters;" but to the consideration of these
resemblances we shall have to recur. It may even be given as a general rule,
that the less any part of the organisation is concerned with special habits, the
more important it becomes for classification. As an instance: Owen, in speaking
of the dugong, says, "The generative organs being those which are most remotely
related to the habits and food of an animal, I have always regarded as affording
very clear indications of its true affinities. We are least likely in the
modifications of these organs to mistake a merely adaptive for an essential
character." So with plants, how remarkable it is that the organs of vegetation,
on which their whole life depends, are of little signification, excepting in the
first main divisions; whereas the organs of reproduction, with their product the
seed, are of paramount importance! We must not, therefore, in classifying, trust
to resemblances in parts of the organisation, however important they may be for
the welfare of the being in relation to the outer world. Perhaps from this cause
it has partly arisen, that almost all naturalists lay the greatest stress on
resemblances in organs of high vital or physiological importance. No doubt this
view of the classificatory importance of organs which are important is
generally, but by no means always, true. But their importance for
classification, I believe, depends on their greater constancy throughout large
groups of species; and this constancy depends on such organs having generally
been subjected to less change in the adaptation of the species to their
conditions of life. That the mere physiological importance of an organ does not
determine its classificatory value, is almost shown by the one fact, that in
allied groups, in which the same organ, as we have every reason to suppose, has
nearly the same physiological value, its classificatory value is widely
different. No naturalist can have worked at any group without being struck with
this fact; and it has been most fully acknowledged in the writings of almost
every author. It will suffice to quote the highest authority, Robert Brown, who
in speaking of certain organs in the Proteaceae, says their generic importance,
"like that of all their parts, not only in this but, as I apprehend, in every
natural family, is very unequal, and in some cases seems to be entirely lost."
Again in another work he says, the genera of the Connaraceae "differ in having
one or more ovaria, in the existence or absence of albumen, in the imbricate or
valvular aestivation. Any one of these characters singly is frequently of more
than generic importance, though here even when all taken together they appear
insufficient to separate Cnestis from Connarus." To give an example amongst
insects, in one great division of the Hymenoptera, the antennae, as Westwood has
remarked, are most constant in structure; in another division they differ much,
and the differences are of quite subordinate value in classification; yet no one
probably will say that the antennae in these two divisions of the same order are
of unequal physiological importance. Any number of instances could be given of
the varying importance for classification of the same important organ within the
same group of beings. Again, no one will say that rudimentary or atrophied
organs are of high physiological or vital importance; yet, undoubtedly, organs
in this condition are often of high value in classification. No one will dispute
that the rudimentary teeth in the upper jaws of young ruminants, and certain
rudimentary bones of the leg, are highly serviceable in exhibiting the close
affinity between Ruminants and Pachyderms. Robert Brown has strongly insisted on
the fact that the rudimentary florets are of the highest importance in the
classification of the Grasses. Numerous instances could be given of characters
derived from parts which must be considered of very trifling physiological
importance, but which are universally admitted as highly serviceable in the
definition of whole groups. For instance, whether or not there is an open
passage from the nostrils to the mouth, the only character, according to Owen,
which absolutely distinguishes fishes and reptiles--the inflection of the angle
of the jaws in Marsupials--the manner in which the wings of insects are
folded--mere colour in certain Algae--mere pubescence on parts of the flower in
grasses--the nature of the dermal covering, as hair or feathers, in the
Vertebrata. If the Ornithorhynchus had been covered with feathers instead of
hair, this external and trifling character would, I think, have been considered
by naturalists as important an aid in determining the degree of affinity of this
strange creature to birds and reptiles, as an approach in structure in any one
internal and important organ. The importance, for classification, of trifling
characters, mainly depends on their being correlated with several other
characters of more or less importance. The value indeed of an aggregate of
characters is very evident in natural history. Hence, as has often been
remarked, a species may depart from its allies in several characters, both of
high physiological importance and of almost universal prevalence, and yet leave
us in no doubt where it should be ranked. Hence, also, it has been found, that a
classification founded on any single character, however important that may be,
has always failed; for no part of the organisation is universally constant. The
importance of an aggregate of characters, even when none are important, alone
explains, I think, that saying of Linnaeus, that the characters do not give the
genus, but the genus gives the characters; for this saying seems founded on an
appreciation of many trifling points of resemblance, too slight to be defined.
Certain plants, belonging to the Malpighiaceae, bear perfect and degraded
flowers; in the latter, as A. de Jussieu has remarked, "the greater number of
the characters proper to the species, to the genus, to the family, to the class,
disappear, and thus laugh at our classification." But when Aspicarpa produced in
France, during several years, only degraded flowers, departing so wonderfully in
a number of the most important points of structure from the proper type of the
order, yet M. Richard sagaciously saw, as Jussieu observes, that this genus
should still be retained amongst the Malpighiaceae. This case seems to me well
to illustrate the spirit with which our classifications are sometimes
necessarily founded. Practically when naturalists are at work, they do not
trouble themselves about the physiological value of the characters which they
use in defining a group, or in allocating any particular species. If they find a
character nearly uniform, and common to a great number of forms, and not common
to others, they use it as one of high value; if common to some lesser number,
they use it as of subordinate value. This principle has been broadly confessed
by some naturalists to be the true one; and by none more clearly than by that
excellent botanist, Aug. St. Hilaire. If certain characters are always found
correlated with others, though no apparent bond of connexion can be discovered
between them, especial value is set on them. As in most groups of animals,
important organs, such as those for propelling the blood, or for aerating it, or
those for propagating the race, are found nearly uniform, they are considered as
highly serviceable in classification; but in some groups of animals all these,
the most important vital organs, are found to offer characters of quite
subordinate value. We can see why characters derived from the embryo should be
of equal importance with those derived from the adult, for our classifications
of course include all ages of each species. But it is by no means obvious, on
the ordinary view, why the structure of the embryo should be more important for
this purpose than that of the adult, which alone plays its full part in the
economy of nature. Yet it has been strongly urged by those great naturalists,
Milne Edwards and Agassiz, that embryonic characters are the most important of
any in the classification of animals; and this doctrine has very generally been
admitted as true. The same fact holds good with flowering plants, of which the
two main divisions have been founded on characters derived from the embryo,--on
the number and position of the embryonic leaves or cotyledons, and on the mode
of development of the plumule and radicle. In our discussion on embryology, we
shall see why such characters are so valuable, on the view of classification
tacitly including the idea of descent. Our classifications are often plainly
influenced by chains of affinities. Nothing can be easier than to define a
number of characters common to all birds; but in the case of crustaceans, such
definition has hitherto been found impossible. There are crustaceans at the
opposite ends of the series, which have hardly a character in common; yet the
species at both ends, from being plainly allied to others, and these to others,
and so onwards, can be recognised as unequivocally belonging to this, and to no
other class of the Articulata. Geographical distribution has often been used,
though perhaps not quite logically, in classification, more especially in very
large groups of closely allied forms. Temminck insists on the utility or even
necessity of this practice in certain groups of birds; and it has been followed
by several entomologists and botanists. Finally, with respect to the comparative
value of the various groups of species, such as orders, sub-orders, families,
sub-families, and genera, they seem to be, at least at present, almost
arbitrary. Several of the best botanists, such as Mr. Bentham and others, have
strongly insisted on their arbitrary value. Instances could be given amongst
plants and insects, of a group of forms, first ranked by practised naturalists
as only a genus, and then raised to the rank of a sub-family or family; and this
has been done, not because further research has detected important structural
differences, at first overlooked, but because numerous allied species, with
slightly different grades of difference, have been subsequently discovered. All
the foregoing rules and aids and difficulties in classification are explained,
if I do not greatly deceive myself, on the view that the natural system is
founded on descent with modification; that the characters which naturalists
consider as showing true affinity between any two or more species, are those
which have been inherited from a common parent, and, in so far, all true
classification is genealogical; that community of descent is the hidden bond
which naturalists have been unconsciously seeking, and not some unknown plan of
creation, or the enunciation of general propositions, and the mere putting
together and separating objects more or less alike. But I must explain my
meaning more fully. I believe that the ARRANGEMENT of the groups within each
class, in due subordination and relation to the other groups, must be strictly
genealogical in order to be natural; but that the AMOUNT of difference in the
several branches or groups, though allied in the same degree in blood to their
common progenitor, may differ greatly, being due to the different degrees of
modification which they have undergone; and this is expressed by the forms being
ranked under different genera, families, sections, or orders. The reader will
best understand what is meant, if he will take the trouble of referring to the
diagram in the fourth chapter. We will suppose the letters A to L to represent
allied genera, which lived during the Silurian epoch, and these have descended
from a species which existed at an unknown anterior period. Species of three of
these genera (A, F, and I) have transmitted modified descendants to the present
day, represented by the fifteen genera (a14 to z14) on the uppermost horizontal
line. Now all these modified descendants from a single species, are represented
as related in blood or descent to the same degree; they may metaphorically be
called cousins to the same millionth degree; yet they differ widely and in
different degrees from each other. The forms descended from A, now broken up
into two or three families, constitute a distinct order from those descended
from I, also broken up into two families. Nor can the existing species,
descended from A, be ranked in the same genus with the parent A; or those from
I, with the parent I. But the existing genus F14 may be supposed to have been
but slightly modified; and it will then rank with the parent-genus F; just as
some few still living organic beings belong to Silurian genera. So that the
amount or value of the differences between organic beings all related to each
other in the same degree in blood, has come to be widely different. Nevertheless
their genealogical ARRANGEMENT remains strictly true, not only at the present
time, but at each successive period of descent. All the modified descendants
from A will have inherited something in common from their common parent, as will
all the descendants from I; so will it be with each subordinate branch of
descendants, at each successive period. If, however, we choose to suppose that
any of the descendants of A or of I have been so much modified as to have more
or less completely lost traces of their parentage, in this case, their places in
a natural classification will have been more or less completely lost,--as
sometimes seems to have occurred with existing organisms. All the descendants of
the genus F, along its whole line of descent, are supposed to have been but
little modified, and they yet form a single genus. But this genus, though much
isolated, will still occupy its proper intermediate position; for F originally
was intermediate in character between A and I, and the several genera descended
from these two genera will have inherited to a certain extent their characters.
This natural arrangement is shown, as far as is possible on paper, in the
diagram, but in much too simple a manner. If a branching diagram had not been
used, and only the names of the groups had been written in a linear series, it
would have been still less possible to have given a natural arrangement; and it
is notoriously not possible to represent in a series, on a flat surface, the
affinities which we discover in nature amongst the beings of the same group.
Thus, on the view which I hold, the natural system is genealogical in its
arrangement, like a pedigree; but the degrees of modification which the
different groups have undergone, have to be expressed by ranking them under
different so-called genera, sub-families, families, sections, orders, and
classes. It may be worth while to illustrate this view of classification, by
taking the case of languages. If we possessed a perfect pedigree of mankind, a
genealogical arrangement of the races of man would afford the best
classification of the various languages now spoken throughout the world; and if
all extinct languages, and all intermediate and slowly changing dialects, had to
be included, such an arrangement would, I think, be the only possible one. Yet
it might be that some very ancient language had altered little, and had given
rise to few new languages, whilst others (owing to the spreading and subsequent
isolation and states of civilisation of the several races, descended from a
common race) had altered much, and had given rise to many new languages and
dialects. The various degrees of difference in the languages from the same
stock, would have to be expressed by groups subordinate to groups; but the
proper or even only possible arrangement would still be genealogical; and this
would be strictly natural, as it would connect together all languages, extinct
and modern, by the closest affinities, and would give the filiation and origin
of each tongue. In confirmation of this view, let us glance at the
classification of varieties, which are believed or known to have descended from
one species. These are grouped under species, with sub-varieties under
varieties; and with our domestic productions, several other grades of difference
are requisite, as we have seen with pigeons. The origin of the existence of
groups subordinate to groups, is the same with varieties as with species,
namely, closeness of descent with various degrees of modification. Nearly the
same rules are followed in classifying varieties, as with species. Authors have
insisted on the necessity of classing varieties on a natural instead of an
artificial system; we are cautioned, for instance, not to class two varieties of
the pine-apple together, merely because their fruit, though the most important
part, happens to be nearly identical; no one puts the swedish and common turnips
together, though the esculent and thickened stems are so similar. Whatever part
is found to be most constant, is used in classing varieties: thus the great
agriculturist Marshall says the horns are very useful for this purpose with
cattle, because they are less variable than the shape or colour of the body,
etc.; whereas with sheep the horns are much less serviceable, because less
constant. In classing varieties, I apprehend if we had a real pedigree, a
genealogical classification would be universally preferred; and it has been
attempted by some authors. For we might feel sure, whether there had been more
or less modification, the principle of inheritance would keep the forms together
which were allied in the greatest number of points. In tumbler pigeons, though
some sub-varieties differ from the others in the important character of having a
longer beak, yet all are kept together from having the common habit of tumbling;
but the short-faced breed has nearly or quite lost this habit; nevertheless,
without any reasoning or thinking on the subject, these tumblers are kept in the
same group, because allied in blood and alike in some other respects. If it
could be proved that the Hottentot had descended from the Negro, I think he
would be classed under the Negro group, however much he might differ in colour
and other important characters from negroes. With species in a state of nature,
every naturalist has in fact brought descent into his classification; for he
includes in his lowest grade, or that of a species, the two sexes; and how
enormously these sometimes differ in the most important characters, is known to
every naturalist: scarcely a single fact can be predicated in common of the
males and hermaphrodites of certain cirripedes, when adult, and yet no one
dreams of separating them. The naturalist includes as one species the several
larval stages of the same individual, however much they may differ from each
other and from the adult; as he likewise includes the so-called alternate
generations of Steenstrup, which can only in a technical sense be considered as
the same individual. He includes monsters; he includes varieties, not solely
because they closely resemble the parent-form, but because they are descended
from it. He who believes that the cowslip is descended from the primrose, or
conversely, ranks them together as a single species, and gives a single
definition. As soon as three Orchidean forms (Monochanthus, Myanthus, and
Catasetum), which had previously been ranked as three distinct genera, were
known to be sometimes produced on the same spike, they were immediately included
as a single species. But it may be asked, what ought we to do, if it could be
proved that one species of kangaroo had been produced, by a long course of
modification, from a bear? Ought we to rank this one species with bears, and
what should we do with the other species? The supposition is of course
preposterous; and I might answer by the argumentum ad hominem, and ask what
should be done if a perfect kangaroo were seen to come out of the womb of a
bear? According to all analogy, it would be ranked with bears; but then
assuredly all the other species of the kangaroo family would have to be classed
under the bear genus. The whole case is preposterous; for where there has been
close descent in common, there will certainly be close resemblance or affinity.
