20 CAUSES OP THE STERILITY [Chap. IX. 



crosses are incidental on unknown diflEerences 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 resem- 

 blance and dissimilarity between organic beings is at- 

 tempted 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 diffi- 

 culty is as important for the endurance and stability of 

 specific forms, as in the case of grafting it is unimpor- 

 tant 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 an3"of h^Sds. 

 might have Tieenslowly acquired "through the natural 

 selection of sliglitly lessened degrees of' fertility, which, 

 like any other variation, spontaneously appeared in cer- 

 tain individuals of one variety when crossed with those 

 of another variety. For it would clearly be advantage- 

 ous 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 sepa- 

 rated species to have been rendered mutually sterile, 

 and consequently this could not have been effected 

 through natural selection; but it may perhaps be argued. 



