HYBRIDISM 41 



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 considerations, however, render the fertility of 

 domestic varieties less remarkable. In the first place, it 

 may be observed that the amount of external difference 

 beween 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 toward modi- 

 fying the reproductive system in a manner leading to 

 mutual sterility, that we have good grounds for admitting 

 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 toward 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 while still retaining 

 the capacity of fertilizing, and being fertilized by, other 

 species. If the Pallasian doctrine of the elimination of 

 sterility through long-continued domestication be ad- 

 mitted, and it can hardly be rejected, it becomes in the 