As descent has universally been used in classing together the individuals of the
same species, though the males and females and larvae are sometimes extremely
different; and as it has been used in classing varieties which have undergone a
certain, and sometimes a considerable amount of modification, may not this same
element of descent have been unconsciously used in grouping species under
genera, and genera under higher groups, though in these cases the modification
has been greater in degree, and has taken a longer time to complete? I believe
it has thus been unconsciously used; and only thus can I understand the several
rules and guides which have been followed by our best systematists. We have no
written pedigrees; we have to make out community of descent by resemblances of
any kind. Therefore we choose those characters which, as far as we can judge,
are the least likely to have been modified in relation to the conditions of life
to which each species has been recently exposed. Rudimentary structures on this
view are as good as, or even sometimes better than, other parts of the
organisation. We care not how trifling a character may be--let it be the mere
inflection of the angle of the jaw, the manner in which an insect's wing is
folded, whether the skin be covered by hair or feathers--if it prevail
throughout many and different species, especially those having very different
habits of life, it assumes high value; for we can account for its presence in so
many forms with such different habits, only by its inheritance from a common
parent. We may err in this respect in regard to single points of structure, but
when several characters, let them be ever so trifling, occur together throughout
a large group of beings having different habits, we may feel almost sure, on the
theory of descent, that these characters have been inherited from a common
ancestor. And we know that such correlated or aggregated characters have
especial value in classification. We can understand why a species or a group of
species may depart, in several of its most important characteristics, from its
allies, and yet be safely classed with them. This may be safely done, and is
often done, as long as a sufficient number of characters, let them be ever so
unimportant, betrays the hidden bond of community of descent. Let two forms have
not a single character in common, yet if these extreme forms are connected
together by a chain of intermediate groups, we may at once infer their community
of descent, and we put them all into the same class. As we find organs of high
physiological importance--those which serve to preserve life under the most
diverse conditions of existence--are generally the most constant, we attach
especial value to them; but if these same organs, in another group or section of
a group, are found to differ much, we at once value them less in our
classification. We shall hereafter, I think, clearly see why embryological
characters are of such high classificatory importance. Geographical distribution
may sometimes be brought usefully into play in classing large and
widely-distributed genera, because all the species of the same genus, inhabiting
any distinct and isolated region, have in all probability descended from the
same parents. We can understand, on these views, the very important distinction
between real affinities and analogical or adaptive resemblances. Lamarck first
called attention to this distinction, and he has been ably followed by Macleay
and others. The resemblance, in the shape of the body and in the fin-like
anterior limbs, between the dugong, which is a pachydermatous animal, and the
whale, and between both these mammals and fishes, is analogical. Amongst insects
there are innumerable instances: thus Linnaeus, misled by external appearances,
actually classed an homopterous insect as a moth. We see something of the same
kind even in our domestic varieties, as in the thickened stems of the common and
swedish turnip. The resemblance of the greyhound and racehorse is hardly more
fanciful than the analogies which have been drawn by some authors between very
distinct animals. On my view of characters being of real importance for
classification, only in so far as they reveal descent, we can clearly understand
why analogical or adaptive character, although of the utmost importance to the
welfare of the being, are almost valueless to the systematist. For animals,
belonging to two most distinct lines of descent, may readily become adapted to
similar conditions, and thus assume a close external resemblance; but such
resemblances will not reveal--will rather tend to conceal their
blood-relationship to their proper lines of descent. We can also understand the
apparent paradox, that the very same characters are analogical when one class or
order is compared with another, but give true affinities when the members of the
same class or order are compared one with another: thus the shape of the body
and fin-like limbs are only analogical when whales are compared with fishes,
being adaptations in both classes for swimming through the water; but the shape
of the body and fin-like limbs serve as characters exhibiting true affinity
between the several members of the whale family; for these cetaceans agree in so
many characters, great and small, that we cannot doubt that they have inherited
their general shape of body and structure of limbs from a common ancestor. So it
is with fishes. As members of distinct classes have often been adapted by
successive slight modifications to live under nearly similar circumstances,--to
inhabit for instance the three elements of land, air, and water,--we can perhaps
understand how it is that a numerical parallelism has sometimes been observed
between the sub-groups in distinct classes. A naturalist, struck by a
parallelism of this nature in any one class, by arbitrarily raising or sinking
the value of the groups in other classes (and all our experience shows that this
valuation has hitherto been arbitrary), could easily extend the parallelism over
a wide range; and thus the septenary, quinary, quaternary, and ternary
classifications have probably arisen. As the modified descendants of dominant
species, belonging to the larger genera, tend to inherit the advantages, which
made the groups to which they belong large and their parents dominant, they are
almost sure to spread widely, and to seize on more and more places in the
economy of nature. The larger and more dominant groups thus tend to go on
increasing in size; and they consequently supplant many smaller and feebler
groups. Thus we can account for the fact that all organisms, recent and extinct,
are included under a few great orders, under still fewer classes, and all in one
great natural system. As showing how few the higher groups are in number, and
how widely spread they are throughout the world, the fact is striking, that the
discovery of Australia has not added a single insect belonging to a new order;
and that in the vegetable kingdom, as I learn from Dr. Hooker, it has added only
two or three orders of small size. In the chapter on geological succession I
attempted to show, on the principle of each group having generally diverged much
in character during the long-continued process of modification, how it is that
the more ancient forms of life often present characters in some slight degree
intermediate between existing groups. A few old and intermediate parent-forms
having occasionally transmitted to the present day descendants but little
modified, will give to us our so-called osculant or aberrant groups. The more
aberrant any form is, the greater must be the number of connecting forms which
on my theory have been exterminated and utterly lost. And we have some evidence
of aberrant forms having suffered severely from extinction, for they are
generally represented by extremely few species; and such species as do occur are
generally very distinct from each other, which again implies extinction. The
genera Ornithorhynchus and Lepidosiren, for example, would not have been less
aberrant had each been represented by a dozen species instead of by a single
one; but such richness in species, as I find after some investigation, does not
commonly fall to the lot of aberrant genera. We can, I think, account for this
fact only by looking at aberrant forms as failing groups conquered by more
successful competitors, with a few members preserved by some unusual coincidence
of favourable circumstances. Mr. Waterhouse has remarked that, when a member
belonging to one group of animals exhibits an affinity to a quite distinct
group, this affinity in most cases is general and not special: thus, according
to Mr. Waterhouse, of all Rodents, the bizcacha is most nearly related to
Marsupials; but in the points in which it approaches this order, its relations
are general, and not to any one marsupial species more than to another. As the
points of affinity of the bizcacha to Marsupials are believed to be real and not
merely adaptive, they are due on my theory to inheritance in common. Therefore
we must suppose either that all Rodents, including the bizcacha, branched off
from some very ancient Marsupial, which will have had a character in some degree
intermediate with respect to all existing Marsupials; or that both Rodents and
Marsupials branched off from a common progenitor, and that both groups have
since undergone much modification in divergent directions. On either view we may
suppose that the bizcacha has retained, by inheritance, more of the character of
its ancient progenitor than have other Rodents; and therefore it will not be
specially related to any one existing Marsupial, but indirectly to all or nearly
all Marsupials, from having partially retained the character of their common
progenitor, or of an early member of the group. On the other hand, of all
Marsupials, as Mr. Waterhouse has remarked, the phascolomys resembles most
nearly, not any one species, but the general order of Rodents. In this case,
however, it may be strongly suspected that the resemblance is only analogical,
owing to the phascolomys having become adapted to habits like those of a Rodent.
The elder De Candolle has made nearly similar observations on the general nature
of the affinities of distinct orders of plants. On the principle of the
multiplication and gradual divergence in character of the species descended from
a common parent, together with their retention by inheritance of some characters
in common, we can understand the excessively complex and radiating affinities by
which all the members of the same family or higher group are connected together.
For the common parent of a whole family of species, now broken up by extinction
into distinct groups and sub-groups, will have transmitted some of its
characters, modified in various ways and degrees, to all; and the several
species will consequently be related to each other by circuitous lines of
affinity of various lengths (as may be seen in the diagram so often referred
to), mounting up through many predecessors. As it is difficult to show the
blood-relationship between the numerous kindred of any ancient and noble family,
even by the aid of a genealogical tree, and almost impossible to do this without
this aid, we can understand the extraordinary difficulty which naturalists have
experienced in describing, without the aid of a diagram, the various affinities
which they perceive between the many living and extinct members of the same
great natural class. Extinction, as we have seen in the fourth chapter, has
played an important part in defining and widening the intervals between the
several groups in each class. We may thus account even for the distinctness of
whole classes from each other--for instance, of birds from all other vertebrate
animals--by the belief that many ancient forms of life have been utterly lost,
through which the early progenitors of birds were formerly connected with the
early progenitors of the other vertebrate classes. There has been less entire
extinction of the forms of life which once connected fishes with batrachians.
