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Show 256 . ~ : HYBRIDISM. CHAP. VIII. of incipient fertilisation. F.r?m this ~xtreme d~gree of sterility we have self-fertilised hybnds produCing a greater and greater number of seeds up to perfect fer- Wil~ . I-Iybrids from two species which are ve7y difficult to cross and which rarely produce any offspring, are generally' very sterile; but the parallelism betwe~~ the difficulty of making a first cross, and the stenhty. of the hybrids thus produced-two classes of facts whwh are generally confounded toget~er-is by no me.ans strict. There are many cases, in whwh two pure specieH 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 ver~ 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 tt·rm systematic affinity is meant, the .res~mblance between species in structure and in constitutwn, more especially in the structure of parts which. are ?f high physiological importance and which differ httle Ill the allied species. Now the fertility of first crosses CHAP. VIII. LAWS OF STERILITY. 257 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. But the correspondence between systematic affinity and the facility of crossing is by no means strict. A multitude of eases 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 N icotiana have been more large I y crossed than the species of almost any other genus; but Gartner found that N. acuminata, which is not a particularly distinct species, obstinately failed to fertilise, or to be fertilised by, no less than eight other species of Nicotiana. Very many analogous facts could be given. No one has been able to point out what kind, or what amount, of difference in any recognisable character 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 differences 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 crosser with ease. |