lity, and produce numerous hybrid offspring,
yet these hybrids are remarkably sterile. On the other hand, there are
species which can be crossed very rarely, or with extreme difficulty,
but the hybrids, when at last produced, are very fertile. Even within
the limits of the same genus, for instance in Dianthus, these two
opposite cases occur.

The fertility, both of first crosses and of hybrids, is more easily
affected by unfavourable conditions, than is that of pure species. But
the fertility of first crosses is likewise innately variable; for it
is not always the same in degree when the same two species are crossed
under the same circumstances; it depends in part upon the constitution
of the individuals which happen to have been chosen for the experiment.
So it is with hybrids, for their degree of fertility is often found to
differ greatly in the several individuals raised from seed out of the
same capsule and exposed to the same conditions.

By the term systematic affinity is meant, the general resemblance
between species in structure and constitution. Now the fertility
of first crosses, and of the hybrids produced from them, is largely
governed by their systematic affinity. This is clearly shown by hybrids
never having been raised between species ranked by systematists in
distinct families; and on the other hand, by very closely allied
species generally uniting with facility. But the correspondence between
systematic affinity and the facility of crossing is by no means strict.
A multitude of cases could be given of very closely allied species which
will not unite, or only with extreme difficulty; and on the other hand
of very distinct species which unite with the utmost facility. In
the same family there may be a genus, as Dianthus, in which very many
species can most readily be crossed; and another genus, as Silene,
in which the most persevering efforts have failed to produce between
extremely close species a single hybrid. Even within the limits of the
same genus, we meet with this same difference; for instance, the many
species of Nicotiana have been more largely crossed than the species of
almost any other genus; but Gartner found that N. acuminata, which is
not a particularly distinct species, obstinately failed to fertilise, or
to be fertilised, by no less than eight other species of Nicotiana. Many
analogous facts could be given.

No one has been able to point out what kind or what amount of
difference, in