There has been still less in some other classes, as in that of the Crustacea,
for here the most wonderfully diverse forms are still tied together by a long,
but broken, chain of affinities. Extinction has only separated groups: it has by
no means made them; for if every form which has ever lived on this earth were
suddenly to reappear, though it would be quite impossible to give definitions by
which each group could be distinguished from other groups, as all would blend
together by steps as fine as those between the finest existing varieties,
nevertheless a natural classification, or at least a natural arrangement, would
be possible. We shall see this by turning to the diagram: the letters, A to L,
may represent eleven Silurian genera, some of which have produced large groups
of modified descendants. Every intermediate link between these eleven genera and
their primordial parent, and every intermediate link in each branch and
sub-branch of their descendants, may be supposed to be still alive; and the
links to be as fine as those between the finest varieties. In this case it would
be quite impossible to give any definition by which the several members of the
several groups could be distinguished from their more immediate parents; or
these parents from their ancient and unknown progenitor. Yet the natural
arrangement in the diagram would still hold good; and, on the principle of
inheritance, all the forms descended from A, or from I, would have something in
common. In a tree we can specify this or that branch, though at the actual fork
the two unite and blend together. We could not, as I have said, define the
several groups; but we could pick out types, or forms, representing most of the
characters of each group, whether large or small, and thus give a general idea
of the value of the differences between them. This is what we should be driven
to, if we were ever to succeed in collecting all the forms in any class which
have lived throughout all time and space. We shall certainly never succeed in
making so perfect a collection: nevertheless, in certain classes, we are tending
in this direction; and Milne Edwards has lately insisted, in an able paper, on
the high importance of looking to types, whether or not we can separate and
define the groups to which such types belong. Finally, we have seen that natural
selection, which results from the struggle for existence, and which almost
inevitably induces extinction and divergence of character in the many
descendants from one dominant parent-species, explains that great and universal
feature in the affinities of all organic beings, namely, their subordination in
group under group. We use the element of descent in classing the individuals of
both sexes and of all ages, although having few characters in common, under one
species; we use descent in classing acknowledged varieties, however different
they may be from their parent; and I believe this element of descent is the
hidden bond of connexion which naturalists have sought under the term of the
Natural System. On this idea of the natural system being, in so far as it has
been perfected, genealogical in its arrangement, with the grades of difference
between the descendants from a common parent, expressed by the terms genera,
families, orders, etc., we can understand the rules which we are compelled to
follow in our classification. We can understand why we value certain
resemblances far more than others; why we are permitted to use rudimentary and
useless organs, or others of trifling physiological importance; why, in
comparing one group with a distinct group, we summarily reject analogical or
adaptive characters, and yet use these same characters within the limits of the
same group. We can clearly see how it is that all living and extinct forms can
be grouped together in one great system; and how the several members of each
class are connected together by the most complex and radiating lines of
affinities. We shall never, probably, disentangle the inextricable web of
affinities between the members of any one class; but when we have a distinct
object in view, and do not look to some unknown plan of creation, we may hope to
make sure but slow progress. MORPHOLOGY. We have seen that the members of the
same class, independently of their habits of life, resemble each other in the
general plan of their organisation. This resemblance is often expressed by the
term "unity of type;" or by saying that the several parts and organs in the
different species of the class are homologous. The whole subject is included
under the general name of Morphology. This is the most interesting department of
natural history, and may be said to be its very soul. What can be more curious
than that the hand of a man, formed for grasping, that of a mole for digging,
the leg of the horse, the paddle of the porpoise, and the wing of the bat,
should all be constructed on the same pattern, and should include the same
bones, in the same relative positions? Geoffroy St. Hilaire has insisted
strongly on the high importance of relative connexion in homologous organs: the
parts may change to almost any extent in form and size, and yet they always
remain connected together in the same order. We never find, for instance, the
bones of the arm and forearm, or of the thigh and leg, transposed. Hence the
same names can be given to the homologous bones in widely different animals. We
see the same great law in the construction of the mouths of insects: what can be
more different than the immensely long spiral proboscis of a sphinx-moth, the
curious folded one of a bee or bug, and the great jaws of a beetle?--yet all
these organs, serving for such different purposes, are formed by infinitely
numerous modifications of an upper lip, mandibles, and two pairs of maxillae.
Analogous laws govern the construction of the mouths and limbs of crustaceans.
So it is with the flowers of plants. Nothing can be more hopeless than to
attempt to explain this similarity of pattern in members of the same class, by
utility or by the doctrine of final causes. The hopelessness of the attempt has
been expressly admitted by Owen in his most interesting work on the 'Nature of
Limbs.' On the ordinary view of the independent creation of each being, we can
only say that so it is;--that it has so pleased the Creator to construct each
animal and plant. The explanation is manifest on the theory of the natural
selection of successive slight modifications,--each modification being
profitable in some way to the modified form, but often affecting by correlation
of growth other parts of the organisation. In changes of this nature, there will
be little or no tendency to modify the original pattern, or to transpose parts.
The bones of a limb might be shortened and widened to any extent, and become
gradually enveloped in thick membrane, so as to serve as a fin; or a webbed foot
might have all its bones, or certain bones, lengthened to any extent, and the
membrane connecting them increased to any extent, so as to serve as a wing: yet
in all this great amount of modification there will be no tendency to alter the
framework of bones or the relative connexion of the several parts. If we suppose
that the ancient progenitor, the archetype as it may be called, of all mammals,
had its limbs constructed on the existing general pattern, for whatever purpose
they served, we can at once perceive the plain signification of the homologous
construction of the limbs throughout the whole class. So with the mouths of
insects, we have only to suppose that their common progenitor had an upper lip,
mandibles, and two pair of maxillae, these parts being perhaps very simple in
form; and then natural selection will account for the infinite diversity in
structure and function of the mouths of insects. Nevertheless, it is conceivable
that the general pattern of an organ might become so much obscured as to be
finally lost, by the atrophy and ultimately by the complete abortion of certain
parts, by the soldering together of other parts, and by the doubling or
multiplication of others,--variations which we know to be within the limits of
possibility. In the paddles of the extinct gigantic sea-lizards, and in the
mouths of certain suctorial crustaceans, the general pattern seems to have been
thus to a certain extent obscured. There is another and equally curious branch
of the present subject; namely, the comparison not of the same part in different
members of a class, but of the different parts or organs in the same individual.
Most physiologists believe that the bones of the skull are homologous with--that
is correspond in number and in relative connexion with--the elemental parts of a
certain number of vertebrae. The anterior and posterior limbs in each member of
the vertebrate and articulate classes are plainly homologous. We see the same
law in comparing the wonderfully complex jaws and legs in crustaceans. It is
familiar to almost every one, that in a flower the relative position of the
sepals, petals, stamens, and pistils, as well as their intimate structure, are
intelligible on the view that they consist of metamorphosed leaves, arranged in
a spire. In monstrous plants, we often get direct evidence of the possibility of
one organ being transformed into another; and we can actually see in embryonic
crustaceans and in many other animals, and in flowers, that organs, which when
mature become extremely different, are at an early stage of growth exactly
alike. How inexplicable are these facts on the ordinary view of creation! Why
should the brain be enclosed in a box composed of such numerous and such
extraordinarily shaped pieces of bone? As Owen has remarked, the benefit derived
from the yielding of the separate pieces in the act of parturition of mammals,
will by no means explain the same construction in the skulls of birds. Why
should similar bones have been created in the formation of the wing and leg of a
bat, used as they are for such totally different purposes? Why should one
crustacean, which has an extremely complex mouth formed of many parts,
consequently always have fewer legs; or conversely, those with many legs have
simpler mouths? Why should the sepals, petals, stamens, and pistils in any
individual flower, though fitted for such widely different purposes, be all
constructed on the same pattern? On the theory of natural selection, we can
satisfactorily answer these questions. In the vertebrata, we see a series of
internal vertebrae bearing certain processes and appendages; in the articulata,
we see the body divided into a series of segments, bearing external appendages;
and in flowering plants, we see a series of successive spiral whorls of leaves.
An indefinite repetition of the same part or organ is the common characteristic
(as Owen has observed) of all low or little-modified forms; therefore we may
readily believe that the unknown progenitor of the vertebrata possessed many
vertebrae; the unknown progenitor of the articulata, many segments; and the
unknown progenitor of flowering plants, many spiral whorls of leaves. We have
formerly seen that parts many times repeated are eminently liable to vary in
number and structure; consequently it is quite probable that natural selection,
during a long-continued course of modification, should have seized on a certain
number of the primordially similar elements, many times repeated, and have
adapted them to the most diverse purposes. And as the whole amount of
modification will have been effected by slight successive steps, we need not
wonder at discovering in such parts or organs, a certain degree of fundamental
resemblance, retained by the strong principle of inheritance. In the great class
of molluscs, though we can homologise the parts of one species with those of
another and distinct species, we can indicate but few serial homologies; that
is, we are seldom enabled to say that one part or organ is homologous with
another in the same individual. And we can understand this fact; for in
molluscs, even in the lowest members of the class, we do not find nearly so much
indefinite repetition of any one part, as we find in the other great classes of
the animal and vegetable kingdoms. Naturalists frequently speak of the skull as
formed of metamorphosed vertebrae: the jaws of crabs as metamorphosed legs; the
stamens and pistils of flowers as metamorphosed leaves; but it would in these
cases probably be more correct, as Professor Huxley has remarked, to speak of
both skull and vertebrae, both jaws and legs, etc.,--as having been
metamorphosed, not one from the other, but from some common element.
Naturalists, however, use such language only in a metaphorical sense: they are
far from meaning that during a long course of descent, primordial organs of any
kind--vertebrae in the one case and legs in the other--have actually been
modified into skulls or jaws. Yet so strong is the appearance of a modification
of this nature having occurred, that naturalists can hardly avoid employing
language having this plain signification. On my view these terms may be used
literally; and the wonderful fact of the jaws, for instance, of a crab retaining
numerous characters, which they would probably have retained through
inheritance, if they had really been metamorphosed during a long course of
descent from true legs, or from some simple appendage, is explained. EMBRYOLOGY.
It has already been casually remarked that certain organs in the individual,
which when mature become widely different and serve for different purposes, are
in the embryo exactly alike. The embryos, also, of distinct animals within the
same class are often strikingly similar: a better proof of this cannot be given,
than a circumstance mentioned by Agassiz, namely, that having forgotten to
ticket the embryo of some vertebrate animal, he cannot now tell whether it be
that of a mammal, bird, or reptile. The vermiform larvae of moths, flies,
beetles, etc., resemble each other much more closely than do the mature insects;
but in the case of larvae, the embryos are active, and have been adapted for
special lines of life. A trace of the law of embryonic resemblance, sometimes
lasts till a rather late age: thus birds of the same genus, and of closely
allied genera, often resemble each other in their first and second plumage; as
we see in the spotted feathers in the thrush group. In the cat tribe, most of
the species are striped or spotted in lines; and stripes can be plainly
distinguished in the whelp of the lion. We occasionally though rarely see
something of this kind in plants: thus the embryonic leaves of the ulex or
furze, and the first leaves of the phyllodineous acaceas, are pinnate or divided
like the ordinary leaves of the leguminosae. The points of structure, in which
the embryos of widely different animals of the same class resemble each other,
often have no direct relation to their conditions of existence. We cannot, for
instance, suppose that in the embryos of the vertebrata the peculiar loop-like
course of the arteries near the branchial slits are related to similar
conditions,--in the young mammal which is nourished in the womb of its mother,
in the egg of the bird which is hatched in a nest, and in the spawn of a frog
under water. We have no more reason to believe in such a relation, than we have
to believe that the same bones in the hand of a man, wing of a bat, and fin of a
porpoise, are related to similar conditions of life. No one will suppose that
the stripes on the whelp of a lion, or the spots on the young blackbird, are of
any use to these animals, or are related to the conditions to which they are
exposed. The case, however, is different when an animal during any part of its
embryonic career is active, and has to provide for itself. The period of
activity may come on earlier or later in life; but whenever it comes on, the
adaptation of the larva to its conditions of life is just as perfect and as
beautiful as in the adult animal. From such special adaptations, the similarity
of the larvae or active embryos of allied animals is sometimes much obscured;
and cases could be given of the larvae of two species, or of two groups of
species, differing quite as much, or even more, from each other than do their
adult parents. In most cases, however, the larvae, though active, still obey
more or less closely the law of common embryonic resemblance. Cirripedes afford
a good instance of this: even the illustrious Cuvier did not perceive that a
barnacle was, as it certainly is, a crustacean; but a glance at the larva shows
this to be the case in an unmistakeable manner. So again the two main divisions
of cirripedes, the pedunculated and sessile, which differ widely in external
appearance, have larvae in all their several stages barely distinguishable. The
embryo in the course of development generally rises in organisation: I use this
expression, though I am aware that it is hardly possible to define clearly what
is meant by the organisation being higher or lower. But no one probably will
dispute that the butterfly is higher than the caterpillar. In some cases,
however, the mature animal is generally considered as lower in the scale than
the larva, as with certain parasitic crustaceans. To refer once again to
cirripedes: the larvae in the first stage have three pairs of legs, a very
simple single eye, and a probosciformed mouth, with which they feed largely, for
they increase much in size. In the second stage, answering to the chrysalis
stage of butterflies, they have six pairs of beautifully constructed natatory
legs, a pair of magnificent compound eyes, and extremely complex antennae; but
they have a closed and imperfect mouth, and cannot feed: their function at this
stage is, to search by their well-developed organs of sense, and to reach by
their active powers of swimming, a proper place on which to become attached and
to undergo their final metamorphosis. When this is completed they are fixed for
life: their legs are now converted into prehensile organs; they again obtain a
well-constructed mouth; but they have no antennae, and their two eyes are now
reconverted into a minute, single, and very simple eye-spot. In this last and
complete state, cirripedes may be considered as either more highly or more lowly
organised than they were in the larval condition. But in some genera the larvae
become developed either into hermaphrodites having the ordinary structure, or
into what I have called complemental males: and in the latter, the development
has assuredly been retrograde; for the male is a mere sack, which lives for a
short time, and is destitute of mouth, stomach, or other organ of importance,
excepting for reproduction. We are so much accustomed to see differences in
structure between the embryo and the adult, and likewise a close similarity in
the embryos of widely different animals within the same class, that we might be
led to look at these facts as necessarily contingent in some manner on growth.
