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Show 226 HYBRIDISM.. [CDAP. Vlll. yields a perfect gradation in the number of seeds prcr duced, up to nearly complete or even quite cornplete fertility · and as we have seen, in certain abnormal cases, even to an 'exces~ of fertility, beyond that which the plant's own pollen will produce. So in hybrids themselves, there are some which never have produced, and probably never would produce, even with the pollen of either pure parent, a single fertile seed: but in soJne 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 witherine; of the flower is well known to be a sign of incipient fertilisation. From this extreme degree of sterHity 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 di£- . ficulty of making a first cross, and the sterility of the hybrids thus produced-two classes of facts which are generally confounded together-is by no means strict. 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 canoe 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, f0r 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 circurnstances, 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 sarne capsule and exposed to exactly the same conditions. CHAP. VIII.] LAWS OF STERILITY. 227 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· trom them, is largely governed by their systematic affinity. This is clearly sho:vn 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 f~cility. But t~~ correspon~ence between system~t1c affinity a~d the facility of crossing is by no n1eans stnct. A multitude of cases could be given of very closely allied species which will not unite or only ~it~ extrem~ diffi~ulty ; . and .on the other hand of very distinct speme~ which unite w1th the utmost facility. In the. same family ther~ may be a genus, as Dianthus, in whiCh very many sp~Cies c~n mo.st 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 instace, the many species of Nicotiana have be~n more largel:r. crossed than the species o~ almost any other genus; ~ut Gartn~r ~ound th3:t N. am~minata, which is not a partiCularly distinct speCies, obstinately failed to fertilise or to ~e fertilised by, no less than eight other species of Nicotlana. Very many analogous facts could be given. No one has been able to point out what kind or what amou?t, of difference in any recognisable ch~racter is suffiCient to prevent two species crossing. It can be shown that plants most widely different in habit and gener~l 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. Ann'ual and perennial plants, deciduous and evergreen trees plants inhabiti~g different stations and fitted for extrem~ly different climates, can often be crossed. with ease. By a r~ciprocal cross bet'Yeen two species, I mean the case, for mstance, of a stalhon-hoFse being :first crossed |