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Show 208 SEXUAL SELECTION : BIRDS. P.AitT 11. the male ; and on this view, all the facts become clear; for the males would probably be most charmed or excited by the females which were the most attractive to them by their briO'htor colours, other ornaments, or vocal powers. Sox~al selection would then soon do its work, steadily adding to the attractions of the females; the males aud the you11g being left not at all, or but little modified. CLASS III. When the adult male resembles the adult female, the young of both sexes have a peculiar first plumage of thei1· own.-In this class both sexes ·\vhon adult resemble each other, and differ from the young. This occurs with many birds of many kinds. The male robin can hardly be distinguished from the female, but the young are widely different with their mottled duskyolive and brown plumage. Tho male and female of tho splendid scarlet Ibis are alike, whilst the young are brown; and the scarlet-colour, though common to both sexes, is apparently a sexual character, for it is not well developed with birds under confinement, in the same manner as often occurs in the case of brilliantly coloured male birds. "With many species of herons the young differ greatly from the adults, and their summer plumage, though common to both sexes, clearly has a nuptial character. Young swans are slate-coloured, whilst the mature birds are pure white ; but it would be superfluous to give additional instances. These differences between the young and the old apparently depond, as in the two last classes, on tho young having retained a former or ancient state of plumage, which has been exchanged for a new plumage by the old of both sexes. When the adults are brightly coloured, we may conclude from the remarks just made in 1·elation to the scarlet ibis and to many herons, and from the analogy of the species in the first class, that such colours have been CHAP. XVI. TilE YOUNG LIKE BOTH ADULTS. 209 acquired through se.xual selection by the nearly mature males; but that, differently from what occurs in the two first classes, the transmission, though limited to the same age, has not been limited to the same sex:. Consequent~ y both sexes when mature resemble each other and drffer from the young. CLASS IV. When the adult male resembles the adult female, the young of both sexes in their first plumage resemble the adults.-In this class the young and the adnlts of both sexes, whether brilliantly or obscurely coloured, resemble each other. Such cases arc, I think, ~ore common than those in the last class. We have m England instances in the kingfisher, some woodpeckers, t~e jay, magpie, crow, and many small dullcoloured ~u~s, s~ch. as the hedge-warbler or kitty-wren. But th~ s1m1lanty m plumage between tho young and ~he ol~ 1~ n~ve~· abso;~tely complete, and graduates away mto d.lSSimilanty. Ihus the young of some members of the kmgfisher family are not only less vividly coloured than the adults, but many of the feathers on the lower surf~ce are edged with brown,27-a vestige probably of a fmmer s~ate of the plumage. Frequentlv in the same ?roup of buds, even within the same genu~, for instance man Australian genus of parrokeets (Platycercus) the young of some species closely resemble whilst' the young of other species differ considerabl; from their parents of both sexes, which are alike.28 Both sexes and .the young of the common jay are closely similar· b~It m tho Canada jay (Perisoreus canadensis) the youn~ dlf'fm·. so much. from their parents that they were formerly descnbed as d1stinct species. 29 f ~hJoBr?on, 'Birds of India,' vol. i. p. 222, 228. Gould's 'Handbook 0 28 o 1rds o~ Australia,' vol. i. 12-.1:, 130. 29 Gould, Ibid. vol. ii. p. 37, 46, 56. Audubon, 'Ornith. Biography,' vol. ii. p. 55. VOL. II. p |