| → applied to the stigma of some one species of the same genus, 1872 |
| of the same genus applied to the stigma of some one species, 1859 1860 1861 1866 |
| of the same genus applied to the stigma of some one of the species, 1869 |
|
| → The hybrids raised 1872 |
| Hybrids 1859 1860 1861 1866 1869 |
|
| → as in the genus Verbascum, can 1869 1872 |
| can 1859 1860 1861 1866 |
|
| → in degree when 1872 |
| when 1859 1860 1861 1866 1869 |
|
|
fertility, the pollen of
species
→applied to the stigma of some one species of the same genus,
yields a perfect gradation in the number of seeds produced, up to nearly complete or even quite complete fertility; and, as we have seen, in certain abnormal cases, even to an excess of fertility, beyond that which the
own pollen
So in hybrids themselves, there are some which never have produced, and probably never would produce, even with the pollen of
pure
a single fertile seed: but in some of these cases a first trace of fertility may be detected, by the pollen of one of the pure parent-species causing the flower of the hybrid to wither earlier than it otherwise would have done; and the early withering of the flower is well known to be a sign of incipient
From this extreme degree of sterility we have self-fertilised hybrids producing a greater and greater number of seeds up to perfect fertility. |
|
|
→The hybrids raised
from two species which are very difficult to cross, and which rarely produce any offspring, are generally very sterile; but the parallelism between the difficulty of making a first cross, and the sterility of the hybrids thus
classes of facts which are generally confounded
by no means strict. There are many cases, in which two pure
→as in the genus Verbascum, 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
of pure species. But the
of
is likewise innately variable; for it is not always the same
→in degree when
the same two species are crossed under the same
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
between species in structure and
Now the fertility of first
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
|
