64 NATURAL SELECTION. [Chap. IV. 



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 modifica- 

 tions, 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 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 manifestly 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 indivi- 

 dual 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 un- 

 usual degree of isolation to check immigration, is necessary in order 

 that new and unoccupied places should be left, for natural selec- 

 tion 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 struc- 

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

 others ; and still further modifications of the same kind would often 

 still further increase the advantage, as long as the species continued 

 under the same conditions of life and profited by similar means of 

 subsistence and defence. No country can be named in which all 



