| 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.
Chapter 4-Natural Selection
Charles Darwin
The Origin of Species |