But there is no obvious reason why, for instance, the wing of a bat, or the fin
of a porpoise, should not have been sketched out with all the parts in proper
proportion, as soon as any structure became visible in the embryo. And in some
whole groups of animals and in certain members of other groups, the embryo does
not at any period differ widely from the adult: thus Owen has remarked in regard
to cuttle-fish, "there is no metamorphosis; the cephalopodic character is
manifested long before the parts of the embryo are completed;" and again in
spiders, "there is nothing worthy to be called a metamorphosis." The larvae of
insects, whether adapted to the most diverse and active habits, or quite
inactive, being fed by their parents or placed in the midst of proper nutriment,
yet nearly all pass through a similar worm-like stage of development; but in
some few cases, as in that of Aphis, if we look to the admirable drawings by
Professor Huxley of the development of this insect, we see no trace of the
vermiform stage. How, then, can we explain these several facts in
embryology,--namely the very general, but not universal difference in structure
between the embryo and the adult;--of parts in the same individual embryo, which
ultimately become very unlike and serve for diverse purposes, being at this
early period of growth alike;--of embryos of different species within the same
class, generally, but not universally, resembling each other;--of the structure
of the embryo not being closely related to its conditions of existence, except
when the embryo becomes at any period of life active and has to provide for
itself;--of the embryo apparently having sometimes a higher organisation than
the mature animal, into which it is developed. I believe that all these facts
can be explained, as follows, on the view of descent with modification. It is
commonly assumed, perhaps from monstrosities often affecting the embryo at a
very early period, that slight variations necessarily appear at an equally early
period. But we have little evidence on this head--indeed the evidence rather
points the other way; for it is notorious that breeders of cattle, horses, and
various fancy animals, cannot positively tell, until some time after the animal
has been born, what its merits or form will ultimately turn out. We see this
plainly in our own children; we cannot always tell whether the child will be
tall or short, or what its precise features will be. The question is not, at
what period of life any variation has been caused, but at what period it is
fully displayed. The cause may have acted, and I believe generally has acted,
even before the embryo is formed; and the variation may be due to the male and
female sexual elements having been affected by the conditions to which either
parent, or their ancestors, have been exposed. Nevertheless an effect thus
caused at a very early period, even before the formation of the embryo, may
appear late in life; as when an hereditary disease, which appears in old age
alone, has been communicated to the offspring from the reproductive element of
one parent. Or again, as when the horns of cross-bred cattle have been affected
by the shape of the horns of either parent. For the welfare of a very young
animal, as long as it remains in its mother's womb, or in the egg, or as long as
it is nourished and protected by its parent, it must be quite unimportant
whether most of its characters are fully acquired a little earlier or later in
life. It would not signify, for instance, to a bird which obtained its food best
by having a long beak, whether or not it assumed a beak of this particular
length, as long as it was fed by its parents. Hence, I conclude, that it is
quite possible, that each of the many successive modifications, by which each
species has acquired its present structure, may have supervened at a not very
early period of life; and some direct evidence from our domestic animals
supports this view. But in other cases it is quite possible that each successive
modification, or most of them, may have appeared at an extremely early period. I
have stated in the first chapter, that there is some evidence to render it
probable, that at whatever age any variation first appears in the parent, it
tends to reappear at a corresponding age in the offspring. Certain variations
can only appear at corresponding ages, for instance, peculiarities in the
caterpillar, cocoon, or imago states of the silk-moth; or, again, in the horns
of almost full-grown cattle. But further than this, variations which, for all
that we can see, might have appeared earlier or later in life, tend to appear at
a corresponding age in the offspring and parent. I am far from meaning that this
is invariably the case; and I could give a good many cases of variations (taking
the word in the largest sense) which have supervened at an earlier age in the
child than in the parent. These two principles, if their truth be admitted,
will, I believe, explain all the above specified leading facts in embryology.
But first let us look at a few analogous cases in domestic varieties. Some
authors who have written on Dogs, maintain that the greyhound and bulldog,
though appearing so different, are really varieties most closely allied, and
have probably descended from the same wild stock; hence I was curious to see how
far their puppies differed from each other: I was told by breeders that they
differed just as much as their parents, and this, judging by the eye, seemed
almost to be the case; but on actually measuring the old dogs and their six-days
old puppies, I found that the puppies had not nearly acquired their full amount
of proportional difference. So, again, I was told that the foals of cart and
race-horses differed as much as the full-grown animals; and this surprised me
greatly, as I think it probable that the difference between these two breeds has
been wholly caused by selection under domestication; but having had careful
measurements made of the dam and of a three-days old colt of a race and heavy
cart-horse, I find that the colts have by no means acquired their full amount of
proportional difference. As the evidence appears to me conclusive, that the
several domestic breeds of Pigeon have descended from one wild species, I
compared young pigeons of various breeds, within twelve hours after being
hatched; I carefully measured the proportions (but will not here give details)
of the beak, width of mouth, length of nostril and of eyelid, size of feet and
length of leg, in the wild stock, in pouters, fantails, runts, barbs, dragons,
carriers, and tumblers. Now some of these birds, when mature, differ so
extraordinarily in length and form of beak, that they would, I cannot doubt, be
ranked in distinct genera, had they been natural productions. But when the
nestling birds of these several breeds were placed in a row, though most of them
could be distinguished from each other, yet their proportional differences in
the above specified several points were incomparably less than in the full-grown
birds. Some characteristic points of difference--for instance, that of the width
of mouth--could hardly be detected in the young. But there was one remarkable
exception to this rule, for the young of the short-faced tumbler differed from
the young of the wild rock-pigeon and of the other breeds, in all its
proportions, almost exactly as much as in the adult state. The two principles
above given seem to me to explain these facts in regard to the later embryonic
stages of our domestic varieties. Fanciers select their horses, dogs, and
pigeons, for breeding, when they are nearly grown up: they are indifferent
whether the desired qualities and structures have been acquired earlier or later
in life, if the full-grown animal possesses them. And the cases just given, more
especially that of pigeons, seem to show that the characteristic differences
which give value to each breed, and which have been accumulated by man's
selection, have not generally first appeared at an early period of life, and
have been inherited by the offspring at a corresponding not early period. But
the case of the short-faced tumbler, which when twelve hours old had acquired
its proper proportions, proves that this is not the universal rule; for here the
characteristic differences must either have appeared at an earlier period than
usual, or, if not so, the differences must have been inherited, not at the
corresponding, but at an earlier age. Now let us apply these facts and the above
two principles--which latter, though not proved true, can be shown to be in some
degree probable--to species in a state of nature. Let us take a genus of birds,
descended on my theory from some one parent-species, and of which the several
new species have become modified through natural selection in accordance with
their diverse habits. Then, from the many slight successive steps of variation
having supervened at a rather late age, and having been inherited at a
corresponding age, the young of the new species of our supposed genus will
manifestly tend to resemble each other much more closely than do the adults,
just as we have seen in the case of pigeons. We may extend this view to whole
families or even classes. The fore-limbs, for instance, which served as legs in
the parent-species, may become, by a long course of modification, adapted in one
descendant to act as hands, in another as paddles, in another as wings; and on
the above two principles--namely of each successive modification supervening at
a rather late age, and being inherited at a corresponding late age--the
fore-limbs in the embryos of the several descendants of the parent-species will
still resemble each other closely, for they will not have been modified. But in
each individual new species, the embryonic fore-limbs will differ greatly from
the fore-limbs in the mature animal; the limbs in the latter having undergone
much modification at a rather late period of life, and having thus been
converted into hands, or paddles, or wings. Whatever influence long-continued
exercise or use on the one hand, and disuse on the other, may have in modifying
an organ, such influence will mainly affect the mature animal, which has come to
its full powers of activity and has to gain its own living; and the effects thus
produced will be inherited at a corresponding mature age. Whereas the young will
remain unmodified, or be modified in a lesser degree, by the effects of use and
disuse. In certain cases the successive steps of variation might supervene, from
causes of which we are wholly ignorant, at a very early period of life, or each
step might be inherited at an earlier period than that at which it first
appeared. In either case (as with the short-faced tumbler) the young or embryo
would closely resemble the mature parent-form. We have seen that this is the
rule of development in certain whole groups of animals, as with cuttle-fish and
spiders, and with a few members of the great class of insects, as with Aphis.
With respect to the final cause of the young in these cases not undergoing any
metamorphosis, or closely resembling their parents from their earliest age, we
can see that this would result from the two following contingencies; firstly,
from the young, during a course of modification carried on for many generations,
having to provide for their own wants at a very early stage of development, and
secondly, from their following exactly the same habits of life with their
parents; for in this case, it would be indispensable for the existence of the
species, that the child should be modified at a very early age in the same
manner with its parents, in accordance with their similar habits. Some further
explanation, however, of the embryo not undergoing any metamorphosis is perhaps
requisite. If, on the other hand, it profited the young to follow habits of life
in any degree different from those of their parent, and consequently to be
constructed in a slightly different manner, then, on the principle of
inheritance at corresponding ages, the active young or larvae might easily be
rendered by natural selection different to any conceivable extent from their
parents. Such differences might, also, become correlated with successive stages
of development; so that the larvae, in the first stage, might differ greatly
from the larvae in the second stage, as we have seen to be the case with
cirripedes. The adult might become fitted for sites or habits, in which organs
of locomotion or of the senses, etc., would be useless; and in this case the
final metamorphosis would be said to be retrograde. As all the organic beings,
extinct and recent, which have ever lived on this earth have to be classed
together, and as all have been connected by the finest gradations, the best, or
indeed, if our collections were nearly perfect, the only possible arrangement,
would be genealogical. Descent being on my view the hidden bond of connexion
which naturalists have been seeking under the term of the natural system. On
this view we can understand how it is that, in the eyes of most naturalists, the
structure of the embryo is even more important for classification than that of
the adult. For the embryo is the animal in its less modified state; and in so
far it reveals the structure of its progenitor. In two groups of animal, however
much they may at present differ from each other in structure and habits, if they
pass through the same or similar embryonic stages, we may feel assured that they
have both descended from the same or nearly similar parents, and are therefore
in that degree closely related. Thus, community in embryonic structure reveals
community of descent. It will reveal this community of descent, however much the
structure of the adult may have been modified and obscured; we have seen, for
instance, that cirripedes can at once be recognised by their larvae as belonging
to the great class of crustaceans. As the embryonic state of each species and
group of species partially shows us the structure of their less modified ancient
progenitors, we can clearly see why ancient and extinct forms of life should
resemble the embryos of their descendants,--our existing species. Agassiz
believes this to be a law of nature; but I am bound to confess that I only hope
to see the law hereafter proved true. It can be proved true in those cases alone
in which the ancient state, now supposed to be represented in many embryos, has
not been obliterated, either by the successive variations in a long course of
modification having supervened at a very early age, or by the variations having
been inherited at an earlier period than that at which they first appeared. It
should also be borne in mind, that the supposed law of resemblance of ancient
forms of life to the embryonic stages of recent forms, may be true, but yet,
owing to the geological record not extending far enough back in time, may remain
for a long period, or for ever, incapable of demonstration. Thus, as it seems to
me, the leading facts in embryology, which are second in importance to none in
natural history, are explained on the principle of slight modifications not
appearing, in the many descendants from some one ancient progenitor, at a very
early period in the life of each, though perhaps caused at the earliest, and
being inherited at a corresponding not early period. Embryology rises greatly in
interest, when we thus look at the embryo as a picture, more or less obscured,
of the common parent-form of each great class of animals. RUDIMENTARY,
ATROPHIED, OR ABORTED ORGANS. Organs or parts in this strange condition, bearing
the stamp of inutility, are extremely common throughout nature. For instance,
rudimentary mammae are very general in the males of mammals: I presume that the
"bastard-wing" in birds may be safely considered as a digit in a rudimentary
state: in very many snakes one lobe of the lungs is rudimentary; in other snakes
there are rudiments of the pelvis and hind limbs. Some of the cases of
rudimentary organs are extremely curious; for instance, the presence of teeth in
foetal whales, which when grown up have not a tooth in their heads; and the
presence of teeth, which never cut through the gums, in the upper jaws of our
unborn calves. It has even been stated on good authority that rudiments of teeth
can be detected in the beaks of certain embryonic birds. Nothing can be plainer
than that wings are formed for flight, yet in how many insects do we see wings
so reduced in size as to be utterly incapable of flight, and not rarely lying
under wing-cases, firmly soldered together! The meaning of rudimentary organs is
often quite unmistakeable: for instance there are beetles of the same genus (and
even of the same species) resembling each other most closely in all respects,
one of which will have full-sized wings, and another mere rudiments of membrane;
and here it is impossible to doubt, that the rudiments represent wings.
