ave 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, &c, which seeded much more freely when fertilised
with the pollen of distinct species, than when self-fertilised with their
own pollen. {263}

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, for, as just
remarked, in the union of two pure species the male and female sexual
elements are perfect, whereas in hybrids they are imperfect