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Books: On the Origin of Species

C >> Charles Darwin >> On the Origin of Species





CHAPTER 4.

NATURAL SELECTION.

Natural Selection: its power compared with man's selection, its power
on characters of trifling importance, its power at all ages and on
both sexes.
Sexual Selection.
On the generality of intercrosses between individuals of the same
species.
Circumstances favourable and unfavourable to Natural Selection,
namely, intercrossing, isolation, number of individuals.
Slow action.
Extinction caused by Natural Selection.
Divergence of Character, related to the diversity of inhabitants of
any small area, and to naturalisation.
Action of Natural Selection, through Divergence of Character and
Extinction, on the descendants from a common parent.
Explains the Grouping of all organic beings.

How will the struggle for existence, discussed too briefly in the last
chapter, act in regard to variation? Can the principle of selection,
which we have seen is so potent in the hands of man, apply in nature?
I think we shall see that it can act most effectually. Let it be borne
in mind in what an endless number of strange peculiarities our
domestic productions, and, in a lesser degree, those under nature,
vary; and how strong the hereditary tendency is. Under domestication,
it may be truly said that the whole organisation becomes in some
degree plastic. Let it be borne in mind how infinitely complex and
close-fitting are the mutual relations of all organic beings to each
other and to their physical conditions of life. Can it, then, be
thought improbable, seeing that variations useful to man have
undoubtedly occurred, that other variations useful in some way to each
being in the great and complex battle of life, should sometimes occur
in the course of thousands of generations? If such do occur, can we
doubt (remembering that many more individuals are born than can
possibly survive) that individuals having any advantage, however
slight, over others, would have the best chance of surviving and of
procreating their kind? On the other hand, we may feel sure that any
variation in the least degree injurious would be rigidly destroyed.
This preservation of favourable variations and the rejection of
injurious variations, I call Natural Selection. Variations neither
useful nor injurious would not be affected by natural selection, and
would be left a fluctuating element, as perhaps we see in the species
called polymorphic.

We shall best understand the probable course of natural selection by
taking the case of a country undergoing some physical change, for
instance, of climate. The proportional numbers of its inhabitants
would almost immediately undergo a change, and some species might
become extinct. We may conclude, from what we have seen of the
intimate and complex manner in which the inhabitants of each country
are bound together, that any change in the numerical proportions of
some of the inhabitants, independently of the change of climate
itself, would most seriously affect many of the others. If the country
were open on its borders, new forms would certainly immigrate, and
this also would seriously disturb the relations of some of the former
inhabitants. Let it be remembered how powerful the influence of a
single introduced tree or mammal has been shown to be. But in the case
of an island, or of a country partly surrounded by barriers, into
which new and better adapted forms could not freely enter, we should
then have places in the economy of nature which would assuredly be
better filled up, if some of the original inhabitants were in some
manner modified; for, had the area been open to immigration, these
same places would have been seized on by intruders. In such case,
every slight modification, which in the course of ages chanced to
arise, and which in any way favoured the individuals of any of the
species, by better adapting them to their altered conditions, would
tend to be preserved; and natural selection would thus have free scope
for the work of improvement.

We have reason to believe, as stated in the first chapter, that a
change in the conditions of life, by specially acting on the
reproductive system, causes or increases variability; and in the
foregoing case the conditions of life are supposed to have undergone a
change, and this would manifestly be favourable to natural selection,
by giving a better chance of profitable variations occurring; and
unless profitable variations do occur, natural selection can do
nothing. Not that, as I believe, any extreme amount of variability is
necessary; as man can certainly produce great results by adding up in
any given direction mere individual differences, so could Nature, but
far more easily, from having incomparably longer time at her disposal.
Nor do I believe that any great physical change, as of climate, or any
unusual degree of isolation to check immigration, is actually
necessary to produce new and unoccupied places for natural selection
to fill up by modifying and improving some of the varying inhabitants.
For as all the inhabitants of each country are struggling together
with nicely balanced forces, extremely slight modifications in the
structure or habits of one inhabitant would often give it an advantage
over others; and still further modifications of the same kind would
often still further increase the advantage. No country can be named in
which all the native inhabitants are now so perfectly adapted to each
other and to the physical conditions under which they live, that none
of them could anyhow be improved; for in all countries, the natives
have been so far conquered by naturalised productions, that they have
allowed foreigners to take firm possession of the land. And as
foreigners have thus everywhere beaten some of the natives, we may
safely conclude that the natives might have been modified with
advantage, so as to have better resisted such intruders.

