lutely 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 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 Sagaret
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 currant, whereas the currant will take, though with
difficulty, on the gooseberry.

We have seen that the sterility of hybrids which have their reproductive
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 when on their own roots. We are reminded by this latter
fact of the extraordinary cases of Hippeastrum, Passiflora, etc., which
seed much more freely when fertilised with the pollen of a distinct
species than when fertilised with pollen from the same plant.

We thus see that, although there is a clear and great difference between
the mere adhesion of grafted stocks and the union of the male and female
elements in the act of reproduction, yet that there is a rude degree of
parallelism in the results of grafting and of crossing distinct species.
And as we must look at the curious and complex laws governing the
facility with which trees can be grafted on each other as incidental on
unknown differences in their vegetative systems, so I believe that the
still more complex laws gover