Darwin s Artificial and Natural Selection 153 



santly occurred with many species, for a multitude are mutu- 

 ally quite barren." 



Darwin points out the interesting parallel existing between 

 the results of intercrossing, and those of grafting together 

 parts of different species. 



"As the capacity of one plant to be grafted or budded on 

 another is unimportant 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 adapted 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 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 distant 

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

 genus. Even different varieties of the pear take with differ- 

 ent degrees of facility on the quince; so do different varieties 

 of the apricot and peach on certain varieties of the plum." 



" We thus see, that although there is a clear and great 

 difference between the mere adhesion of grafted stocks, and 



