20 ■ CAUSES OF THE STEEILITY [Chap. IX. 



facility of first crosses are incidental on unknown 

 differences in their reproductive systems. These 

 differences in both cases, follow to a certain extent; as 

 might have been expected, systematic affinity, by which 

 term every kind of resemblance and dissimilarity 

 between organic beings is attempted to be expressed. 

 The facts by no means seem to indicate that the greater 

 or lesser difficulty of either grafting or crossing various 

 species has been a special endowment ; although in the 

 case of crossing, the difficulty is as important for the 

 endurance and stability of specific forms, as in the case 

 of grafting it is unimportant for their welfare. 



Origin and Causes of the Sterility of first Crosses 

 and of Hybrids. 



At one time it appeared to me probable, as it has to 

 others, that the sterility of first crosses and of hybrids 

 might have been slowly acquired through the natural 

 selection of slightly lessened degrees of fertility, which, 

 like any other variation, spontaneously appeared in 

 certain individuals of one variety when crossed with 

 those of another variety. For it would clearly be 

 advantageous to two varieties or incipient species, if 

 they could be kept from blending, on the same 

 principle that, when man is selecting at the same time 

 two varieties, it is necessary that he should keep them 

 separate. In the first place, it may be remarked that 

 species inhabiting distinct regions are often sterile 

 when crossed; now it could clearly have been of no 

 advantage to such separated species to have been 

 rendered mutually sterile, and consequently this could 

 not have been effected through natural selection ; but 