As man can produce and certainly has produced a great result by his
methodical and unconscious means of selection, what may not nature
effect? Man can act only on external and visible characters: nature
cares nothing for appearances, except in so far as they may be useful
to any being. She can act on every internal organ, on every shade of
constitutional difference, on the whole machinery of life. Man selects
only for his own good; Nature only for that of the being which she
tends. Every selected character is fully exercised by her; and the
being is placed under well-suited conditions of life. Man keeps the
natives of many climates in the same country; he seldom exercises each
selected character in some peculiar and fitting manner; he feeds a
long and a short beaked pigeon on the same food; he does not exercise
a long-backed or long-legged quadruped in any peculiar manner; he
exposes sheep with long and short wool to the same climate. He does
not allow the most vigorous males to struggle for the females. He does
not rigidly destroy all inferior animals, but protects during each
varying season, as far as lies in his power, all his productions. He
often begins his selection by some half-monstrous form; or at least by
some modification prominent enough to catch his eye, or to be plainly
useful to him. Under nature, the slightest difference of structure or
constitution may well turn the nicely-balanced scale in the struggle
for life, and so be preserved. How fleeting are the wishes and efforts
of man! how short his time! and consequently how poor will his
products be, compared with those accumulated by nature during whole
geological periods. Can we wonder, then, that nature's productions
should be far "truer" in character than man's productions; that they
should be infinitely better adapted to the most complex conditions of
life, and should plainly bear the stamp of far higher workmanship?

It may be said that natural selection is daily and hourly
scrutinising, throughout the world, every variation, even the
slightest; rejecting that which is bad, preserving and adding up all
that is good; silently and insensibly working, whenever and wherever
opportunity offers, at the improvement of each organic being in
relation to its organic and inorganic conditions of life. We see
nothing of these slow changes in progress, until the hand of time has
marked the long lapse of ages, and then so imperfect is our view into
long past geological ages, that we only see that the forms of life are
now different from what they formerly were.

Although natural selection can act only through and for the good of
each being, yet characters and structures, which we are apt to
consider as of very trifling importance, may thus be acted on. When we
see leaf-eating insects green, and bark-feeders mottled-grey; the
alpine ptarmigan white in winter, the red-grouse the colour of
heather, and the black-grouse that of peaty earth, we must believe
that these tints are of service to these birds and insects in
preserving them from danger. Grouse, if not destroyed at some period
of their lives, would increase in countless numbers; they are known to
suffer largely from birds of prey; and hawks are guided by eyesight to
their prey,--so much so, that on parts of the Continent persons are
warned not to keep white pigeons, as being the most liable to
destruction. Hence I can see no reason to doubt that natural selection
might be most effective in giving the proper colour to each kind of
grouse, and in keeping that colour, when once acquired, true and
constant. Nor ought we to think that the occasional destruction of an
animal of any particular colour would produce little effect: we should
remember how essential it is in a flock of white sheep to destroy
every lamb with the faintest trace of black. In plants the down on the
fruit and the colour of the flesh are considered by botanists as
characters of the most trifling importance: yet we hear from an
excellent horticulturist, Downing, that in the United States
smooth-skinned fruits suffer far more from a beetle, a curculio, than
those with down; that purple plums suffer far more from a certain
disease than yellow plums; whereas another disease attacks
yellow-fleshed peaches far more than those with other coloured flesh.
If, with all the aids of art, these slight differences make a great
difference in cultivating the several varieties, assuredly, in a state
of nature, where the trees would have to struggle with other trees and
with a host of enemies, such differences would effectually settle
which variety, whether a smooth or downy, a yellow or purple fleshed
fruit, should succeed.

