NATURAL SELECTION 123 



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 to- 

 gether, that any change in the numerical proportions 

 of the inhabitants, independently of the change of cli- 

 mate 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 sur- 

 rounded 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 bet- 

 ter 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, flight modifications, 

 which in any way favored the individuals of any spe- 

 cies, 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 woald man- 

 ifestly 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 for- 

 gotten that mere individual difi'erences are included. As 



