64 THE ORIGIN OF SPECIES 



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, inde- 

 pendently 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 modi- 

 fications, 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 tendency to increased 

 variability; and in the foregoing cases the conditions have 

 changed, and this would manifestly be favorable to natural selec- 

 tion, 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 can produce a 

 great result with his domestic animals and plants by adding up in 

 any given direction individual differences, so could natural selec- 

 tion, 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 in order that new and unoccupied places should be 

 left for natural selection to fill up by improving some of the vary- 

 ing inhabitants. For as all the inhabitants of each country are 

 struggling together with nicely balanced forces, extremely slight 

 modifications in the structure 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 the native inhabitants are now so per- 



