OF FIRST CROSSES AND OF UYBIUDS. 201 



other as incidental on unknown differences in their vege- 

 tative systems, so I believe that the still more complex 

 laws governing the facility of first crosses are inciaental on 

 unknown differences in their reproductive systems. These 

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

 might have been expected, systematic affinity, bv 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 diffi- 

 culty 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 re- 

 marked 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 rendered 

 sterile with some one compatriot, sterility with other spe- 

 cies would follow as a necessary contingency. In tlie 

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

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

 ciprocal crosses the male element of one form should have 

 been rendered utterly impotent on a second form, while at 

 the same time the male element of this second form is en- 



