OF FIRST GROSSES AND OF HYBRIDS. 289 



species? Why should some species cross with facility and 

 yet produce very sterile hybrids; and other species cross 

 with extreme difficulty, and yet produce fairly fertile 

 hybrids? Why should there often be so great a differ- 

 ence in the result of a reciprocal cross between the same 

 two species? Why, it may even be asked, has the pro- 

 duction of hybrids been permitted? To grant to species 

 the special power of producing hybrids, and then to stop 

 their further propagation by different degrees of sterility, 

 not strictly related to the facility of the first union between 

 their parents, seems a strange arrangement. 



The foregoing rules and facts, on the other hand, appear 

 to me clearly to indicate that the sterility, both of first 

 crosses and of hybrids, is simply incidental or dependent on 

 unknown differences in their reproductive systems; the 

 differences being of so peculiar and limited a nature, that, 

 in reciprocal crosses between the same two species, the 

 male sexual element of the one will often freely act on the 

 female sexual element of the other, but not in a reversed 

 direction. It will be advisable to explain a little more 

 fully, by an example, what I mean by sterility being inci- 

 dental on other differences, and not a specially endowed 

 quality. As the capacity of one plant to be grafted or 

 budded on another is unimportant for their welfare in a 

 state of nature, I presume that no one will suppose that 

 this capacity is a specially endowed quality, but will admit 

 that it is incidental on differences in the laws of growth of 

 the two plants. We can sometimes see the reason why one 

 tree will not take on another from differences in their rate of 

 growth, in the hardness of their wood, in the period of the 

 flow or nature of their sap, etc. ; but in a multitude of 

 cases we can assign no reason wliatever. Great diversity 

 in the size of two j)lants, one being woody and the other 

 herbaceous, one being evergreen and the other deciduous, 

 and adaptation to widely different climates, do not always 

 prevent the two grafting together. As in hybridization, so 

 with grafting, the capacity is limited by systematic aflftnity, 

 for no one has been able to graft together trees belonging 

 to quite distinct families; and, on the other hand, closely 

 allied species and varieties of the same species, can usually, 

 but not invariably, be grafted with ease. But this capacity, 

 as in hybridization, is by no means absolutely governed by 



