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Show 161 SEXUAL SELECTION: BIRDS. PART If. sion that as vocal and instrumental organs are of special service only to the males during their courtsh~p, these organs were developed through sexual selec.tJOn a~d continued use in this sex alone-the successive vanations and the effects of us() having been from the first limited in their transmission in a greater or less degree-to the male offspring. . l\iany analogous cases could be a~vanced; for m-stance the plumes on the head, wh1ch are ge_nerally lono-er in the male than in the female, sometimes of equ~l length in both sexes, and occasio~ally absent. in tho female -these several cases sometimes occurrmg in the sam~ group of birds. It would be difficult to account for a difference of this kind between the sexes on the principle of the female having been benefited ?Y possessing a slightly shorter crest than the ~ale, and Its consequent diminution or complete suppressiOn through natural selection. But I will take a more favourable case, namely, the length of the tail. The long t:ain of the peacock would have been not only inconvement but dangerous to the peahen during the period of incubation and whilst accompanying her young. Hence there is not the least a priori improbability in the development of her tail having been checked through natural selection. But the females of various pheasants, which apparently are exposed on their open nests to as much danger as the peahen, have tails of considerable length. The females as well as the males of the Menura superba have long tails, and they build a domed nest, which is a great anomaly in so large a bird. Naturalists have wondered how the female :M:enura could manage her tail during incubation ; but it been dangerous to them during incubation. He adds, that a similar view mo.y possibly account for the inferiority of the female to the male in plumage. Cu.AP. XV. LENGTH OF TAIL IN FEMALE. 165 is now known 7 that she "enters the nest head first, " and then turns round with her tail sometimes over "'her back, but more often bent round by her side. " Thus in time the tail becomes quite askew, and is a "''tolerable guide to the length of time the bird has been sitting." Both sexes of an Australian kingfisher (Panysiptera sylvia) have the middle tail-feathers greatly lengthened ; and as the female makes her nest in a hole, these feathers become, as I am informed by Mr. R. B. Sharpe, much crumpled during nidi£cation. In these two cases the great length of the tail-feathers must be in some degree inconvenient to the female; and as in both species the tail-feathers of the female are somewhat shorter than those of the male, it might be argued that their full development had been prevented through natural selection. Judging from these .cases, if with the peahen, the development of the tail l1ad been checked only when it became inconveniently or dangerously long, she would have acquired a much longer tail than she actually possesses; for her tail is not neatly so long, relatively to the size of her body, as that of many female pheasants, nor longer than that of the female turkey. It must also be borne in mind, that in accordance wjth this view as soon as the tail of the peahen became dangerously long, and its development was consequently checked, she would have continually reacted on her male progeny, and thus have p~·evented t.he peacock from acquiring his present magmficent tram. We may therefore infer that the length of the tail in the peacock and its shortness in the peahen are the result of the requisite variations in the male having been from the first transmitted to the male offspring alone. 7 1\:Ir. Ramsay, in 'Proc. Zoolog. Soc.' 18()8, p. 50. |