Darwin's Artificial and Natural Selection 1 53 
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 
spectally 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 
