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Show 290 THE PRINCIPLES OF PART II. the horns are developed later in life in this breed than in ordinary sheep in which both sexes are horned. But with domesticated sheep the presence or absence of horns is not a firmly fixed character; a certain proportion of the merino ewes bearing small horns, and some of the rams being hornless; whHst with ordinary sheep hornless ewes are occasionally produced. . . In most of the species of the splend1d family of tbe Pheasants, the males differ conspicuously from the females, and they acquire their ornaments at a rat~1er late period of life. The eared pheasant ( Cros~opt~lon auriturn ), however, offers a remarkable exception, fot· both sexes possess the fine caudal plumes, the large e.artufts and the crimson velvet about the head; and I find on enquiry in the Zoological Gardens that all these characters in accordance with our rule, appear very early in life. The adult male can, however, be distinguished from the adult female by one character, ~amely by the presence of spurs ; and conformably with our rule, these do not begin to be developed, as I am assured by :M:r. Bartlett, before the age of six months, and even at this age, can hardly be distinguished in the two sexes.28 The male and female Peacock differ con-period oflife, conformably with our rule, than in the Welch sheep, in which both sexes are horned. - 2s In the common peacock (Pavo cristatus) the male alone possesses spurs, whilst both sexes of the Java peacock (P. muticus) offer the unusual case of being furnished with spurs. Hence I fully expected that in the latter species they would have been developed earlier in life than in the common peacock; but M. Hegt of Amsterdam informs me, that with young birds of the previous year, belonging to bo.th species, compared on April 23rd, 1869, there was no difference in the development of the spurs. The spurs, however, were as yet represented merely by slight knobs or elevations: I presume that I should have been informed if any difference in the rate of development had subsequently be~n observed. 'CHAP. Vlll. SEXUAL SELECTION. 2D1 spicuously from each other in almost every part of thei.r plumage, except in the elegant head-crest, which is ?om_mon to both sexes; and this is developed very early m hfe, long before the other ornaments which are confined to the male. The wild-duck offers an analogous case, for the beautiful green speculum on the wings is common to both sexes, though duller and somewhat :Smaller in the female, and it is developed early in life, whilst the curled tail-feathers and other ornaments peculiar to the male are developed later.29 Between such extreme cases of close sexual resemblance and wide dissimilarity, as those of the Crossoptilon and peacock, many intermediate ones could be given, in which the characters follow in their order .of development our two rules. As most insects emerge from their pupal state in a mature condition, it is doubtful whether the period of development determines the transference <Of their characters to one or both sexes. But we do not know that the coloured scales, for instance, in two species of butterflies, in one of which the sexes differ in colour, whilst in the other they are alike, are developed at the same relative age in the cocoon. Nor do we know whether .all the scales are simultaneously developed on the wings 29 In some other species of the Duck Family the speculum in the 'two sexes differs in a greater degree; but I have not been able to discover whether its full development occurs later in life in the males of such specie~, than in the male of the common duck, as ought to be the case accordmg to our rule. With the allied Mergus cucullatus we have however, a case of this kind : the two sexes differ conspicuously i~ ?eneral pl~m~ge, and to a considerable degree in the speculum, whicll Is pure wlnte m the male and greyish-white in the female. ' Now the you~g males at first resemble, in all respects, the female, and have a grey1sh-white s~cculum, but this becomes pure white at an earlier age than that at wln~h the ad~lt male acquires his other more stronglymarked sexual differences m pluma"'e: see Audubon 'OrnitholoO'ical .Biography,' vol. iii. 1835, p. 24D-25o."' ' "' u 2 |