24 THE ORIGIN OF SPECIES 



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

 of the flow or nature of their sap, etc. ; but in a multi- 

 tude 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 hybridization, 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 hybridiza- 

 tion, 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 facil- 

 ity 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 species in being 

 grafted together. As in reciprocal crosses, the facility of 

 effecting a union is often very far from equal, so it some- 

 times is in grafting; the common gooseberry, for instance, 

 cannot be grafted on the currant, whereas the currant will 

 take, though with difficulty, on the gooseberry. 



