100 NATURAL SELECTION, [Chap. IV. 



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 in- 

 habitants, independently of the change of climate itself, 

 would seriously affect the others. If the country were 

 open on its borders, new forms would certainly immi- 

 grate, 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 immi- 

 gration, 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 fur the work of improvement. 



We lur 1 reason to 1 elieve, 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 mani- 

 festly be favourable 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 



