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Show 180 SEXUAL SELECTION: BIRDS. PAU.T II. furnished with a tuft of bristles on the ?reast, but ~n two-year-old birds the tuft is about four mches long m the male and hardly apparent in the female; w~e~, however, the latter has reached her fourth year, It IS from four to £. ve inches in length.29 In these cases, the femaJes follow a normal course of development in ultimately becoming like the ma~es; ~nd such cases must not be confounded with those m whiCh diseased or old females assume masculine characters, or with those in which perfectly fertile females, whilst young, acquire through variation or some unknown cause the characters of the male.30 But all these cases have so much in common that they depend, according to the hypothesis of pangenesis, on gemmules derived f~·om each part of the male being present,. though laten~, m the female; their clevelopment followmg o~ some s~Ight change in the elective affinities of her constituent tissues. · A few words must be added on changes of plumage in relation to the season of the year. From reasons formerly assigned there can be little doubt that the elegant plumes, long pendant feathers, crests, &c., of ecrrets herons and many other birds, which are deve-loo ped ' and re't ained only during the summer, serve exclusively for ornamental or nuptial purpose~, though 2o On Ardetta, Translation of Cnvier·s 'Rcgne Animal,' hy Mr. Blyth, footnote, p. 15D. On tho Peregrine Falcon, Mr. Blyth, in Charlesworth's 'Mag. of Nttt. Ilist.' vol. i. 1837, p. 30!. On Diorurus, 'Ibis,' 1863, p. 44. On tlw Platalea, 'Ibis,' vol. vi. 186-1, p. 366. On the Bombycilla, Audubon's 'Ornitholog. Biography,' vol. i. p. 229. On tho Palroornis, sec, also, J erdon, 'Birds of India,' vol. i. p. 263. On the wild turkey, Audubon, ibid. vol. i. p. 15: I hoar from Judge Caton that in Illinois the female very rarely acquires a tuft. 30 Mr. Blyth has recorded (Translation of Cuvier's 'Regne Animal,' p. 158) various instances with Lanius, Rnticilla, Linaria, and Anas. Audubon has also recorded a similar case (' Ornith. Biog.' vol. v. p. 519) with Tymnga i.l!stiva. CHAP. XV. SUMMER PLUMAGE. 181 common to both sexes. The female is thus rendered more conspicuous during the period of incubation than during the winter; but such birds as herons and egrets would be able to defend themselves. As, however, plumes would probably be inconvenient and certainly of no use during the winter, it is possible that the habit of moulting twice in the year may have been gradually acquired through natural selection for the sake of casting off inconvenient ornaments during the winter. But this view cannot be extended to the many waders, in which the summer and winter plumages differ very little in colour. vVith defenceless species, in which either both sexes or the males alone become extremely conspicuous during the breeding-season,or when the males acquire at this season such long wing or tail-feathers as to impede their flight, as with Oosmetornis and Vidua,-it certainly at first appears highly probable that the second moult has been gained for the special purpose of throwing off these ornaments. We must, however, remember that many birds, such as Birds of Paradise, the Argus pheasant and peacock, do not cast their plumes during the winter; and it can hardly be maintained that there is somethino- in the constitution of these birds, at least of the Gallinacero, rendering a double moult impossible, for the ptarmigan moults thrice in the year.31 Hence it must be considcrecl as doubtful whether the many species which moult their ornamental plumes or lose their bright eolours during the winter, have acqnired this habit on uccount of the inconvenience or danger which they would otherwise have suffered. I conclude, therefore, that the habit of moulting twice in the year was in most or all cases first acquired aJ See Gould's 'Birds of Great Britain.' |
