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 {81}
(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 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 {82} places would have been seized on
by intruders. In such case, every slight modification, which in the course
of ages c