NATURAL SELECTION. 71 



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 pro- 

 portional 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 seri- 

 ously disturb the relations of some of the former inhabit- 

 ants. 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 tend- 

 ency 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 indi- 

 vidual 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, is necessary iu 



