230 EVOLUTION IN NATURE AND UNDER DOMESTICATION. 



be remembered that no strict intentional isolation need be 

 practised in a group of individuals, which nevertheless, by the 

 very fact of being cultivated may be sufficiently isolated to 

 constitute a new species. 



If the natives of some uncultivated country catch a num- 

 ber of young wolves or jackals, and make them live in their 

 midst, they take no pains to prevent their mating with their 

 wild relatives. But, notwitstanding the fact that occasional 

 matings of tame wolves and wild ones occur, the number of in- 

 stances in which tame animals mate with tame is sufficiently 

 larger than the number of instances of crosses between wild 

 and tame, from the very fact that the tame ones live in close 

 proximity to man, to make a species out of the tame group. 



If one takes even a common wild-plant like Ray-grass or 

 Thyme into cultivation, the very massing of the individuals in 

 his fields constitutes an isolation, sufficiently close to insure 

 that the group will become pure for its own genotype, as its to- 

 tal potential variability must have been a fraction of that of 

 the whole species. Crosses with wild-growing plants will occur, 

 but if we contrast the close proximity in which the cultivated 

 plants grow to the scattered stand of the wild plants, we see 

 that the very massing is an effective means of isolation. 



Selection cannot influence a group unless this presents some 

 potential variability, and unless it is isolated from the multitude. 

 In cultivation both conditions are met. 



Why should the potential variation in cultivated plants and 

 animals be high, and therefore make them amenable to change 

 by selection ? We saw, that the very fact of their being taken 

 into cultivation made it probable, that the group has a poten- 

 tial variability smaller than that of the whole species. The 

 answer to the apparent paradox is that species in which the 

 potential variability is low, have no good chance of being a 

 success as cultivated animals or plants. For it is significant to 

 note, that it is not those animals or plants which promise the 

 greatest return per unit of area, which have attained the great- 

 est successes under cultivation, but it is those groups with 