Rudimentary organs sometimes retain their potentiality, and are merely not
developed: this seems to be the case with the mammae of male mammals, for many
instances are on record of these organs having become well developed in
full-grown males, and having secreted milk. So again there are normally four
developed and two rudimentary teats in the udders of the genus Bos, but in our
domestic cows the two sometimes become developed and give milk. In individual
plants of the same species the petals sometimes occur as mere rudiments, and
sometimes in a well-developed state. In plants with separated sexes, the male
flowers often have a rudiment of a pistil; and Kolreuter found that by crossing
such male plants with an hermaphrodite species, the rudiment of the pistil in
the hybrid offspring was much increased in size; and this shows that the
rudiment and the perfect pistil are essentially alike in nature. An organ
serving for two purposes, may become rudimentary or utterly aborted for one,
even the more important purpose; and remain perfectly efficient for the other.
Thus in plants, the office of the pistil is to allow the pollen-tubes to reach
the ovules protected in the ovarium at its base. The pistil consists of a stigma
supported on the style; but in some Compositae, the male florets, which of
course cannot be fecundated, have a pistil, which is in a rudimentary state, for
it is not crowned with a stigma; but the style remains well developed, and is
clothed with hairs as in other compositae, for the purpose of brushing the
pollen out of the surrounding anthers. Again, an organ may become rudimentary
for its proper purpose, and be used for a distinct object: in certain fish the
swim-bladder seems to be rudimentary for its proper function of giving buoyancy,
but has become converted into a nascent breathing organ or lung. Other similar
instances could be given. Rudimentary organs in the individuals of the same
species are very liable to vary in degree of development and in other respects.
Moreover, in closely allied species, the degree to which the same organ has been
rendered rudimentary occasionally differs much. This latter fact is well
exemplified in the state of the wings of the female moths in certain groups.
Rudimentary organs may be utterly aborted; and this implies, that we find in an
animal or plant no trace of an organ, which analogy would lead us to expect to
find, and which is occasionally found in monstrous individuals of the species.
Thus in the snapdragon (antirrhinum) we generally do not find a rudiment of a
fifth stamen; but this may sometimes be seen. In tracing the homologies of the
same part in different members of a class, nothing is more common, or more
necessary, than the use and discovery of rudiments. This is well shown in the
drawings given by Owen of the bones of the leg of the horse, ox, and rhinoceros.
It is an important fact that rudimentary organs, such as teeth in the upper jaws
of whales and ruminants, can often be detected in the embryo, but afterwards
wholly disappear. It is also, I believe, a universal rule, that a rudimentary
part or organ is of greater size relatively to the adjoining parts in the
embryo, than in the adult; so that the organ at this early age is less
rudimentary, or even cannot be said to be in any degree rudimentary. Hence,
also, a rudimentary organ in the adult, is often said to have retained its
embryonic condition. I have now given the leading facts with respect to
rudimentary organs. In reflecting on them, every one must be struck with
astonishment: for the same reasoning power which tells us plainly that most
parts and organs are exquisitely adapted for certain purposes, tells us with
equal plainness that these rudimentary or atrophied organs, are imperfect and
useless. In works on natural history rudimentary organs are generally said to
have been created "for the sake of symmetry," or in order "to complete the
scheme of nature;" but this seems to me no explanation, merely a restatement of
the fact. Would it be thought sufficient to say that because planets revolve in
elliptic courses round the sun, satellites follow the same course round the
planets, for the sake of symmetry, and to complete the scheme of nature? An
eminent physiologist accounts for the presence of rudimentary organs, by
supposing that they serve to excrete matter in excess, or injurious to the
system; but can we suppose that the minute papilla, which often represents the
pistil in male flowers, and which is formed merely of cellular tissue, can thus
act? Can we suppose that the formation of rudimentary teeth which are
subsequently absorbed, can be of any service to the rapidly growing embryonic
calf by the excretion of precious phosphate of lime? When a man's fingers have
been amputated, imperfect nails sometimes appear on the stumps: I could as soon
believe that these vestiges of nails have appeared, not from unknown laws of
growth, but in order to excrete horny matter, as that the rudimentary nails on
the fin of the manatee were formed for this purpose. On my view of descent with
modification, the origin of rudimentary organs is simple. We have plenty of
cases of rudimentary organs in our domestic productions,--as the stump of a tail
in tailless breeds,--the vestige of an ear in earless breeds,--the reappearance
of minute dangling horns in hornless breeds of cattle, more especially,
according to Youatt, in young animals,--and the state of the whole flower in the
cauliflower. We often see rudiments of various parts in monsters. But I doubt
whether any of these cases throw light on the origin of rudimentary organs in a
state of nature, further than by showing that rudiments can be produced; for I
doubt whether species under nature ever undergo abrupt changes. I believe that
disuse has been the main agency; that it has led in successive generations to
the gradual reduction of various organs, until they have become rudimentary,--as
in the case of the eyes of animals inhabiting dark caverns, and of the wings of
birds inhabiting oceanic islands, which have seldom been forced to take flight,
and have ultimately lost the power of flying. Again, an organ useful under
certain conditions, might become injurious under others, as with the wings of
beetles living on small and exposed islands; and in this case natural selection
would continue slowly to reduce the organ, until it was rendered harmless and
rudimentary. Any change in function, which can be effected by insensibly small
steps, is within the power of natural selection; so that an organ rendered,
during changed habits of life, useless or injurious for one purpose, might
easily be modified and used for another purpose. Or an organ might be retained
for one alone of its former functions. An organ, when rendered useless, may well
be variable, for its variations cannot be checked by natural selection. At
whatever period of life disuse or selection reduces an organ, and this will
generally be when the being has come to maturity and to its full powers of
action, the principle of inheritance at corresponding ages will reproduce the
organ in its reduced state at the same age, and consequently will seldom affect
or reduce it in the embryo. Thus we can understand the greater relative size of
rudimentary organs in the embryo, and their lesser relative size in the adult.
But if each step of the process of reduction were to be inherited, not at the
corresponding age, but at an extremely early period of life (as we have good
reason to believe to be possible) the rudimentary part would tend to be wholly
lost, and we should have a case of complete abortion. The principle, also, of
economy, explained in a former chapter, by which the materials forming any part
or structure, if not useful to the possessor, will be saved as far as is
possible, will probably often come into play; and this will tend to cause the
entire obliteration of a rudimentary organ. As the presence of rudimentary
organs is thus due to the tendency in every part of the organisation, which has
long existed, to be inherited--we can understand, on the genealogical view of
classification, how it is that systematists have found rudimentary parts as
useful as, or even sometimes more useful than, parts of high physiological
importance. Rudimentary organs may be compared with the letters in a word, still
retained in the spelling, but become useless in the pronunciation, but which
serve as a clue in seeking for its derivation. On the view of descent with
modification, we may conclude that the existence of organs in a rudimentary,
imperfect, and useless condition, or quite aborted, far from presenting a
strange difficulty, as they assuredly do on the ordinary doctrine of creation,
might even have been anticipated, and can be accounted for by the laws of
inheritance. SUMMARY. In this chapter I have attempted to show, that the
subordination of group to group in all organisms throughout all time; that the
nature of the relationship, by which all living and extinct beings are united by
complex, radiating, and circuitous lines of affinities into one grand system;
the rules followed and the difficulties encountered by naturalists in their
classifications; the value set upon characters, if constant and prevalent,
whether of high vital importance, or of the most trifling importance, or, as in
rudimentary organs, of no importance; the wide opposition in value between
analogical or adaptive characters, and characters of true affinity; and other
such rules;--all naturally follow on the view of the common parentage of those
forms which are considered by naturalists as allied, together with their
modification through natural selection, with its contingencies of extinction and
divergence of character. In considering this view of classification, it should
be borne in mind that the element of descent has been universally used in
ranking together the sexes, ages, and acknowledged varieties of the same
species, however different they may be in structure. If we extend the use of
this element of descent,--the only certainly known cause of similarity in
organic beings,--we shall understand what is meant by the natural system: it is
genealogical in its attempted arrangement, with the grades of acquired
difference marked by the terms varieties, species, genera, families, orders, and
classes. On this same view of descent with modification, all the great facts in
Morphology become intelligible,--whether we look to the same pattern displayed
in the homologous organs, to whatever purpose applied, of the different species
of a class; or to the homologous parts constructed on the same pattern in each
individual animal and plant. On the principle of successive slight variations,
not necessarily or generally supervening at a very early period of life, and
being inherited at a corresponding period, we can understand the great leading
facts in Embryology; namely, the resemblance in an individual embryo of the
homologous parts, which when matured will become widely different from each
other in structure and function; and the resemblance in different species of a
class of the homologous parts or organs, though fitted in the adult members for
purposes as different as possible. Larvae are active embryos, which have become
specially modified in relation to their habits of life, through the principle of
modifications being inherited at corresponding ages. On this same principle--and
bearing in mind, that when organs are reduced in size, either from disuse or
selection, it will generally be at that period of life when the being has to
provide for its own wants, and bearing in mind how strong is the principle of
inheritance--the occurrence of rudimentary organs and their final abortion,
present to us no inexplicable difficulties; on the contrary, their presence
might have been even anticipated. The importance of embryological characters and
of rudimentary organs in classification is intelligible, on the view that an
arrangement is only so far natural as it is genealogical. Finally, the several
classes of facts which have been considered in this chapter, seem to me to
proclaim so plainly, that the innumerable species, genera, and families of
organic beings, with which this world is peopled, have all descended, each
within its own class or group, from common parents, and have all been modified
in the course of descent, that I should without hesitation adopt this view, even
if it were unsupported by other facts or arguments. CHAPTER 14. RECAPITULATION
AND CONCLUSION. Recapitulation of the difficulties on the theory of Natural
Selection. Recapitulation of the general and special circumstances in its
favour. Causes of the general belief in the immutability of species. How far the
theory of natural selection may be extended. Effects of its adoption on the
study of Natural history. Concluding remarks. As this whole volume is one long
argument, it may be convenient to the reader to have the leading facts and
inferences briefly recapitulated. That many and grave objections may be advanced
against the theory of descent with modification through natural selection, I do
not deny. I have endeavoured to give to them their full force. Nothing at first
can appear more difficult to believe than that the more complex organs and
instincts should have been perfected, not by means superior to, though analogous
with, human reason, but by the accumulation of innumerable slight variations,
each good for the individual possessor. Nevertheless, this difficulty, though
appearing to our imagination insuperably great, cannot be considered real if we
admit the following propositions, namely,--that gradations in the perfection of
any organ or instinct, which we may consider, either do now exist or could have
existed, each good of its kind,--that all organs and instincts are, in ever so
slight a degree, variable,--and, lastly, that there is a struggle for existence
leading to the preservation of each profitable deviation of structure or
instinct. The truth of these propositions cannot, I think, be disputed. It is,
no doubt, extremely difficult even to conjecture by what gradations many
structures have been perfected, more especially amongst broken and failing
groups of organic beings; but we see so many strange gradations in nature, as is
proclaimed by the canon, "Natura non facit saltum," that we ought to be
extremely cautious in saying that any organ or instinct, or any whole being,
could not have arrived at its present state by many graduated steps. There are,
it must be admitted, cases of special difficulty on the theory of natural
selection; and one of the most curious of these is the existence of two or three
defined castes of workers or sterile females in the same community of ants; but
I have attempted to show how this difficulty can be mastered. With respect to
the almost universal sterility of species when first crossed, which forms so
remarkable a contrast with the almost universal fertility of varieties when
crossed, I must refer the reader to the recapitulation of the facts given at the
end of the eighth chapter, which seem to me conclusively to show that this
sterility is no more a special endowment than is the incapacity of two trees to
be grafted together, but that it is incidental on constitutional differences in
the reproductive systems of the intercrossed species. We see the truth of this
conclusion in the vast difference in the result, when the same two species are
crossed reciprocally; that is, when one species is first used as the father and
then as the mother. The fertility of varieties when intercrossed and of their
mongrel offspring cannot be considered as universal; nor is their very general
fertility surprising when we remember that it is not likely that either their
constitutions or their reproductive systems should have been profoundly
modified. Moreover, most of the varieties which have been experimentised on have
been produced under domestication; and as domestication apparently tends to
eliminate sterility, we ought not to expect it also to produce sterility. The
sterility of hybrids is a very different case from that of first crosses, for
their reproductive organs are more or less functionally impotent; whereas in
first crosses the organs on both sides are in a perfect condition. As we
continually see that organisms of all kinds are rendered in some degree sterile
from their constitutions having been disturbed by slightly different and new
conditions of life, we need not feel surprise at hybrids being in some degree
sterile, for their constitutions can hardly fail to have been disturbed from
being compounded of two distinct organisations. This parallelism is supported by
another parallel, but directly opposite, class of facts; namely, that the vigour
and fertility of all organic beings are increased by slight changes in their
conditions of life, and that the offspring of slightly modified forms or
varieties acquire from being crossed increased vigour and fertility. So that, on
the one hand, considerable changes in the conditions of life and crosses between
greatly modified forms, lessen fertility; and on the other hand, lesser changes
in the conditions of life and crosses between less modified forms, increase
fertility. Turning to geographical distribution, the difficulties encountered on
the theory of descent with modification are grave enough. All the individuals of
the same species, and all the species of the same genus, or even higher group,
must have descended from common parents; and therefore, in however distant and
isolated parts of the world they are now found, they must in the course of
successive generations have passed from some one part to the others. We are
