CHAPTER IV - NATURAL SELECTION;
OR THE SURVIVAL OF THE FITTEST
How will the struggle for existence, briefly discussed 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 under
nature? I think we shall see that it can act most efficiently. Let the
endless number of slight variations and individual differences
occurring in our domestic productions, and, in a lesser degree, in
those under nature, be borne in mind; as well as the strength of the
hereditary tendency. Under domestication, it may be truly said that
the whole organisation becomes in some degree plastic. But the
variability, which we almost universally meet with in our domestic
productions, is not directly produced, as Hooker and Asa Gray have
well remarked, by man; he can neither originate varieties, nor prevent
their occurrence; he can preserve and accumulate such as do occur.
Unintentionally he exposes organic beings to new and changing
conditions of life, and variability ensues; but similar changes of
conditions might and do occur under nature. Let it also 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; and consequently what infinitely varied diversities of
structure might be of use to each being under changing 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 occur in the course of many successive 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 individual
differences and variations, and the destruction of those which are
injurious, I have called Natural Selection, or the Survival of the
Fittest. Variations neither useful nor injurious would not be affected
by natural selection, and would be left either a fluctuating
element, as perhaps we see in certain polymorphic species, or would
ultimately become fixed, owing to the nature of the organism and the
nature of the conditions.
Several writers have misapprehended or objected to the term
Natural Selection. Some have even imagined that natural selection
induces variability, whereas it implies only the preservation of
such variations as arise and are beneficial to the being under its
conditions of life. No one objects to agriculturists speaking of the
potent effects of man's selection; and in this case the individual
differences given by nature, which man for some object selects, must
of necessity first occur. Others have objected that the term selection
implies conscious choice in the animals which become modified; and
it has even been urged that, as plants have no volition, natural
selection is not applicable to them! In the literal sense of the word,
no doubt, natural selection is a false term; but who ever objected
to chemists speaking of the elective affinities of the various
elements?- and yet an acid cannot strictly be said to elect the base
with which it in preference combines. It has been said that I speak of
natural selection as an active power or Deity; but who objects to an
author speaking of the attraction of gravity as ruling the movements
of the planets? Every one knows what is meant and is implied by such
metaphorical expressions; and they are almost necessary for brevity.
So again it is difficult to avoid personifying the word Nature; but
I mean by Nature, only the aggregate action and product of many
natural laws, and by laws the sequence of events as ascertained by us.
With a little familiarity such superficial objections will be
forgotten.
We shall best understand the probable course of natural selection by
taking the case of a country undergoing some slight physical change,
for instance, of climate. The proportional numbers of its
inhabitants will almost immediately undergo a change, and some species
will probably 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 the inhabitants, independently of the change of climate
itself, would seriously affect the others. If the country were open on
its borders, new forms would certainly immigrate, and this would
likewise 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 cases,
slight modifications, which in any way favoured the individuals of any
species, by better adapting them to their altered conditions, would
tend to be preserved; and natural selection would have free scope
for the work of improvement.
We have good reason to believe, as shown in the first chapter,
that changes in the conditions of life give a tendency to increased
variability; and in the foregoing cases the conditions have changed,
and this would manifestly be favourable to natural selection, by
affording a better chance of the occurrence of profitable
variations. Unless such occur, natural selection can do nothing. Under
the term of "variations," it must never be forgotten that mere
individual differences are included. As man can produce a great result
with his domestic animals and plants by adding up in any given
direction individual differences, so could natural selection, but
far more easily from having incomparably longer time for action. Nor
do I believe that any great physical change, as of climate, or any
unusual degree of isolation to check immigration, is necessary in
order that new and unoccupied places should be left, for natural
selection to fill up by 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 species would often give it an advantage
over others; and still further modifications of the same kind would
often still further increase the advantage, as long as the species
continued under the same conditions of life and profited by similar
means of subsistence and defence. 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 be still better adapted or improved; for in all
countries, the natives have been so far conquered by naturalised
productions, that they have allowed some foreigners to take firm
possession of the land. And as foreigners have thus in every country
beaten some of the natives, we may safely conclude that the natives
might have been modified with advantage, so as to have better resisted
the intruders.