In looking at many small points of difference between species, which,
as far as our ignorance permits us to judge, seem to be quite
unimportant, we must not forget that climate, food, etc., probably
produce some slight and direct effect. It is, however, far more
necessary to bear in mind that there are many unknown laws of
correlation of growth, which, when one part of the organisation is
modified through variation, and the modifications are accumulated by
natural selection for the good of the being, will cause other
modifications, often of the most unexpected nature.

As we see that those variations which under domestication appear at
any particular period of life, tend to reappear in the offspring at
the same period;--for instance, in the seeds of the many varieties of
our culinary and agricultural plants; in the caterpillar and cocoon
stages of the varieties of the silkworm; in the eggs of poultry, and
in the colour of the down of their chickens; in the horns of our sheep
and cattle when nearly adult;--so in a state of nature, natural
selection will be enabled to act on and modify organic beings at any
age, by the accumulation of profitable variations at that age, and by
their inheritance at a corresponding age. If it profit a plant to have
its seeds more and more widely disseminated by the wind, I can see no
greater difficulty in this being effected through natural selection,
than in the cotton-planter increasing and improving by selection the
down in the pods on his cotton-trees. Natural selection may modify and
adapt the larva of an insect to a score of contingencies, wholly
different from those which concern the mature insect. These
modifications will no doubt affect, through the laws of correlation,
the structure of the adult; and probably in the case of those insects
which live only for a few hours, and which never feed, a large part of
their structure is merely the correlated result of successive changes
in the structure of their larvae. So, conversely, modifications in the
adult will probably often affect the structure of the larva; but in
all cases natural selection will ensure that modifications consequent
on other modifications at a different period of life, shall not be in
the least degree injurious: for if they became so, they would cause
the extinction of the species.

Natural selection will modify the structure of the young in relation
to the parent, and of the parent in relation to the young. In social
animals it will adapt the structure of each individual for the benefit
of the community; if each in consequence profits by the selected
change. What natural selection cannot do, is to modify the structure
of one species, without giving it any advantage, for the good of
another species; and though statements to this effect may be found in
works of natural history, I cannot find one case which will bear
investigation. A structure used only once in an animal's whole life,
if of high importance to it, might be modified to any extent by
natural selection; for instance, the great jaws possessed by certain
insects, and used exclusively for opening the cocoon--or the hard tip
to the beak of nestling birds, used for breaking the egg. It has been
asserted, that of the best short-beaked tumbler-pigeons more perish in
the egg than are able to get out of it; so that fanciers assist in the
act of hatching. Now, if nature had to make the beak of a full-grown
pigeon very short for the bird's own advantage, the process of
modification would be very slow, and there would be simultaneously the
most rigorous selection of the young birds within the egg, which had
the most powerful and hardest beaks, for all with weak beaks would
inevitably perish: or, more delicate and more easily broken shells
might be selected, the thickness of the shell being known to vary like
every other structure.

SEXUAL SELECTION.

Inasmuch as peculiarities often appear under domestication in one sex
and become hereditarily attached to that sex, the same fact probably
occurs under nature, and if so, natural selection will be able to
modify one sex in its functional relations to the other sex, or in
relation to wholly different habits of life in the two sexes, as is
sometimes the case with insects. And this leads me to say a few words
on what I call Sexual Selection. This depends, not on a struggle for
existence, but on a struggle between the males for possession of the
females; the result is not death to the unsuccessful competitor, but
few or no offspring. Sexual selection is, therefore, less rigorous
than natural selection. Generally, the most vigorous males, those
which are best fitted for their places in nature, will leave most
progeny. But in many cases, victory will depend not on general vigour,
but on having special weapons, confined to the male sex. A hornless
stag or spurless cock would have a poor chance of leaving offspring.
Sexual selection by always allowing the victor to breed might surely
give indomitable courage, length to the spur, and strength to the wing
to strike in the spurred leg, as well as the brutal cock-fighter, who
knows well that he can improve his breed by careful selection of the
best cocks. How low in the scale of nature this law of battle
descends, I know not; male alligators have been described as fighting,
bellowing, and whirling round, like Indians in a war-dance, for the
possession of the females; male salmons have been seen fighting all
day long; male stag-beetles often bear wounds from the huge mandibles
of other males. The war is, perhaps, severest between the males of
polygamous animals, and these seem oftenest provided with special
weapons. The males of carnivorous animals are already well armed;
though to them and to others, special means of defence may be given
through means of sexual selection, as the mane to the lion, the
shoulder-pad to the boar, and the hooked jaw to the male salmon; for
the shield may be as important for victory, as the sword or spear.