often wholly unable even to conjecture how this could have been effected. Yet,
as we have reason to believe that some species have retained the same specific
form for very long periods, enormously long as measured by years, too much
stress ought not to be laid on the occasional wide diffusion of the same
species; for during very long periods of time there will always be a good chance
for wide migration by many means. A broken or interrupted range may often be
accounted for by the extinction of the species in the intermediate regions. It
cannot be denied that we are as yet very ignorant of the full extent of the
various climatal and geographical changes which have affected the earth during
modern periods; and such changes will obviously have greatly facilitated
migration. As an example, I have attempted to show how potent has been the
influence of the Glacial period on the distribution both of the same and of
representative species throughout the world. We are as yet profoundly ignorant
of the many occasional means of transport. With respect to distinct species of
the same genus inhabiting very distant and isolated regions, as the process of
modification has necessarily been slow, all the means of migration will have
been possible during a very long period; and consequently the difficulty of the
wide diffusion of species of the same genus is in some degree lessened. As on
the theory of natural selection an interminable number of intermediate forms
must have existed, linking together all the species in each group by gradations
as fine as our present varieties, it may be asked, Why do we not see these
linking forms all around us? Why are not all organic beings blended together in
an inextricable chaos? With respect to existing forms, we should remember that
we have no right to expect (excepting in rare cases) to discover DIRECTLY
connecting links between them, but only between each and some extinct and
supplanted form. Even on a wide area, which has during a long period remained
continuous, and of which the climate and other conditions of life change
insensibly in going from a district occupied by one species into another
district occupied by a closely allied species, we have no just right to expect
often to find intermediate varieties in the intermediate zone. For we have
reason to believe that only a few species are undergoing change at any one
period; and all changes are slowly effected. I have also shown that the
intermediate varieties which will at first probably exist in the intermediate
zones, will be liable to be supplanted by the allied forms on either hand; and
the latter, from existing in greater numbers, will generally be modified and
improved at a quicker rate than the intermediate varieties, which exist in
lesser numbers; so that the intermediate varieties will, in the long run, be
supplanted and exterminated. On this doctrine of the extermination of an
infinitude of connecting links, between the living and extinct inhabitants of
the world, and at each successive period between the extinct and still older
species, why is not every geological formation charged with such links? Why does
not every collection of fossil remains afford plain evidence of the gradation
and mutation of the forms of life? We meet with no such evidence, and this is
the most obvious and forcible of the many objections which may be urged against
my theory. Why, again, do whole groups of allied species appear, though
certainly they often falsely appear, to have come in suddenly on the several
geological stages? Why do we not find great piles of strata beneath the Silurian
system, stored with the remains of the progenitors of the Silurian groups of
fossils? For certainly on my theory such strata must somewhere have been
deposited at these ancient and utterly unknown epochs in the world's history. I
can answer these questions and grave objections only on the supposition that the
geological record is far more imperfect than most geologists believe. It cannot
be objected that there has not been time sufficient for any amount of organic
change; for the lapse of time has been so great as to be utterly inappreciable
by the human intellect. The number of specimens in all our museums is absolutely
as nothing compared with the countless generations of countless species which
certainly have existed. We should not be able to recognise a species as the
parent of any one or more species if we were to examine them ever so closely,
unless we likewise possessed many of the intermediate links between their past
or parent and present states; and these many links we could hardly ever expect
to discover, owing to the imperfection of the geological record. Numerous
existing doubtful forms could be named which are probably varieties; but who
will pretend that in future ages so many fossil links will be discovered, that
naturalists will be able to decide, on the common view, whether or not these
doubtful forms are varieties? As long as most of the links between any two
species are unknown, if any one link or intermediate variety be discovered, it
will simply be classed as another and distinct species. Only a small portion of
the world has been geologically explored. Only organic beings of certain classes
can be preserved in a fossil condition, at least in any great number. Widely
ranging species vary most, and varieties are often at first local,--both causes
rendering the discovery of intermediate links less likely. Local varieties will
not spread into other and distant regions until they are considerably modified
and improved; and when they do spread, if discovered in a geological formation,
they will appear as if suddenly created there, and will be simply classed as new
species. Most formations have been intermittent in their accumulation; and their
duration, I am inclined to believe, has been shorter than the average duration
of specific forms. Successive formations are separated from each other by
enormous blank intervals of time; for fossiliferous formations, thick enough to
resist future degradation, can be accumulated only where much sediment is
deposited on the subsiding bed of the sea. During the alternate periods of
elevation and of stationary level the record will be blank. During these latter
periods there will probably be more variability in the forms of life; during
periods of subsidence, more extinction. With respect to the absence of
fossiliferous formations beneath the lowest Silurian strata, I can only recur to
the hypothesis given in the ninth chapter. That the geological record is
imperfect all will admit; but that it is imperfect to the degree which I
require, few will be inclined to admit. If we look to long enough intervals of
time, geology plainly declares that all species have changed; and they have
changed in the manner which my theory requires, for they have changed slowly and
in a graduated manner. We clearly see this in the fossil remains from
consecutive formations invariably being much more closely related to each other,
than are the fossils from formations distant from each other in time. Such is
the sum of the several chief objections and difficulties which may justly be
urged against my theory; and I have now briefly recapitulated the answers and
explanations which can be given to them. I have felt these difficulties far too
heavily during many years to doubt their weight. But it deserves especial notice
that the more important objections relate to questions on which we are
confessedly ignorant; nor do we know how ignorant we are. We do not know all the
possible transitional gradations between the simplest and the most perfect
organs; it cannot be pretended that we know all the varied means of Distribution
during the long lapse of years, or that we know how imperfect the Geological
Record is. Grave as these several difficulties are, in my judgment they do not
overthrow the theory of descent with modification. Now let us turn to the other
side of the argument. Under domestication we see much variability. This seems to
be mainly due to the reproductive system being eminently susceptible to changes
in the conditions of life; so that this system, when not rendered impotent,
fails to reproduce offspring exactly like the parent-form. Variability is
governed by many complex laws,--by correlation of growth, by use and disuse, and
by the direct action of the physical conditions of life. There is much
difficulty in ascertaining how much modification our domestic productions have
undergone; but we may safely infer that the amount has been large, and that
modifications can be inherited for long periods. As long as the conditions of
life remain the same, we have reason to believe that a modification, which has
already been inherited for many generations, may continue to be inherited for an
almost infinite number of generations. On the other hand we have evidence that
variability, when it has once come into play, does not wholly cease; for new
varieties are still occasionally produced by our most anciently domesticated
productions. Man does not actually produce variability; he only unintentionally
exposes organic beings to new conditions of life, and then nature acts on the
organisation, and causes variability. But man can and does select the variations
given to him by nature, and thus accumulate them in any desired manner. He thus
adapts animals and plants for his own benefit or pleasure. He may do this
methodically, or he may do it unconsciously by preserving the individuals most
useful to him at the time, without any thought of altering the breed. It is
certain that he can largely influence the character of a breed by selecting, in
each successive generation, individual differences so slight as to be quite
inappreciable by an uneducated eye. This process of selection has been the great
agency in the production of the most distinct and useful domestic breeds. That
many of the breeds produced by man have to a large extent the character of
natural species, is shown by the inextricable doubts whether very many of them
are varieties or aboriginal species. There is no obvious reason why the
principles which have acted so efficiently under domestication should not have
acted under nature. In the preservation of favoured individuals and races,
during the constantly-recurrent Struggle for Existence, we see the most powerful
and ever-acting means of selection. The struggle for existence inevitably
follows from the high geometrical ratio of increase which is common to all
organic beings. This high rate of increase is proved by calculation, by the
effects of a succession of peculiar seasons, and by the results of
naturalisation, as explained in the third chapter. More individuals are born
than can possibly survive. A grain in the balance will determine which
individual shall live and which shall die,--which variety or species shall
increase in number, and which shall decrease, or finally become extinct. As the
individuals of the same species come in all respects into the closest
competition with each other, the struggle will generally be most severe between
them; it will be almost equally severe between the varieties of the same
species, and next in severity between the species of the same genus. But the
struggle will often be very severe between beings most remote in the scale of
nature. The slightest advantage in one being, at any age or during any season,
over those with which it comes into competition, or better adaptation in however
slight a degree to the surrounding physical conditions, will turn the balance.
With animals having separated sexes there will in most cases be a struggle
between the males for possession of the females. The most vigorous individuals,
or those which have most successfully struggled with their conditions of life,
will generally leave most progeny. But success will often depend on having
special weapons or means of defence, or on the charms of the males; and the
slightest advantage will lead to victory. As geology plainly proclaims that each
land has undergone great physical changes, we might have expected that organic
beings would have varied under nature, in the same way as they generally have
varied under the changed conditions of domestication. And if there be any
variability under nature, it would be an unaccountable fact if natural selection
had not come into play. It has often been asserted, but the assertion is quite
incapable of proof, that the amount of variation under nature is a strictly
limited quantity. Man, though acting on external characters alone and often
capriciously, can produce within a short period a great result by adding up mere
individual differences in his domestic productions; and every one admits that
there are at least individual differences in species under nature. But, besides
such differences, all naturalists have admitted the existence of varieties,
which they think sufficiently distinct to be worthy of record in systematic
works. No one can draw any clear distinction between individual differences and
slight varieties; or between more plainly marked varieties and sub-species, and
species. Let it be observed how naturalists differ in the rank which they assign
to the many representative forms in Europe and North America. If then we have
under nature variability and a powerful agent always ready to act and select,
why should we doubt that variations in any way useful to beings, under their
excessively complex relations of life, would be preserved, accumulated, and
inherited? Why, if man can by patience select variations most useful to himself,
should nature fail in selecting variations useful, under changing conditions of
life, to her living products? What limit can be put to this power, acting during
long ages and rigidly scrutinising the whole constitution, structure, and habits
of each creature,--favouring the good and rejecting the bad? I can see no limit
to this power, in slowly and beautifully adapting each form to the most complex
relations of life. The theory of natural selection, even if we looked no further
than this, seems to me to be in itself probable. I have already recapitulated,
as fairly as I could, the opposed difficulties and objections: now let us turn
to the special facts and arguments in favour of the theory. On the view that
species are only strongly marked and permanent varieties, and that each species
first existed as a variety, we can see why it is that no line of demarcation can
be drawn between species, commonly supposed to have been produced by special
acts of creation, and varieties which are acknowledged to have been produced by
secondary laws. On this same view we can understand how it is that in each
region where many species of a genus have been produced, and where they now
flourish, these same species should present many varieties; for where the
manufactory of species has been active, we might expect, as a general rule, to
find it still in action; and this is the case if varieties be incipient species.
Moreover, the species of the larger genera, which afford the greater number of
varieties or incipient species, retain to a certain degree the character of
varieties; for they differ from each other by a less amount of difference than
do the species of smaller genera. The closely allied species also of the larger
genera apparently have restricted ranges, and they are clustered in little
groups round other species--in which respects they resemble varieties. These are
strange relations on the view of each species having been independently created,
but are intelligible if all species first existed as varieties. As each species
tends by its geometrical ratio of reproduction to increase inordinately in
number; and as the modified descendants of each species will be enabled to
increase by so much the more as they become more diversified in habits and
structure, so as to be enabled to seize on many and widely different places in
the economy of nature, there will be a constant tendency in natural selection to
preserve the most divergent offspring of any one species. Hence during a
long-continued course of modification, the slight differences, characteristic of
varieties of the same species, tend to be augmented into the greater differences
characteristic of species of the same genus. New and improved varieties will
inevitably supplant and exterminate the older, less improved and intermediate
varieties; and thus species are rendered to a large extent defined and distinct
objects. Dominant species belonging to the larger groups tend to give birth to
new and dominant forms; so that each large group tends to become still larger,
and at the same time more divergent in character. But as all groups cannot thus
succeed in increasing in size, for the world would not hold them, the more
dominant groups beat the less dominant. This tendency in the large groups to go
on increasing in size and diverging in character, together with the almost
inevitable contingency of much extinction, explains the arrangement of all the
forms of life, in groups subordinate to groups, all within a few great classes,
which we now see everywhere around us, and which has prevailed throughout all
time. This grand fact of the grouping of all organic beings seems to me utterly
inexplicable on the theory of creation. As natural selection acts solely by
accumulating slight, successive, favourable variations, it can produce no great
or sudden modification; it can act only by very short and slow steps. Hence the
canon of "Natura non facit saltum," which every fresh addition to our knowledge
tends to make more strictly correct, is on this theory simply intelligible. We
can plainly see why nature is prodigal in variety, though niggard in innovation.
