, 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 wou