As man can produce, and certainly has produced, a great result by
his methodical and unconscious means of selection, what may not
natural selection effect? Man can act only on external and visible
characters: Nature, if I may be allowed to personify the natural
preservation or survival of the fittest, cares nothing for
appearances, except in so far as they are 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, as is implied by the
fact of their selection. 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 the eye or to be plainly useful to him. Under
nature, the slightest differences 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 be his results,
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 metaphorically be said that natural selection is daily and
hourly scrutinising, throughout the world, the slightest variations;
rejecting those that are bad, preserving and adding up all that are
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 lapse of ages, and then so imperfect is our view into
long-past geological ages, that we see only that the forms of life are
now different from what they formerly were.
In order that any great amount of modification should be effected in
a species, a variety when once formed must again, perhaps after a long
interval of time, vary or present individual differences of the same
favourable nature as before; and these must be again preserved, and so
onwards step by step. Seeing that individual differences of the same
kind perpetually recur, this can hardly be considered as an
unwarrantable assumption. But whether it is true, we can judge only by
seeing how far the hypothesis accords with and explains the general
phenomena of nature. On the other hand, the ordinary belief that the
amount of possible variation is a strictly limited quantity is
likewise a simple assumption.
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, 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 natural selection might
be 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 a
lamb with the faintest trace of black. We have seen how the colour
of the hogs, which feed on the "paint-root" in Virginia, determines
whether they shall live or die. 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 quite
unimportant, we must not forget that climate, food, &c., have no doubt
produced some direct effect. It is also necessary to bear in mind
that, owing to the law of correlation, when one part varies, and the
variations are accumulated through natural selection, other
modifications, often of the most unexpected nature, will ensue.
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 shape, size, and flavour of the
seeds of the many varieties of our culinary and agricultural plants;
in the caterpillar and cocoon stages of the varieties of the
silk-worm; 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
variations profitable 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; and these modifications
may affect, through correlation, the structure of the adult. So,
conversely, modifications in the adult may affect the structure of the
larva; but in all cases natural selection will ensure that they
shall not be injurious: for if they were so, the species would
become extinct.
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 whole community, if the community 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 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, used exclusively for opening the cocoon- or the hard tip to
the beak of unhatched birds, used for breaking the egg. It has been
asserted, that of the best short-beaked tumbler-pigeons a greater
number 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 all 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.
It may be well here to remark that with all beings there must be
much fortuitous destruction, which can have little or no influence
on the course of natural selection. For instance a vast number of eggs
or seeds are annually devoured, and these could be modified through
natural selection only if they varied in some manner which protected
them from their enemies. Yet many of these eggs or seeds would
perhaps, if not destroyed, have yielded individuals better adapted
to their conditions of life than any of these which happened to
survive. So again a vast number of mature animals and plants,
whether or not they be the best adapted to their conditions, must be
annually destroyed by accidental causes, which would not be in the
least degree mitigated by certain changes of structure or constitution
which would in other ways be beneficial to the species. But let the
destruction of the adults be ever so heavy, if the number which can
exist in any district be not wholly kept down by such causes,- or
again let the destruction of eggs or seeds be so great that only a
hundredth or a thousandth part are developed,- yet of those which do
survive, the best adapted individuals, supposing that there is any
variability in favourable direction, will tend to propagate their kind
in larger numbers than the less well adapted. If the numbers be wholly
kept down by the causes just indicated, as will often have been the
case, natural selection will be powerless in certain beneficial
directions; but this is no valid objection to its efficiency at
other times and in other ways; for we are far from having any reason
to suppose that many species ever undergo modification and improvement
at the same time in the same area.
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