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.
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