But why this should be a law of nature if each species has been independently
created, no man can explain. Many other facts are, as it seems to me, explicable
on this theory. How strange it is that a bird, under the form of woodpecker,
should have been created to prey on insects on the ground; that upland geese,
which never or rarely swim, should have been created with webbed feet; that a
thrush should have been created to dive and feed on sub-aquatic insects; and
that a petrel should have been created with habits and structure fitting it for
the life of an auk or grebe! and so on in endless other cases. But on the view
of each species constantly trying to increase in number, with natural selection
always ready to adapt the slowly varying descendants of each to any unoccupied
or ill-occupied place in nature, these facts cease to be strange, or perhaps
might even have been anticipated. As natural selection acts by competition, it
adapts the inhabitants of each country only in relation to the degree of
perfection of their associates; so that we need feel no surprise at the
inhabitants of any one country, although on the ordinary view supposed to have
been specially created and adapted for that country, being beaten and supplanted
by the naturalised productions from another land. Nor ought we to marvel if all
the contrivances in nature be not, as far as we can judge, absolutely perfect;
and if some of them be abhorrent to our ideas of fitness. We need not marvel at
the sting of the bee causing the bee's own death; at drones being produced in
such vast numbers for one single act, and being then slaughtered by their
sterile sisters; at the astonishing waste of pollen by our fir-trees; at the
instinctive hatred of the queen bee for her own fertile daughters; at
ichneumonidae feeding within the live bodies of caterpillars; and at other such
cases. The wonder indeed is, on the theory of natural selection, that more cases
of the want of absolute perfection have not been observed. The complex and
little known laws governing variation are the same, as far as we can see, with
the laws which have governed the production of so-called specific forms. In both
cases physical conditions seem to have produced but little direct effect; yet
when varieties enter any zone, they occasionally assume some of the characters
of the species proper to that zone. In both varieties and species, use and
disuse seem to have produced some effect; for it is difficult to resist this
conclusion when we look, for instance, at the logger-headed duck, which has
wings incapable of flight, in nearly the same condition as in the domestic duck;
or when we look at the burrowing tucutucu, which is occasionally blind, and then
at certain moles, which are habitually blind and have their eyes covered with
skin; or when we look at the blind animals inhabiting the dark caves of America
and Europe. In both varieties and species correlation of growth seems to have
played a most important part, so that when one part has been modified other
parts are necessarily modified. In both varieties and species reversions to
long-lost characters occur. How inexplicable on the theory of creation is the
occasional appearance of stripes on the shoulder and legs of the several species
of the horse-genus and in their hybrids! How simply is this fact explained if we
believe that these species have descended from a striped progenitor, in the same
manner as the several domestic breeds of pigeon have descended from the blue and
barred rock-pigeon! On the ordinary view of each species having been
independently created, why should the specific characters, or those by which the
species of the same genus differ from each other, be more variable than the
generic characters in which they all agree? Why, for instance, should the colour
of a flower be more likely to vary in any one species of a genus, if the other
species, supposed to have been created independently, have differently coloured
flowers, than if all the species of the genus have the same coloured flowers? If
species are only well-marked varieties, of which the characters have become in a
high degree permanent, we can understand this fact; for they have already varied
since they branched off from a common progenitor in certain characters, by which
they have come to be specifically distinct from each other; and therefore these
same characters would be more likely still to be variable than the generic
characters which have been inherited without change for an enormous period. It
is inexplicable on the theory of creation why a part developed in a very unusual
manner in any one species of a genus, and therefore, as we may naturally infer,
of great importance to the species, should be eminently liable to variation;
but, on my view, this part has undergone, since the several species branched off
from a common progenitor, an unusual amount of variability and modification, and
therefore we might expect this part generally to be still variable. But a part
may be developed in the most unusual manner, like the wing of a bat, and yet not
be more variable than any other structure, if the part be common to many
subordinate forms, that is, if it has been inherited for a very long period; for
in this case it will have been rendered constant by long-continued natural
selection. Glancing at instincts, marvellous as some are, they offer no greater
difficulty than does corporeal structure on the theory of the natural selection
of successive, slight, but profitable modifications. We can thus understand why
nature moves by graduated steps in endowing different animals of the same class
with their several instincts. I have attempted to show how much light the
principle of gradation throws on the admirable architectural powers of the
hive-bee. Habit no doubt sometimes comes into play in modifying instincts; but
it certainly is not indispensable, as we see, in the case of neuter insects,
which leave no progeny to inherit the effects of long-continued habit. On the
view of all the species of the same genus having descended from a common parent,
and having inherited much in common, we can understand how it is that allied
species, when placed under considerably different conditions of life, yet should
follow nearly the same instincts; why the thrush of South America, for instance,
lines her nest with mud like our British species. On the view of instincts
having been slowly acquired through natural selection we need not marvel at some
instincts being apparently not perfect and liable to mistakes, and at many
instincts causing other animals to suffer. If species be only well-marked and
permanent varieties, we can at once see why their crossed offspring should
follow the same complex laws in their degrees and kinds of resemblance to their
parents,--in being absorbed into each other by successive crosses, and in other
such points,--as do the crossed offspring of acknowledged varieties. On the
other hand, these would be strange facts if species have been independently
created, and varieties have been produced by secondary laws. If we admit that
the geological record is imperfect in an extreme degree, then such facts as the
record gives, support the theory of descent with modification. New species have
come on the stage slowly and at successive intervals; and the amount of change,
after equal intervals of time, is widely different in different groups. The
extinction of species and of whole groups of species, which has played so
conspicuous a part in the history of the organic world, almost inevitably
follows on the principle of natural selection; for old forms will be supplanted
by new and improved forms. Neither single species nor groups of species reappear
when the chain of ordinary generation has once been broken. The gradual
diffusion of dominant forms, with the slow modification of their descendants,
causes the forms of life, after long intervals of time, to appear as if they had
changed simultaneously throughout the world. The fact of the fossil remains of
each formation being in some degree intermediate in character between the
fossils in the formations above and below, is simply explained by their
intermediate position in the chain of descent. The grand fact that all extinct
organic beings belong to the same system with recent beings, falling either into
the same or into intermediate groups, follows from the living and the extinct
being the offspring of common parents. As the groups which have descended from
an ancient progenitor have generally diverged in character, the progenitor with
its early descendants will often be intermediate in character in comparison with
its later descendants; and thus we can see why the more ancient a fossil is, the
oftener it stands in some degree intermediate between existing and allied
groups. Recent forms are generally looked at as being, in some vague sense,
higher than ancient and extinct forms; and they are in so far higher as the
later and more improved forms have conquered the older and less improved organic
beings in the struggle for life. Lastly, the law of the long endurance of allied
forms on the same continent,--of marsupials in Australia, of edentata in
America, and other such cases,--is intelligible, for within a confined country,
the recent and the extinct will naturally be allied by descent. Looking to
geographical distribution, if we admit that there has been during the long
course of ages much migration from one part of the world to another, owing to
former climatal and geographical changes and to the many occasional and unknown
means of dispersal, then we can understand, on the theory of descent with
modification, most of the great leading facts in Distribution. We can see why
there should be so striking a parallelism in the distribution of organic beings
throughout space, and in their geological succession throughout time; for in
both cases the beings have been connected by the bond of ordinary generation,
and the means of modification have been the same. We see the full meaning of the
wonderful fact, which must have struck every traveller, namely, that on the same
continent, under the most diverse conditions, under heat and cold, on mountain
and lowland, on deserts and marshes, most of the inhabitants within each great
class are plainly related; for they will generally be descendants of the same
progenitors and early colonists. On this same principle of former migration,
combined in most cases with modification, we can understand, by the aid of the
Glacial period, the identity of some few plants, and the close alliance of many
others, on the most distant mountains, under the most different climates; and
likewise the close alliance of some of the inhabitants of the sea in the
northern and southern temperate zones, though separated by the whole
intertropical ocean. Although two areas may present the same physical conditions
of life, we need feel no surprise at their inhabitants being widely different,
if they have been for a long period completely separated from each other; for as
the relation of organism to organism is the most important of all relations, and
as the two areas will have received colonists from some third source or from
each other, at various periods and in different proportions, the course of
modification in the two areas will inevitably be different. On this view of
migration, with subsequent modification, we can see why oceanic islands should
be inhabited by few species, but of these, that many should be peculiar. We can
clearly see why those animals which cannot cross wide spaces of ocean, as frogs
and terrestrial mammals, should not inhabit oceanic islands; and why, on the
other hand, new and peculiar species of bats, which can traverse the ocean,
should so often be found on islands far distant from any continent. Such facts
as the presence of peculiar species of bats, and the absence of all other
mammals, on oceanic islands, are utterly inexplicable on the theory of
independent acts of creation. The existence of closely allied or representative
species in any two areas, implies, on the theory of descent with modification,
that the same parents formerly inhabited both areas; and we almost invariably
find that wherever many closely allied species inhabit two areas, some identical
species common to both still exist. Wherever many closely allied yet distinct
species occur, many doubtful forms and varieties of the same species likewise
occur. It is a rule of high generality that the inhabitants of each area are
related to the inhabitants of the nearest source whence immigrants might have
been derived. We see this in nearly all the plants and animals of the Galapagos
archipelago, of Juan Fernandez, and of the other American islands being related
in the most striking manner to the plants and animals of the neighbouring
American mainland; and those of the Cape de Verde archipelago and other African
islands to the African mainland. It must be admitted that these facts receive no
explanation on the theory of creation. The fact, as we have seen, that all past
and present organic beings constitute one grand natural system, with group
subordinate to group, and with extinct groups often falling in between recent
groups, is intelligible on the theory of natural selection with its
contingencies of extinction and divergence of character. On these same
principles we see how it is, that the mutual affinities of the species and
genera within each class are so complex and circuitous. We see why certain
characters are far more serviceable than others for classification;--why
adaptive characters, though of paramount importance to the being, are of hardly
any importance in classification; why characters derived from rudimentary parts,
though of no service to the being, are often of high classificatory value; and
why embryological characters are the most valuable of all. The real affinities
of all organic beings are due to inheritance or community of descent. The
natural system is a genealogical arrangement, in which we have to discover the
lines of descent by the most permanent characters, however slight their vital
importance may be. The framework of bones being the same in the hand of a man,
wing of a bat, fin of the porpoise, and leg of the horse,--the same number of
vertebrae forming the neck of the giraffe and of the elephant,--and innumerable
other such facts, at once explain themselves on the theory of descent with slow
and slight successive modifications. The similarity of pattern in the wing and
leg of a bat, though used for such different purpose,--in the jaws and legs of a
crab,--in the petals, stamens, and pistils of a flower, is likewise intelligible
on the view of the gradual modification of parts or organs, which were alike in
the early progenitor of each class. On the principle of successive variations
not always supervening at an early age, and being inherited at a corresponding
not early period of life, we can clearly see why the embryos of mammals, birds,
reptiles, and fishes should be so closely alike, and should be so unlike the
adult forms. We may cease marvelling at the embryo of an air-breathing mammal or
bird having branchial slits and arteries running in loops, like those in a fish
which has to breathe the air dissolved in water, by the aid of well-developed
branchiae. Disuse, aided sometimes by natural selection, will often tend to
reduce an organ, when it has become useless by changed habits or under changed
conditions of life; and we can clearly understand on this view the meaning of
rudimentary organs. But disuse and selection will generally act on each
creature, when it has come to maturity and has to play its full part in the
struggle for existence, and will thus have little power of acting on an organ
during early life; hence the organ will not be much reduced or rendered
rudimentary at this early age. The calf, for instance, has inherited teeth,
which never cut through the gums of the upper jaw, from an early progenitor
having well-developed teeth; and we may believe, that the teeth in the mature
animal were reduced, during successive generations, by disuse or by the tongue
and palate having been fitted by natural selection to browse without their aid;
whereas in the calf, the teeth have been left untouched by selection or disuse,
and on the principle of inheritance at corresponding ages have been inherited
from a remote period to the present day. On the view of each organic being and
each separate organ having been specially created, how utterly inexplicable it
is that parts, like the teeth in the embryonic calf or like the shrivelled wings
under the soldered wing-covers of some beetles, should thus so frequently bear
the plain stamp of inutility! Nature may be said to have taken pains to reveal,
by rudimentary organs and by homologous structures, her scheme of modification,
which it seems that we wilfully will not understand. I have now recapitulated
the chief facts and considerations which have thoroughly convinced me that
species have changed, and are still slowly changing by the preservation and
accumulation of successive slight favourable variations. Why, it may be asked,
have all the most eminent living naturalists and geologists rejected this view
of the mutability of species? It cannot be asserted that organic beings in a
state of nature are subject to no variation; it cannot be proved that the amount
of variation in the course of long ages is a limited quantity; no clear
distinction has been, or can be, drawn between species and well-marked
varieties. It cannot be maintained that species when intercrossed are invariably
sterile, and varieties invariably fertile; or that sterility is a special
endowment and sign of creation. The belief that species were immutable
productions was almost unavoidable as long as the history of the world was
thought to be of short duration; and now that we have acquired some idea of the
lapse of time, we are too apt to assume, without proof, that the geological
record is so perfect that it would have afforded us plain evidence of the
mutation of species, if they had undergone mutation. But the chief cause of our
natural unwillingness to admit that one species has given birth to other and
distinct species, is that we are always slow in admitting any great change of
which we do not see the intermediate steps. The difficulty is the same as that
felt by so many geologists, when Lyell first insisted that long lines of inland
cliffs had been formed, and great valleys excavated, by the slow action of the
coast-waves. The mind cannot possibly grasp the full meaning of the term of a
hundred million years; it cannot add up and perceive the full effects of many
slight variations, accumulated during an almost infinite number of generations.
