250 THE ORIGIN OF SPECIES 



for their 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, 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 deciduous, and adaptation to widely different climates, 

 do not always prevent the two grafting together. As in hybridiza- 

 tion, 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 hybridization, 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 differ- 

 ent 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 an union is often very far from 

 equal, so it sometimes is in grafting. The common gooseberry, for 

 instance, cannot be grafted on the current, whereas the current 

 will take, though with difficulty, on the gooseberry. 



We have seen that the sterility of hybrids which have their re- 

 productive organs in an imperfect condition, is a different case 

 from the difficulty of uniting two pure species which have their 

 reproductive organs perfect; yet these two distinct classes of 

 cases run to a large extent parallel. Something analogous occurs 

 in grafting; for Thouin found that three species of Robinia, which 

 seeded freely on their own roots, and which could be grafted with 

 no great difficulty on a fourth species, when thus grafted were 

 rendered barren. On the other hand, certain species of Sorbus, 

 when grafted on other species, yielded twice as much fruit as 



