172 HYBRIDISM. Chap. XIX. 



exposed ; and this difference in susceptibility is clearly in- 

 cidental on other and unknown differences in their organisa- 

 tion. So again the capacity in different kinds of trees to be 

 grafted on each other, or on a third species, differs much, and 

 is of no advantage to these trees, but is incidental on struc- 

 tural or functional differences in their woody tissues. We 

 need not feel surprise at sterility incidentally resulting from 

 crosses between distinct species, — the modified descendants of 

 a common progenitor, — when we bear in mind how easily the 

 reproductive system is affected by various causes — often by 

 extremely slight changes in the conditions of life, by too close 

 interbreeding, and by other agencies. It is well to bear in 

 mind such cases as that of the Passiflora alata, which re- 

 covered its self-fertility from being grafted on a distinct 

 species — the cases of plants which normally or abnormally 

 are self-impotent, but can readily be fertilissed 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 ? W^e can, to a certain extent, give a satisfac- 

 tory 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 differ- 

 ences 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 sub- 

 jected have had so little tendency towards modifying the 

 reproductive system in a manner leading to mutual sterilit}^, 

 that we have very good grounds for admitting the directly 

 opposite doctrine of Pallas, namely, that such conditio is 



