Chap. XIX. HYBRIDISM. 189 



being grafted on a distinct species — the cases of plants which 

 normally or abnormally are self-impotent, but can readily be 

 fertilised by the pollen of a distinct species — and lastly the 

 cases of individual domesticated animals which evince towards 

 each other sexual incompatibility. 



We now at last come to the immediate point under dis- 

 cussion : how is it that, with some few exceptions in the case of 

 plants, domesticated varieties, such as those of the dog, fowl, 

 pigeon, several fruit-trees, and culinary vegetables, which differ 

 from each other in external characters more than many species, 

 are perfectly fertile when crossed, or even fertile in excess, 

 whilst closely allied species are almost invariably in some 

 degree sterile ? We can, to a certain extent, give a satisfactory 

 answer to this question. Passing over the fact 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, we know that with 

 species the cause lies exclusively in differences in their sexual 

 constitution. Now the 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 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 would have been in some degree sterile when crossed, 

 become perfectly fertile together. With plants, so far is culti- 

 vation from giving a tendency towards mutual sterility, that in 

 several well-authenticated cases, already often alluded to, certain. 

 species have been affected in a very different manner, for they 

 have become self-impotent, whilst still retaining the capacity 

 of fertilising, and being fertilised by, distinct species. If the 

 Pallasian doctrine of the elimination of sterility through long- 

 continued domestication be admitted, and it can hardly be 

 rejected, it becomes in the highest degree improbable that similar 

 circumstances should commonly both induce and eliminate the 

 same tendency ; though in certain cases, with species having a 

 peculiar constitution, sterility might occasionally be thus in- 



