afting together. 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 capacity, as in hybridisation, 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 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 very different
case from the difficulty of uniting two pure species, which have their
reproductive organs perfect; yet these two distinct cases run to a
certain 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
another 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 case of Hippeastrum, Lobelia, etc.,
which seeded much more freely when fertilised with the pollen of
distinct species, than when self-fertilised with their own pollen.

We thus see, that although there is a clear and fundamental difference
between the mere adhesion of grafted stocks, and the union of the male
and female