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Show 184 HYBRIDISM. CHAP. XIX. test of lessened fertility, both in first crosses and in hybrids, is no safe criterion of specific distinction; secondly, because we uiay conclude that there must be some unk~own l~w or bond connecting the infertility of illegitimate umons with that ~f their illeo·itimate offspring, and we are thus led to extend this view to first crosses and hybrids; thirdly, beca.us~ we ~nd, an.d this seems to me of especial importance, that w1th tr1morplnc plants three forms of the same species exist, which when cro.ssed in a particular manner are infertile, and yet the.se forms di~er in no respect from each other, except in thmr reprodu.ct~ve oro·ans -as in .the relative lenoth of the stamens [and pistils, 0 ' ,, 0 • • h in the size, form, and colour of the pollen-grams, m t e struc-ture of the stioma, and in the number and size of the seeds. 0 • . • ·with these differences and no others, either m orgamsahon or constitution, we find that the illegitimate unions and the illegitimate progeny of these three forms are more or less sterile, and closely resemble in a whole series of relations the first unions and hybrid offspring of distinct species. From this we may infer that the sterility of species when crossed and of their hybrid progeny is likewise in all probability exclusively due to differences confined to the reproductive system. We have indeed been brought to a similar conclusion by observing that the sterility of crossed species does not strictly coincide with their systematic affinity, that is, with the sum of their external resemblances; nor does it coincide with their similarity in general constitution. But we are more especially led to this same conclusion by considering reciprocal crosses, in which the male of one species cannot be united, or can be united with extreme difficulty, with the female of a second species, whilst the converse cross can be effected with perfect facility ; for this difference in the facility of making reciprocal crosses, and in the fertility of their offspring, must be attributed either to the ~ale or female element in the first species having been differentiated with reference to the sexual element of the second species in a higher degree than in the converse case. In so complex a su~ject as Hybridism it is of considerable importance thus to arrive at a definitive conclusion, namely, that the sterility which almost invariably follows the union of distinct CHAP. XIX. ITYBRIDISM. 185 species depends exclusively on differences m their sexual constitution. On the principle which makes it necessary for man, whilst he is selecting and improving his domestic varieties, to keep them separate, it would clearly be advantageous to varieties in a state of nature, that is to incipient species, if they could be kept from blending, either through sexual aversion, or by becoming mutually sterile. Hence it at one time appeared to me probable, as it has to others, that this sterility might have been acquired through natural selection. On this view we must suppose that a shade of lessened fertility first spontaneously appeared, like any other modification, in certain individuals of a species when crossed with other individuals of the same species; and that successive slight degrees of infertility, from being advantageous, were slowly accumulated. This appears all the more probable, if we admit that the structural differences between the forms of dimorphic and trimorphic plants, as the length and curvature of the pistil, &c., have been co-adapted through natural selection; for if this be admitted, we can hardly avoid extending the same conclusion to their mutual infertility. Sterility moreover has been acquired through natural selection for other and widely different purposes, as with neuter insects in reference to their social economy. In the case of plants, the flowers on the cirqumference of the truss in the guelder-rose (Viburnum opulus) and those on the summit of the spike in the feather-hyacinth (Muscari comosum) have been rendered conspicuous, and apparently in consequence sterile, in order that insects might easily discover and visit the other flowers. But when we endeavour to apply the principle of natural selection to the acquirement by distinct species of mutual sterility, we meet with great difficulties. In the first place, it may be remarked that separate regions are often inhabited by groups of species or by single species, which when brought together and crossed are found to be more or less sterile; now it could clearly have been of no advantage to such separated species to have been rendered mutually sterile, and consequently this could not have been effected through natural selection ; but it may perhaps be argued, that, if a species were rendered sterile with |