Amongst birds, the contest is often of a more peaceful character. All
those who have attended to the subject, believe that there is the
severest rivalry between the males of many species to attract by
singing the females. The rock-thrush of Guiana, birds of Paradise, and
some others, congregate; and successive males display their gorgeous
plumage and perform strange antics before the females, which standing
by as spectators, at last choose the most attractive partner. Those
who have closely attended to birds in confinement well know that they
often take individual preferences and dislikes: thus Sir R. Heron has
described how one pied peacock was eminently attractive to all his hen
birds. It may appear childish to attribute any effect to such
apparently weak means: I cannot here enter on the details necessary to
support this view; but if man can in a short time give elegant
carriage and beauty to his bantams, according to his standard of
beauty, I can see no good reason to doubt that female birds, by
selecting, during thousands of generations, the most melodious or
beautiful males, according to their standard of beauty, might produce
a marked effect. I strongly suspect that some well-known laws with
respect to the plumage of male and female birds, in comparison with
the plumage of the young, can be explained on the view of plumage
having been chiefly modified by sexual selection, acting when the
birds have come to the breeding age or during the breeding season; the
modifications thus produced being inherited at corresponding ages or
seasons, either by the males alone, or by the males and females; but I
have not space here to enter on this subject.

Thus it is, as I believe, that when the males and females of any
animal have the same general habits of life, but differ in structure,
colour, or ornament, such differences have been mainly caused by
sexual selection; that is, individual males have had, in successive
generations, some slight advantage over other males, in their weapons,
means of defence, or charms; and have transmitted these advantages to
their male offspring. Yet, I would not wish to attribute all such
sexual differences to this agency: for we see peculiarities arising
and becoming attached to the male sex in our domestic animals (as the
wattle in male carriers, horn-like protuberances in the cocks of
certain fowls, etc.), which we cannot believe to be either useful to
the males in battle, or attractive to the females. We see analogous
cases under nature, for instance, the tuft of hair on the breast of
the turkey-cock, which can hardly be either useful or ornamental to
this bird;--indeed, had the tuft appeared under domestication, it
would have been called a monstrosity.

ILLUSTRATIONS OF THE ACTION OF NATURAL SELECTION.

In order to make it clear how, as I believe, natural selection acts, I
must beg permission to give one or two imaginary illustrations. Let us
take the case of a wolf, which preys on various animals, securing some
by craft, some by strength, and some by fleetness; and let us suppose
that the fleetest prey, a deer for instance, had from any change in
the country increased in numbers, or that other prey had decreased in
numbers, during that season of the year when the wolf is hardest
pressed for food. I can under such circumstances see no reason to
doubt that the swiftest and slimmest wolves would have the best chance
of surviving, and so be preserved or selected,--provided always that
they retained strength to master their prey at this or at some other
period of the year, when they might be compelled to prey on other
animals. I can see no more reason to doubt this, than that man can
improve the fleetness of his greyhounds by careful and methodical
selection, or by that unconscious selection which results from each
man trying to keep the best dogs without any thought of modifying the
breed.

Even without any change in the proportional numbers of the animals on
which our wolf preyed, a cub might be born with an innate tendency to
pursue certain kinds of prey. Nor can this be thought very improbable;
for we often observe great differences in the natural tendencies of
our domestic animals; one cat, for instance, taking to catch rats,
another mice; one cat, according to Mr. St. John, bringing home winged
game, another hares or rabbits, and another hunting on marshy ground
and almost nightly catching woodcocks or snipes. The tendency to catch
rats rather than mice is known to be inherited. Now, if any slight
innate change of habit or of structure benefited an individual wolf,
it would have the best chance of surviving and of leaving offspring.
Some of its young would probably inherit the same habits or structure,
and by the repetition of this process, a new variety might be formed
which would either supplant or coexist with the parent-form of wolf.
Or, again, the wolves inhabiting a mountainous district, and those
frequenting the lowlands, would naturally be forced to hunt different
prey; and from the continued preservation of the individuals best
fitted for the two sites, two varieties might slowly be formed. These
varieties would cross and blend where they met; but to this subject of
intercrossing we shall soon have to return. I may add, that, according
to Mr. Pierce, there are two varieties of the wolf inhabiting the
Catskill Mountains in the United States, one with a light
greyhound-like form, which pursues deer, and the other more bulky,
with shorter legs, which more frequently attacks the shepherd's
flocks.

