100 NATURAL SELECTION. [Chap. IV. 



will almost immediately undergo a change, and some 

 species will probably become extinct. We may con- 

 clude, from what we have seen of the intimate and com- 

 plex 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 ailect the 

 others. If the country were open on its borders, new 

 forms would certainly immigrate, and this would like- 

 wise 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 fav- 

 oured 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 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 forgot- 

 ten that mere individual differences are included. As 



