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Show 158 DARWINISM CHAP. But a.n insect visiting, first, a long-styled plant, woul<l d~pos~t the pollen on the stigma of ~n~ther plant of th.e.same lnnd 1f it were next visited; and tlus IS proba.bly the 1eason why the wild short-styled plants were found to be almost. ~lways most productive of seed, since they must be all fer~1h. eel by the other form, whereas the long-styled plants might often be fertilised by their own form. The wh_ole a.rra,ngeme~t, however, ensures cross-fertilisation ; a.nd th1s, as Mr. D~rwm h:ts shown by copious experiments, adds bot~ to the v1gour a,nd fertility of almost all plants as well as ammals. Besides the primrose family, many other pbnts of several di tinct natural orders pr-esent similar phenomena, one or two of the most curious of which must be referred to. The beautiful crimson flax (Linum gra.ndiflorum) has also two forms, the styles only differing in l01~gth; and ir: this case ~r. Darwin found by numerous cxpcnments, whiCh have smce been repeated and confirmed by other obscrvors, that each form is absolutely sterile with pollen from another phnt of its own form, but abundantly fertile when crossed with a,ny plant of the other form. In this case tho. pollen of tl1c two forms cannot be distinguished under tho microscope ( w boreas that of the two forms of Primula differs in size aml ·hapc), yet it has the remarkable property of b~ing ahsolnt~ly powerless on the stigmas of half the plants ~f 1ts own species. The crosses between the opposite forms, whiCh are fertile, arc termed by Mr. Darwin "legitimate," and those between similar forms, which are sterile, "illegitimate"; and he remarks that we have here, within tho limits of the S<WlC species, a degree of sterility which r~rcly occur~ except between pla.nts or animals not only of different specMs but of different genera. . . . . But there is a.nothcr set of plants, the tnmorphiC, m which the styles and stamens ba.vc each three forms-:-1ong, m.ctlium, and short, and in these it is possible to have mghtccn different crosses. By an elaborate series of experiments it w:"Ls shown that the six legitimate unions-that is, when a plan~ ·was fertilised by pollen from stamens of length corrcspondmg to that of its style in the two other forms-were all abunchntly fertile ; while the twelve illegitimate unions, when a plant w.as fertilised by pollen from stamens of a different length from 1t::; VII ON THE INFERTILITY OF CROSSES 159 own style, in any of the three forms, were either comparatively or wholly sterile. 1 v\' c have here a wonderful amount of constitutional difference of the reproducti vc organs within a single species, greater than usually occurs within the numerous distinct species of a genus or group of genera; and all this diversity appears to have arisen for a purpo. c which has been obtained by many other, and apparently simpler, changes of structure or of function, in other plants. This seems to show us, in the first place, tha.t variations in the mutual relations of the reproductive organs of different individuals must be as frequent u.s strnctural variations have been shown to be; and, also, that sterility in itself can be no test of specific distinctness. But this point will be better considered when we have furth er illustrated and discussed the complex phenomena. of hybridity. Cases of the Fertility of Hybrids, and of the Infertility of Mong1·els. I now propose to adduce a few cases in which it has been proved, by experiment, tha,t hybrids between two distinct species are fertile inte1· se ,· and then to consider w by it is that such cases are so few in number. The common domestic gooi'ic (Anser ferus) and the Chinese goose (A. cygnoides) arevery distinct species, so dist~.nct that some naturalists have placed them in different genera; yet they have bred together, and Mr. Eyton raised from a pair of these hybrids a brood of eight. This fact was confirmed by Mr. Darwin himself, who raised several fine birds from a ].Jair of hybrids which were sent him. 2 In India, according to Mr. Blyth and Captain Hutton, whole flocks of these hybri<l geese arc kept in various parts of the country where neither of the pure parent species exists, and as they are kept for profit they must certainly be fully fertile. Another equally striking case is that of the Indian humped and the common cattle, species which differ osteologically, and also in habits, form, voice, and constitution, so that they are by no means closely allied ; yet Mr. Darwin assures us that he 1 For n full account of these interesting facts and of the various problems to which th ey give rise, tl1e reader must consult Darwin's volume on The Different F01-ms of Flowers in Plants of the sante Species, chaps. i.-iv. 2 See Nature, vol. xxi. p. 207. |