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. Even in first crosses, the greater or lesser difficulty in
effecting a union apparently depends on several distinct causes. There
must sometimes be a physical impossibility in the male element reaching
the ovule, as would be the case with a plant having a pistil too long
for the pollen-tubes to reach the ovarium. It has also been observed
that when pollen of one species is placed on the stigma of a distantly
allied species, though the pollen-tubes protrude, they do not penetrate
the stigmatic surface. Again, the male element may reach the female
element, but be incapable of causing an embryo to be developed, as seems
to have been the case with some of Thuret's experiments on Fuci. No
explanation can be given of these facts, any more than why certain trees
cannot be grafted on others. Lastly, an embryo may be developed, and
then perish at an early period. This latter alternative has not been
sufficiently attended to; but I believe, from observations communicated
to me by Mr. Hewitt, who has had great experience in hybridising
gal