Chap. VIII. COMPARED WITH GRAFTING. 261 



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 speci- 

 ally endowed quality. As the capacity of one plant to 

 be grafted or budded on another is so entirely unim- 

 portant for its 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, &c. ; but in a multitude 

 of cases we can assign no reason whatever. Great di- 

 versity in the size of two plants, one being woody and 

 the other herbaceous, one being evergreen and the 

 other deciduous, and adaptation to widely different 

 climates, does not always prevent the two grafting to- 

 gether. 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 capa- 

 city, as in hybridisation, is by no means absolutely go- 

 verned by systematic affinity. Although many distinct 

 genera within the same family have been grafted to- 

 gether, in other cases species of the same genus will 

 not take on each other. The pear can be grafted far 

 more readily on the quince, which is ranked as a distinct 

 genus, than on the apple, which is a member of the 

 same genus. Even different varieties of the pear take 



