82 NATUKAL SELECTION. Chap. IV. 



places would have been seized on by intruders. In such 

 case, every slight modification, which in the course of 

 ages chanced to arise, and which in any way favoured 

 the individuals of any of the species, by better adapting 

 them to their altered conditions, would tend to be pre- 

 served ; and natural selection would thus have free 

 scope for the work of improvement. 



We have reason to believe, as stated in the first 

 chapter, that a change in the conditions of life, by 

 specially acting on the reproductive system, causes or 

 increases variability; and in the foregoing case the 

 conditions of life are supposed to have undergone a 

 change, and this would manifestly be favourable to 

 natural selection, by giving a better chance of profitable 

 variations occurring ; and unless profitable variations do 

 occur, natural selection can do nothing. Not that, as I 

 believe, any extreme amount of variability is necessary ; 

 as man can certainly produce great results by adding 

 up in any given direction mere individual differences, 

 so could Nature, but far more easily, from having incom- 

 parably longer time at her disposal. Nor do I believe 

 that any great physical change, as of climate, or any 

 unusual degree of isolation to check immigration, is 

 actually necessary to produce new and unoccupied 

 places for natural selection to fill up by modifying 

 and 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 structure or habits of one inha- 

 bitant would often give it an advantage over others; 

 and still further modifications of the same kind would 

 often still further increase the advantage. No country 

 can be named in which all the native inhabitants are 

 now so perfectly adapted to each other and to the 

 physical conditions under which they live, that none of 



