CHAP. IV.] NATURAL SELECTION. 59 



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 im- 

 provement. 



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 favourable to natural 

 selection, by affording a better chance of the occurrence of profit- 

 able variations. Unless such occur, natural selection can do 

 nothing. Under the term of " variations," it must never be for- 

 gotten 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 incom- 

 parably 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, is necessary in order that new and un- 

 occupied places should be left, for natural selection to fill up by 

 improving some of the varying inhabitants. For as all the 

 inhabitants of each country are struggling together with nicely 

 balanced forces, extremely slight modifications in the structure 

 or habits of one species would often give it an advantage over 



