26 THE ORIGIN OF SPECIES 



tempted to be expressed. The facts by no means seem 

 to indicate that the greater or lesser dilTiculty 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 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 cer- 

 tain 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 it may perhaps be argued, that, if a species was ren- 

 dered sterile with some one compatriot, sterility with other 

 species would follow as a necessary contingency. In the 

 second place, it is almost as much opposed to the theory 

 of natural selection as to that of special creation, that in 

 reciprocal crosses the male element of one form should 



