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.
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