Chap. VIIL 
COMPARED WITH GRAFTING. 
261 
pear to me clearly to indicate that the sterility both of 
first crosses and of hybrids is simply incidental or de¬ 
pendent on unknown differences, chiefly in the repro¬ 
ductive systems, of the species which are crossed. The 
differences being of so peculiar and limited a nature, 
that, in reciprocal crosses between two species the male 
sexual element of the one will often freely act on the 
female sexual element of the other, but not in a re¬ 
versed direction. It will be advisable to explain a little 
more fully by an example what I mean by sterility 
being incidental on other differences, and not a speci¬ 
ally endowed quality. As the capacity of one plant to 
be grafted or budded on another is so entirely unim¬ 
portant for its 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, &c.; but in a multitude 
of cases we can assign no reason whatever. Great di¬ 
versity 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, does not always prevent the two grafting to¬ 
gether. As in hybridisation, so with grafting, the 
capacity is limited by systematic affinity, for no one 
has been able to graft trees together 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 capa¬ 
city, as in hybridisation, is by no means absolutely go¬ 
verned by systematic affinity. Although many distinct 
genera within the same family have been grafted to- 
