Chap. IX.] WHEN CROSSED. 85 



the true one, is that they are descended from ahorigi- 

 nally distinct species. Nevertheless the perfect fertil- 

 ity 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 es- 

 pecially 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 ho sure guide to their degree of mutual steril- 

 ity, 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 modify- 

 ing the reproductive system in a manner leading to 

 mutual sterility, that we have good grounds for admit- 

 ting 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 fer- 

 tile 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 op- 

 posite manner, for they have become self-impotent 

 whilst still retaining the capacity of fertilising, and 

 being fertilised by, other species. If the Pallasian doc- 

 trine of the elimination of sterility through long-con- 

 38 



