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
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