102 CIRCUMSTANCES FAVOURABLE Chap. IV. 



is an extremely intricate subject. A large amount of 

 inheritable and diversified variability is favourable, but 

 I believe mere individual differences suffice for the work. 

 A large number of individuals, by giving a better chance 

 for the appearance within any given period of profitable 

 variations, will compensate for a lesser amount of vari- 

 ability in each individual, and is, I believe, an ex- 

 tremely important element of success. Though nature 

 grants vast periods of time for the work of natural 

 selection, she does not grant an indefinite period ; for 

 as all organic beings are striving, it may be said, to 

 seize on each place in the economy of nature, if any one 

 species does not become modified and improved in a 

 corresponding degree with its competitors, it will soon 

 be exterminated. 



In man's methodical selection, a breeder selects for 

 some definite object, and free intercrossing will wholly 

 stop his work. But when many men, without intending 

 to alter the breed, have a nearly common standard of 

 perfection, and all try to get and breed from the best 

 animals, much improvement and modification surely but 

 slowly follow from this unconscious process of selection, 

 notwithstanding a large amount of crossing with inferior 

 animals. Thus it will be in nature ; for within a con- 

 fined area, with some place in its polity not so perfectly 

 occupied as might be, natural selection will always tend 

 to preserve all the individuals varying in the right direc- 

 tion, though in different degrees, so as better to fill up 

 the unoccupied place. But if the area be large, its 

 several districts will almost certainly present different 

 conditions of life ; and then if natural selection be mo- 

 difying and improving a species in the several districts, 

 there will be intercrossing with the other individuals 

 of the same species on the confines of each. And in 

 this case the effects of intercrossing can hardly be coun- 



