82 
NATUEAL 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 
ijip 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 v/hich they live, that none of 
