h 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 the changed, and
this would manifestly be favourable to natural selection, by affording
a better chance of the occurrence of profitable variations. Unl