NATURAL SELECTION 225 
sions; 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 famili- 
arity 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 prob- 
ably 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 pro- 
portions 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 inhabi- 
tants. 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 favored 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 have changed, and this would 
manifestly be favorable to natural selection, by affording a better 
chance of the occurrence of profitable variations. Unless such occur, 
natural selection can do nothing. Under the term of “‘variations,”’ it 
must never be forgotten that mere individual differences are included. 
As man can produce a great result with his domestic animals and plants 
by adding up in any given direction individual differences, so could 
natural selection, but far more easily from having incomparably longer 
time for action. Nor do I believe that any great physical change, as 
of climate, or any unusual degree of isolation to check immigration, 
