312 ORIGIN OF SPECIES 



we must look at the curious and complex laws governing the 

 facility with which trees can be grafted on each other as in- 

 cidental on unknown differences in their vegetative systems, 

 so I believe that the still more complex laws governing the 

 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 vari- 

 ous species has been a special endowment; although in the 

 case of crossing, the difficulty is as important for the endur- 

 ance and stability of specific forms, as in the case of graft- 

 ing 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 dis- 

 tinct 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 rendered 

 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 have been ten- 



