18 LAWS GOVERNING THE STERILITY [Chap. IX. 



nature, I presume that no one will suppose that this 



capacity is a specially endowed qualit}^ 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 adaj)tation 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 tliis capacity, as in hybridisation, is by 



no means absolutely governed by systematic affinity. 



Although many distinct genera witliin 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 



