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Show 134 SEXUAL SEI,ECTION: BIRDS. PAUl' JI. development of a perfect ocellus does not require a long course of variation an<.l selection. With birds and many other animals it seems, from the comparison of allied species, to follow, tlJat circular spots arc often generated by the breaking up and contraction of stripes. In the Tragopan pheasant faint white lines in the female represent the beautiful white spots in the male; 16 and something of the sn,me kind may be observed in the two sexes of the Argus pheasant. However this may be, appearances strongly favour the belief that, on the one hand, a dark spot is often formed by tho colouring-matter being drawn towards a central point from a surrounding zone, which is thus rendered lighter. And, on the other hand, that a white spot is often formed by the colour being driven away from a central point, so that it aecumulates in a surrounding darker zone. In either case an ocellus is the result. The colouring matter seems to be a nearly constant quantity, but is Tedistributed, either centripetally or centrifugally. The feathers of the common guinea-fowl offer a good instance of white spots surrounded by darker zones; and wherever the white spots are large and stand near each other, the surrounding dark zones become confluent. In the same wing-feather of the Argus pheasant dark spots may be seen surrounded by a pale zone, and white spots by a dark zone. Thus tho formation of an ocellus in its simplest state appears to be a simple affair. By what further steps tho more complex ocelli, which wonderful amount of variation in tho coloration and shape of tho wings of this butterfly, in his 'Rhopalocora Africro Australis,' p. 18G. See also an interesting paper by the Rev. II. H. Higgins, on the origin of tho ocelli in the Lepidoptera in the 'Quarterly Journal of Science/ July, 1868, p. 325. 46 Jordon, .' Birds of.Inclia,' vol. iii. p. 517. CIIAI'. XlY. GRADATION OF CHARACTERS. 135 are surrounded by many successive zones of colour, have been generated, I will not pretend to say. But bearing in mind the zoned feathers of the mongrel offspring from differently-coloured fowls, and the extraordinary variability of the ocelli in many Lepidoptera, the formation of these beautiful ornaments can hardly be a highly complex process, and probably depends on some slight and graduated change in the nature of the " tissues. Gradation of Secondary Sexual Oharacters.-Cases of gradation are important for us, as they shew that it is at least possible that highly complex ornaments may have been acquired by small successive steps. In ordel' to discover the actual steps by which the male of any existing bird has acquired his magnificent colours or other ornaments, we ought to behold the long line of his ancient and extinct progenitors ; but this is obviously impossible. Vve may, however, generally gain a clue by comparing all the species of a group, if it be a large one ; for some of them will probably retain, at least in a partial manner, traces of their former characters. Instead of entering on tedious details respecting various groups, in which striking instances of gradation could be given, it seems the best plan to take some one or two strongly-characterised cases, for instance that of the peacock, in order to discover if any light can thus be thrown on the steps by which this bil'd has become so splendidly decorated. 'rhe peacock is chiefly rernarka.ble from the extraordinary length of his tail-coverts; the tail itself not being much elongated. The barbs along nearly the whole length of these feathers stand separate or are decomposed; but this is the case with the feathers of many species, and with some varieties of the |