Let us now take a more complex case. Certain plants excrete a sweet
juice, apparently for the sake of eliminating something injurious from
their sap: this is effected by glands at the base of the stipules in
some Leguminosae, and at the back of the leaf of the common laurel.
This juice, though small in quantity, is greedily sought by insects.
Let us now suppose a little sweet juice or nectar to be excreted by
the inner bases of the petals of a flower. In this case insects in
seeking the nectar would get dusted with pollen, and would certainly
often transport the pollen from one flower to the stigma of another
flower. The flowers of two distinct individuals of the same species
would thus get crossed; and the act of crossing, we have good reason
to believe (as will hereafter be more fully alluded to), would produce
very vigorous seedlings, which consequently would have the best chance
of flourishing and surviving. Some of these seedlings would probably
inherit the nectar-excreting power. Those individual flowers which had
the largest glands or nectaries, and which excreted most nectar, would
be oftenest visited by insects, and would be oftenest crossed; and so
in the long-run would gain the upper hand. Those flowers, also, which
had their stamens and pistils placed, in relation to the size and
habits of the particular insects which visited them, so as to favour
in any degree the transportal of their pollen from flower to flower,
would likewise be favoured or selected. We might have taken the case
of insects visiting flowers for the sake of collecting pollen instead
of nectar; and as pollen is formed for the sole object of
fertilisation, its destruction appears a simple loss to the plant; yet
if a little pollen were carried, at first occasionally and then
habitually, by the pollen-devouring insects from flower to flower, and
a cross thus effected, although nine-tenths of the pollen were
destroyed, it might still be a great gain to the plant; and those
individuals which produced more and more pollen, and had larger and
larger anthers, would be selected.

When our plant, by this process of the continued preservation or
natural selection of more and more attractive flowers, had been
rendered highly attractive to insects, they would, unintentionally on
their part, regularly carry pollen from flower to flower; and that
they can most effectually do this, I could easily show by many
striking instances. I will give only one--not as a very striking case,
but as likewise illustrating one step in the separation of the sexes
of plants, presently to be alluded to. Some holly-trees bear only male
flowers, which have four stamens producing rather a small quantity of
pollen, and a rudimentary pistil; other holly-trees bear only female
flowers; these have a full-sized pistil, and four stamens with
shrivelled anthers, in which not a grain of pollen can be detected.
Having found a female tree exactly sixty yards from a male tree, I put
the stigmas of twenty flowers, taken from different branches, under
the microscope, and on all, without exception, there were
pollen-grains, and on some a profusion of pollen. As the wind had set
for several days from the female to the male tree, the pollen could
not thus have been carried. The weather had been cold and boisterous,
and therefore not favourable to bees, nevertheless every female flower
which I examined had been effectually fertilised by the bees,
accidentally dusted with pollen, having flown from tree to tree in
search of nectar. But to return to our imaginary case: as soon as the
plant had been rendered so highly attractive to insects that pollen
was regularly carried from flower to flower, another process might
commence. No naturalist doubts the advantage of what has been called
the "physiological division of labour;" hence we may believe that it
would be advantageous to a plant to produce stamens alone in one
flower or on one whole plant, and pistils alone in another flower or
on another plant. In plants under culture and placed under new
conditions of life, sometimes the male organs and sometimes the female
organs become more or less impotent; now if we suppose this to occur
in ever so slight a degree under nature, then as pollen is already
carried regularly from flower to flower, and as a more complete
separation of the sexes of our plant would be advantageous on the
principle of the division of labour, individuals with this tendency
more and more increased, would be continually favoured or selected,
until at last a complete separation of the sexes would be effected.