The Origin of Species
Chapter 4: Natural Selection
by Charles Darwin
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 lapses 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, &c., 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,
&c.), 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 in 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, &c., 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 Gärtner, 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 Kölreuter 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 mongrelised? 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, &c.; 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 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 m 2 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 a14; 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 a214
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.