Chap. IX.] WHEN CROSSED. 35 



instance, that certain South American indigenous do- 

 mestic dogs do not readily unite with European dogs, 

 the explanation which will occur to every one, and 

 probably the true one, is that they are descended from 

 aboriginally distinct species. Nevertheless the perfect 

 fertility of so many domestic races, differing widely 

 from each other in appearance, for instance those of the 

 pigeon, or of the cabbage, is a remarkable fact ; more 

 especially when we reflect how many species there are, 

 which, though resembling each other most closely, are 

 utterly sterile when intercrossed. Several considera- 

 tions, however, render the fertility of domestic varieties 

 less remarkable. In the first place, it may be observed 

 that the amount of external difference between two 

 species is no sure guide to their degree of mutual 

 sterility, so that similar differences in the case of 

 varieties would be no sure guide. It is certain that with 

 species the cause lies exclusively in differences in their 

 sexual constitution. Now the varying conditions to 

 which domesticated animals and cultivated plants have 

 been subjected, have had so little tendency towards 

 modifying the reproductive system in a manner leading 

 to mutual sterility, that we have good grounds for ad- 

 mitting the directly opposite doctrine of Pallas, namely, 

 that such conditions generally eliminate this tendency ; 

 so that the domesticated descendants of species, which 

 in their natural state probably would have been in 

 some degree sterile when crossed, become perfectly 

 fertile together. With plants, so far is cultivation from 

 giving a tendency towards sterility between distinct 

 species, that in several well-authenticated cases already 

 alluded to, certain plants have been affected in an 

 opposite manner, for they have become self-impotent 



