| → mare by a male-ass; 1872 |
| male-ass with a mare: 1859 1860 1861 1866 1869 |
|
| → structure or constitution, excepting in their reproductive systems. 1872 |
| whole organisation. 1859 1860 1861 |
| whole organisation, except in their reproductive systems. 1866 1869 |
|
| ↑ 2 blocks not present in 1866 1869 1872; present in 1859 1860 1861 |
| On the other hand, these cases clearly show that the capacity for crossing is connected with constitutional differences imperceptible by us, and confined to the reproductive system.
This difference in the result of reciprocal crosses between the same two species was long ago observed by Kölreuter.
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|
|
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 Gärtner found that N. acuminata, which is not a particularly distinct species, obstinately failed to fertilise, or to be fertilised
no less than eight other species of Nicotiana.
analogous facts could be given. |
|
| No one has been able to point out what
or what
of
in any recognisable
is sufficient to prevent two species crossing. It can be shown that plants most widely different in habit and general appearance, and having strongly marked
in every part of the flower, even in the pollen, in the fruit, and in the cotyledons, can be crossed. Annual and perennial plants, deciduous and evergreen trees, plants inhabiting different stations and fitted for extremely different climates, can often be crossed with ease. |
|
| By a reciprocal cross between two species, I mean the case, for instance, of a
being first crossed
a
and then a
→mare by a male-ass;
these two species may then be said to have been reciprocally crossed. There is often the widest possible difference in the facility of making reciprocal crosses. Such cases are highly important, for they prove that the capacity in any two species to cross is often completely independent of their systematic affinity,
of any
difference in their
→structure or constitution, excepting in their reproductive systems. The diversity of
in reciprocal crosses between the same two species was long ago observed by Kölreuter. ↑ To give an instance: Mirabilis
can easily be fertilised by the pollen of M. longiflora, and the hybrids thus produced are sufficiently fertile; but Kölreuter tried more than two hundred times, during eight following years, to fertilise reciprocally M. longiflora with the pollen of M.
and utterly failed. Several other equally striking cases could be given. Thuret has observed the same fact with certain sea-weeds or Fuci. Gärtner, moreover, found that this difference of
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