234 ON THE ORIGIN OF SPECIES 



by different degrees of sterility, not strictly related to 

 the facility of the first union between their parents, 

 seems to be 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, chiefly in the repro- 

 ductive systems, of the species which are crossed. The 

 differences being of so peculiar and limited a nature, 

 that, in reciprocal crosses between 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 re- 

 versed direction. It will be advisable to explain a little 

 more fully by an example what I mean by sterility being 

 incidental on other differences, and not a specially 

 endowed quality. As the capacity of one plant to be 

 grafted or budded on another is so entirely unimportant 

 for its welfare in a state of nature, I presume that no 

 one will suppose that this capacity is a specially en- 

 dowed 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 whatever. Great diversity in 

 the size of two plants, one being woody and the other 

 herbaceous, one being evergreen and the other de- 

 ciduous, and adaptation to widely different climates, 

 does not always prevent the two grafting together. As 

 in hybridisation, so with grafting, the capacity is 

 limited by systematic affinity, for no one has been able 

 to graft trees together 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 hybridisation, is by no means absolutely governed by 

 systematic affinity. Although many distinct genera 

 within the same family have been grafted together, in 

 other cases species of the same genus will not take on 



