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Show 176 SEX.UAL SELECTIO~ : BIRDS. PART H. plumage between the sexes are of. the same general nature as the occasionally greater chfferonccs. A good illustration of this fact has already been afforded by those kina:fishers in which either the tail alone or the whole bupper surface of the plumage differs in the same manner in the two sexes. Similar cases may be observed with parrots and pigeons. The differences in colour between the sexes of the same species are, also, of the same general nature as the differences in colour between the distinct species of the same group. Fol' when in a group in which the sexes are usually alike, the malo differs considerably from the female, he is not coloured in a quite new style. Hence we may infer that within the same group the special colours of both sexes when they are alike, and the colours of the malo, when be differs slightly or even considerably from the female, have in most cases been determined by the same general cause ; this being sexual selection. It is not probable, as has already been remarked, that differences in colour between the sexes, when very slight, can be of service to the female as a protection. Assuming, however, that they are of service, they might be thought to be ca es of transition; but we have no reason to believe that many species at any one time are undergoing change. Therefore we can hardly admit that the numerous females which differ very slightly in colour from their males are now all commencing to become obscure for the sake of protection. Even if we consider somewhat more marked sexual dif· ferences, is it probable, for instance, that the bead of the female chaffinch, the crimson on the breast of the female bull:finch,-the green of the female greoufinclt,-tho crest of the female golden-crested wren, have all been rendered less bright by the slow process of selection for the sake of protection? I cannot think so ; and still less CHAP. KV. COLOUR AND NIDIFICATION. 1.71 with the slight differences between the sexes of those .birds which build concealed nests. On the other hand the differences in colour between the sexes, whethe; .great or small, may to a large extent be explained on the principle of the successive variations, acquired ~y the males through sexual selection, having been from the first more or less limited in their transmission to the females. That the degree of limitation should differ. in different species of the same group will not :surpnse any one who has studied the laws of inheritance ~or they are so complex that they appear to us in ou; 1gnorance to be capricious in their action.27 .A.~ far as I ?a? discover there are very few .groups ?f l.lJJ:~s contammg a considerable number of species, m whiCh nll have both sexes brilliantly coloured and alike ; but this appears to be the case, a.s I hear fr~m 1\Ir. Sclater, with the 1\'Iusophagre or plaintai. n-e~ters .. Nor do I believe that any large group .e~Is.ts .m w~ICh the sexes of all the species are widely dissimilar m colour: M1:. Wal1ace informs me that the chatt~rers of S. America ( Ootingidm) offer one of the. best mstances; but with some of the species, in wh1~h. the male has a splendid red breast, the female exh1b1ts some red on her breast; and the females of other species shew traces of the green and other colours -of the males. Nevertheless we have a near approaeh to close sexual similarity or dissimilarity throughout several groups: and this, from whu.t has just been said of the fluctuating nature of inheritance, is a somewhat surprising circumstance. But that the same laws should largely prevail with allied animals is not surprising. The domestic fowl has produced a • ~7 .see remarks to this offLot i~ my work on' Varintbn under Domesl! cntwn,' vol. ii. chap. xii. VOL. II. N |
