onfounded together--is by no means strict. There are many
cases, in which two pure species can be united with unusual facility,
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 the fertility of pure
species. But the degree of fertility is likewise innately variable; for
it is not always the same when the same two species are crossed under
the same circumstances, but 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 exactly the same conditions.

By the term systematic affinity is meant, the resemblance between
species in structure and in constitution, more especially in the
structure of parts which are of high physiological importance and which
differ little in the allied species. Now the fertility of first crosses
between species, 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, wh