Although I am fully convinced of the truth of the views given in this volume
under the form of an abstract, I by no means expect to convince experienced
naturalists whose minds are stocked with a multitude of facts all viewed, during
a long course of years, from a point of view directly opposite to mine. It is so
easy to hide our ignorance under such expressions as the "plan of creation,"
"unity of design," etc., and to think that we give an explanation when we only
restate a fact. Any one whose disposition leads him to attach more weight to
unexplained difficulties than to the explanation of a certain number of facts
will certainly reject my theory. A few naturalists, endowed with much
flexibility of mind, and who have already begun to doubt on the immutability of
species, may be influenced by this volume; but I look with confidence to the
future, to young and rising naturalists, who will be able to view both sides of
the question with impartiality. Whoever is led to believe that species are
mutable will do good service by conscientiously expressing his conviction; for
only thus can the load of prejudice by which this subject is overwhelmed be
removed. Several eminent naturalists have of late published their belief that a
multitude of reputed species in each genus are not real species; but that other
species are real, that is, have been independently created. This seems to me a
strange conclusion to arrive at. They admit that a multitude of forms, which
till lately they themselves thought were special creations, and which are still
thus looked at by the majority of naturalists, and which consequently have every
external characteristic feature of true species,--they admit that these have
been produced by variation, but they refuse to extend the same view to other and
very slightly different forms. Nevertheless they do not pretend that they can
define, or even conjecture, which are the created forms of life, and which are
those produced by secondary laws. They admit variation as a vera causa in one
case, they arbitrarily reject it in another, without assigning any distinction
in the two cases. The day will come when this will be given as a curious
illustration of the blindness of preconceived opinion. These authors seem no
more startled at a miraculous act of creation than at an ordinary birth. But do
they really believe that at innumerable periods in the earth's history certain
elemental atoms have been commanded suddenly to flash into living tissues? Do
they believe that at each supposed act of creation one individual or many were
produced? Were all the infinitely numerous kinds of animals and plants created
as eggs or seed, or as full grown? and in the case of mammals, were they created
bearing the false marks of nourishment from the mother's womb? Although
naturalists very properly demand a full explanation of every difficulty from
those who believe in the mutability of species, on their own side they ignore
the whole subject of the first appearance of species in what they consider
reverent silence. It may be asked how far I extend the doctrine of the
modification of species. The question is difficult to answer, because the more
distinct the forms are which we may consider, by so much the arguments fall away
in force. But some arguments of the greatest weight extend very far. All the
members of whole classes can be connected together by chains of affinities, and
all can be classified on the same principle, in groups subordinate to groups.
Fossil remains sometimes tend to fill up very wide intervals between existing
orders. Organs in a rudimentary condition plainly show that an early progenitor
had the organ in a fully developed state; and this in some instances necessarily
implies an enormous amount of modification in the descendants. Throughout whole
classes various structures are formed on the same pattern, and at an embryonic
age the species closely resemble each other. Therefore I cannot doubt that the
theory of descent with modification embraces all the members of the same class.
I believe that animals have descended from at most only four or five
progenitors, and plants from an equal or lesser number. Analogy would lead me
one step further, namely, to the belief that all animals and plants have
descended from some one prototype. But analogy may be a deceitful guide.
Nevertheless all living things have much in common, in their chemical
composition, their germinal vesicles, their cellular structure, and their laws
of growth and reproduction. We see this even in so trifling a circumstance as
that the same poison often similarly affects plants and animals; or that the
poison secreted by the gall-fly produces monstrous growths on the wild rose or
oak-tree. Therefore I should infer from analogy that probably all the organic
beings which have ever lived on this earth have descended from some one
primordial form, into which life was first breathed. When the views entertained
in this volume on the origin of species, or when analogous views are generally
admitted, we can dimly foresee that there will be a considerable revolution in
natural history. Systematists will be able to pursue their labours as at
present; but they will not be incessantly haunted by the shadowy doubt whether
this or that form be in essence a species. This I feel sure, and I speak after
experience, will be no slight relief. The endless disputes whether or not some
fifty species of British brambles are true species will cease. Systematists will
have only to decide (not that this will be easy) whether any form be
sufficiently constant and distinct from other forms, to be capable of
definition; and if definable, whether the differences be sufficiently important
to deserve a specific name. This latter point will become a far more essential
consideration than it is at present; for differences, however slight, between
any two forms, if not blended by intermediate gradations, are looked at by most
naturalists as sufficient to raise both forms to the rank of species. Hereafter
we shall be compelled to acknowledge that the only distinction between species
and well-marked varieties is, that the latter are known, or believed, to be
connected at the present day by intermediate gradations, whereas species were
formerly thus connected. Hence, without quite rejecting the consideration of the
present existence of intermediate gradations between any two forms, we shall be
led to weigh more carefully and to value higher the actual amount of difference
between them. It is quite possible that forms now generally acknowledged to be
merely varieties may hereafter be thought worthy of specific names, as with the
primrose and cowslip; and in this case scientific and common language will come
into accordance. In short, we shall have to treat species in the same manner as
those naturalists treat genera, who admit that genera are merely artificial
combinations made for convenience. This may not be a cheering prospect; but we
shall at least be freed from the vain search for the undiscovered and
undiscoverable essence of the term species. The other and more general
departments of natural history will rise greatly in interest. The terms used by
naturalists of affinity, relationship, community of type, paternity, morphology,
adaptive characters, rudimentary and aborted organs, etc., will cease to be
metaphorical, and will have a plain signification. When we no longer look at an
organic being as a savage looks at a ship, as at something wholly beyond his
comprehension; when we regard every production of nature as one which has had a
history; when we contemplate every complex structure and instinct as the summing
up of many contrivances, each useful to the possessor, nearly in the same way as
when we look at any great mechanical invention as the summing up of the labour,
the experience, the reason, and even the blunders of numerous workmen; when we
thus view each organic being, how far more interesting, I speak from experience,
will the study of natural history become! A grand and almost untrodden field of
inquiry will be opened, on the causes and laws of variation, on correlation of
growth, on the effects of use and disuse, on the direct action of external
conditions, and so forth. The study of domestic productions will rise immensely
in value. A new variety raised by man will be a far more important and
interesting subject for study than one more species added to the infinitude of
already recorded species. Our classifications will come to be, as far as they
can be so made, genealogies; and will then truly give what may be called the
plan of creation. The rules for classifying will no doubt become simpler when we
have a definite object in view. We possess no pedigrees or armorial bearings;
and we have to discover and trace the many diverging lines of descent in our
natural genealogies, by characters of any kind which have long been inherited.
Rudimentary organs will speak infallibly with respect to the nature of long-lost
structures. Species and groups of species, which are called aberrant, and which
may fancifully be called living fossils, will aid us in forming a picture of the
ancient forms of life. Embryology will reveal to us the structure, in some
degree obscured, of the prototypes of each great class. When we can feel assured
that all the individuals of the same species, and all the closely allied species
of most genera, have within a not very remote period descended from one parent,
and have migrated from some one birthplace; and when we better know the many
means of migration, then, by the light which geology now throws, and will
continue to throw, on former changes of climate and of the level of the land, we
shall surely be enabled to trace in an admirable manner the former migrations of
the inhabitants of the whole world. Even at present, by comparing the
differences of the inhabitants of the sea on the opposite sides of a continent,
and the nature of the various inhabitants of that continent in relation to their
apparent means of immigration, some light can be thrown on ancient geography.
The noble science of Geology loses glory from the extreme imperfection of the
record. The crust of the earth with its embedded remains must not be looked at
as a well-filled museum, but as a poor collection made at hazard and at rare
intervals. The accumulation of each great fossiliferous formation will be
recognised as having depended on an unusual concurrence of circumstances, and
the blank intervals between the successive stages as having been of vast
duration. But we shall be able to gauge with some security the duration of these
intervals by a comparison of the preceding and succeeding organic forms. We must
be cautious in attempting to correlate as strictly contemporaneous two
formations, which include few identical species, by the general succession of
their forms of life. As species are produced and exterminated by slowly acting
and still existing causes, and not by miraculous acts of creation and by
catastrophes; and as the most important of all causes of organic change is one
which is almost independent of altered and perhaps suddenly altered physical
conditions, namely, the mutual relation of organism to organism,--the
improvement of one being entailing the improvement or the extermination of
others; it follows, that the amount of organic change in the fossils of
consecutive formations probably serves as a fair measure of the lapse of actual
time. A number of species, however, keeping in a body might remain for a long
period unchanged, whilst within this same period, several of these species, by
migrating into new countries and coming into competition with foreign
associates, might become modified; so that we must not overrate the accuracy of
organic change as a measure of time. During early periods of the earth's
history, when the forms of life were probably fewer and simpler, the rate of
change was probably slower; and at the first dawn of life, when very few forms
of the simplest structure existed, the rate of change may have been slow in an
extreme degree. The whole history of the world, as at present known, although of
a length quite incomprehensible by us, will hereafter be recognised as a mere
fragment of time, compared with the ages which have elapsed since the first
creature, the progenitor of innumerable extinct and living descendants, was
created. In the distant future I see open fields for far more important
researches. Psychology will be based on a new foundation, that of the necessary
acquirement of each mental power and capacity by gradation. Light will be thrown
on the origin of man and his history. Authors of the highest eminence seem to be
fully satisfied with the view that each species has been independently created.
To my mind it accords better with what we know of the laws impressed on matter
by the Creator, that the production and extinction of the past and present
inhabitants of the world should have been due to secondary causes, like those
determining the birth and death of the individual. When I view all beings not as
special creations, but as the lineal descendants of some few beings which lived
long before the first bed of the Silurian system was deposited, they seem to me
to become ennobled. Judging from the past, we may safely infer that not one
living species will transmit its unaltered likeness to a distant futurity. And
of the species now living very few will transmit progeny of any kind to a far
distant futurity; for the manner in which all organic beings are grouped, shows
that the greater number of species of each genus, and all the species of many
genera, have left no descendants, but have become utterly extinct. We can so far
take a prophetic glance into futurity as to foretel that it will be the common
and widely-spread species, belonging to the larger and dominant groups, which
will ultimately prevail and procreate new and dominant species. As all the
living forms of life are the lineal descendants of those which lived long before
the Silurian epoch, we may feel certain that the ordinary succession by
generation has never once been broken, and that no cataclysm has desolated the
whole world. Hence we may look with some confidence to a secure future of
equally inappreciable length. And as natural selection works solely by and for
the good of each being, all corporeal and mental endowments will tend to
progress towards perfection. It is interesting to contemplate an entangled bank,
clothed with many plants of many kinds, with birds singing on the bushes, with
various insects flitting about, and with worms crawling through the damp earth,
and to reflect that these elaborately constructed forms, so different from each
other, and dependent on each other in so complex a manner, have all been
produced by laws acting around us. These laws, taken in the largest sense, being
Growth with Reproduction; Inheritance which is almost implied by reproduction;
Variability from the indirect and direct action of the external conditions of
life, and from use and disuse; a Ratio of Increase so high as to lead to a
Struggle for Life, and as a consequence to Natural Selection, entailing
Divergence of Character and the Extinction of less-improved forms. Thus, from
the war of nature, from famine and death, the most exalted object which we are
capable of conceiving, namely, the production of the higher animals, directly
follows. There is grandeur in this view of life, with its several powers, having
been originally breathed into a few forms or into one; and that, whilst this
planet has gone cycling on according to the fixed law of gravity, from so simple
a beginning endless forms most beautiful and most wonderful have been, and are
being, evolved.