Chap. XII. GENERAL RESULTS. 473 



of sterility and the amount of external difference in the 

 species which are crossed ; and still more clearly in the 

 wide difference in the results of crossing reciprocally 

 the same two species; that is, when species A is 

 crossed with pollen from B, and then B is crossed with 

 pollen from A. Bearing in mind what has just been 

 said on the extreme sensitiveness and delicate affinities 

 of the reproductive system, why should we feel any 

 surprise at the sexual elements of those forms, which 

 we call species, having been differentiated in such a 

 manner that they are incapable or only feebly capable 

 of acting on one another ? We know that species have 

 generally lived under the same conditions, and have 

 retained their own proper characters, for a much longer 

 period than varieties. Long-continued domestication 

 eliminates, as I have shown in my 'Variation under 

 Domestication,' the mutual sterility which distinct 

 species lately taken from a state of nature almost always 

 exhibit when intercrossed; and we can thus understand 

 the fact that the most different domestic races of animals 

 are not mutually sterile. But whether this holds good 

 with cultivated varieties of plants is not known, though 

 some facts indicate that it does. The elimination of 

 sterility through long-continued domestication may 

 probably be attributed to the varying conditions to 

 which our domestic animals have been subjected ; and 

 no doubt it is owing to this same cause that they with- 

 stand great and sudden changes in their conditions of 

 life with far less loss of fertility than do natural species. 

 From these several considerations it appears probable 

 that the difference in the affinities of the sexual 

 elements of distinct species, on which their mutual 

 incapacity for breeding together depends, is caused by 

 their having been habituated for a very long period 

 each to its own conditions, and to the sexual elements 



