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Show 152 SEXUAL SELECTION: BIRDS. PaRT n. passes over sexual selection, ancl asks, " What explana" tion does the law of natural selection give of such " specific varieties as these ? " He answers " none "whatever;" and I quite agree with him. But can this be so confidently said of sexual selection? Seeing in how many ways the tail-feathers of humming-birds differ, why should not the four central feathers have varied in this one species alone, so as to have acquired white tips? The variations may have been gradual, or somewhat abrupt as in the case recently given of the humming-birds near Bogota, in which certain indi'viduals alone have the "central tail-feathers tipped "with beautiful green." In the female of the Uros• ticte I noticed extremely minute or rudimental white tips to the two outer of the four central black tailfeathers ; so that here we have an indication of change of some kind in the plumage of this species. If we grant the possibility of the central tail-feathers of the male varying in whiteness, there is nothing strange in such variations having been sexually selected. The white tips, together with the small white ear-tuftF-:, certainly add, as the Duke of Argyll admits, to the beauty of the male; and whiteness is apparently appreciated by other birds, as may be inferred from such cases as the snowwhite male of the Boll-bird. The statement made by Sir R. Heron should not be forgotten, namely that his peahens, when debarred from access to the pied peacock, would not unite with any other male, and· during that season produced no offspring. Nor is it strange that variations in the tail-feathers of the Urosticte should have been specially selected for the sake of ornament, for the next succeeding genus in the family takes its name of Metallura from the splendour of these feathers. Mr. G:ould, after describing the peculiar plumage of the Uroshcte, adds, "that ornament and variety is the sole CnaP. XIV. GRADATION OF CHARACTERS. 153 "object, I have myself but little doubt." 50 If this be admitted, we can perceive that the males which were decked in the most elegant and novel manner would have gained an advantage, not in the ordinary struggle for life, but in rivalry with other males, and would consequently have left a larger number of offspring to. inherit their newly-acquired beauty. 50 'Introduction to the Tl'ochilidro,' 18Gl, p. 110. |
