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 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 governing the
facility of first crosses, are incidental on unknown differences,
chiefly in their reproductive systems. These differences, in both
cases, follow to a certain extent, as might have been expected,
systematic affinity, by which every kind of resemblance and
dissimilarity between organic beings is attempted to be expressed. The
facts by no means seem to me to indicate that the greater or lesser
difficulty of either grafting or crossing together various species has
been a special endowment; although in the case of crossing, the
difficulty is as important for the endurance and stability of specific
forms, as in the case of grafting it is unimportant for their welfare.
_Causes of the Sterility of first Crosses and of Hybrids_. We may now
look a little closer at the probable causes of the sterility of first
crosses and of hybrids. These two cases are fundamentally different,
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