NATURAL SELECTION 1485 



vored the individuals of any species, by better adapt- 

 ing 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 that changes in the 

 conditions of life give a tendency to increased varia- 

 bility; 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 oc- 

 cur, 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 indi- 

 vidual 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 iso- 

 lation 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 varying 

 inhabitants. For as all the inhabitants of each coun- 

 try 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 the native inhabitants are now 



