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.
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