| → Hybrids 1859 1860 1861 1866 1869 |
| The hybrids raised 1872 |
|
| → can 1859 1860 1861 1866 |
| as in the genus Verbascum, can 1869 1872 |
|
| → when 1859 1860 1861 1866 1869 |
| in degree when 1872 |
|
|
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. |
|
|
→Hybrids
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
→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
→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
|
