18 LAWS GOVERNING THE STERILITY [Chap. IX. 



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 diversity 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, do 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 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 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 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 with different degrees of facility on the 

 quince ; so do different varieties of the apricot and 

 peach on certain varieties of the plum. 



As Gartner found that there was sometimes an 

 innate difference in different individuals of the same 

 two species in crossing ; so Sageret believes this to be 

 the case with different individuals of the same two 



