beings, with which it comes into competition for food or residence, or from
which it has to escape, or on which it preys. This is obvious in the
structure of the teeth and talons of the tiger; and in that of the legs and
claws of the parasite which clings to the hair on the tiger's body. But in
the beautifully plumed seed of the dandelion, and in the flattened and
fringed legs of the water-beetle, the relation seems at first confined to
the elements of air and water. Yet the advantage of plumed seeds no doubt
stands in the closest relation to the land being already thickly clothed by
other plants; so that the seeds may be widely distributed and fall on
unoccupied ground. In the water-beetle, the structure of its legs, so well
adapted for diving, allows it to compete with other aquatic insects, to
hunt for its own prey, and to escape serving as prey to other animals.
The store of nutriment laid up within the seeds of many plants seems at
first sight to have no sort of relation to other plants. But from the
strong growth of young plants produced from such seeds (as peas and beans),
when sown in the midst of long grass, I suspect that the chief use of the
nutriment in the seed is to favour the growth of the young seedling, whilst
struggling with other plants growing vigorously all around.
Look at a plant in the midst of its range, why does it not double or
quadruple its numbers? We know that it can perfectly well withstand a
little more heat or cold, dampness or dryness, for elsewhere it ranges into
slightly hotter or colder, damper or drier districts. In this case we can
clearly see that if we wished in imagination to give the plant the power of
increasing in number, we should have to give it some advantage over its
competitors, or over the animals which preyed on it. On the confines of
its geographical range, a change of constitution with respect to climate
would clearly be an advantage to our plant; but we have reason to believe
that only a few plants or animals range so far, that they are destroyed by
the rigour of the climate alone. Not until we reach the extreme confines
of life, in the arctic regions or on the borders of an utter desert, will
competition cease. The land may be extremely cold or dry, yet there will
be competition between some few species, or between the individuals of the
same species, for the warmest or dampest spots.
Hence, also, we can see that when a plant or animal is placed in a new
country amongst new competitors, though the climate may be exactly the same
as in its former home, yet the conditions of its life will generally be
changed in an essential manner. If we wished to increase its average
numbers in its new home, we should have to modify it in a different way to
what we should have done in its native country; for we should have to give
it some advantage over a different set of competitors or enemies.
It is good thus to try in our imagination to give any form some advantage
over another. Probably in no single instance should we know what to do, so
as to succeed. It will convince us of our ignorance on the mutual
relations of all organic beings; a conviction as necessary, as it seems to
be difficult to acquire. All that we can do, is to keep steadily in mind
that each organic being is striving to increase at a geometrical ratio;
that each at some period of its life, during some season of the year,
during each generation or at intervals, has to struggle for life, and to
suffer great destruction. When we reflect on this struggle, we may console
ourselves with the full belief, that the war of nature is not incessant,
that no fear is felt, that death is generally prompt, and that the
vigorous, the healthy, and the happy survive and multiply.
Chapter IV
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.
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
an
d 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
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, &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