Chap. IV. 
NATURAL SELECTION. 
81 
bering that many more individuals are born than can 
possibly survive) that individuals having any advan¬ 
tage, 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 varia¬ 
tion 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 under¬ 
going 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 
Ave have seen of the intimate and complex manner in 
wliich the inhabitants of each country are bound to¬ 
gether, 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 ori¬ 
ginal inhabitants were in some manner modified; for, 
had the area been open to immigration, these same 
E 3 
