With the peacock even the bones of the tail have been modified to support the heavy tail- coverts. (66. Dr. W. Marshall, 'Uber den Vogelschwanz,' ibid. B. I. Heft 2, 1872.) The body of the Argus is not larger than that of a fowl; yet the length from the end of the beak to the extremity of the tail is no less than five feet three inches (67. Jardine's 'Naturalist Library: Birds,' vol. xiv. p. 166.), and that of the beautifully ocellated secondary wing- feathers nearly three feet. In a small African night-jar (Cosmetornis vexillarius) one of the primary wing-feathers, during the breeding-season, attains a length of twenty-six inches, whilst the bird itself is only ten inches in length.
In another closely-allied genus of night-jars, the shafts of the elongated wing-feathers are naked, except at the extremity, where there is a disc. (68. Sclater, in the 'Ibis,' vol. vi. 1864, p. 114; Livingstone, 'Expedition to the Zambesi,' 1865, p. 66.) Again, in another genus of night-jars, the tail-feathers are even still more prodigiously developed. In general the feathers of the tail are more often elongated than those of the wings, as any great elongation of the latter impedes flight. We thus see that in closely-allied birds ornaments of the same kind have been gained by the males through the development of widely different feathers.
It is a curious fact that the feathers of species belonging to very distinct groups have been modified in almost exactly the same peculiar manner. Thus the wing-feathers in one of the above-mentioned night-jars are bare along the shaft, and terminate in a disc; or are, as they are sometimes called, spoon or racket-shaped. Feathers of this kind occur in the tail of a motmot (Eumomota superciliaris), of a king-fisher, finch, humming-bird, parrot, several Indian drongos (Dicrurus and Edolius, in one of which the disc stands vertically), and in the tail of certain birds of paradise. In these latter birds, similar feathers, beautifully ocellated, ornament the head, as is likewise the case with some gallinaceous birds.
In an Indian bustard (Sypheotides auritus) the feathers forming the ear- tufts, which are about four inches in length, also terminate in discs. (69. Jerdon, 'Birds of India,' vol. iii. p. 620.) It is a most singular fact that the motmots, as Mr. Salvin has clearly shewn (70. 'Proceedings, Zoological Society,' 1873, p. 429.), give to their tail feathers the racket-shape by biting off the barbs, and, further, that this continued mutilation has produced a certain amount of inherited effect.
Again, the barbs of the feathers in various widely-distinct birds are filamentous or plumose, as with some herons, ibises, birds of paradise, and Gallinaceae. In other cases the barbs disappear, leaving the shafts bare from end to end; and these in the tail of the Paradisea apoda attain a length of thirty-four inches (71. Wallace, in 'Annals and Magazine of Natural History,' vol. xx. 1857, p. 416, and in his 'Malay Archipelago,' vol. ii. 1869, p. 390.): in P. Papuana (Fig. 47) they are much shorter and thin. Smaller feathers when thus denuded appear like bristles, as on the breast of the turkey-cock.
As any fleeting fashion in dress comes to be admired by man, so with birds a change of almost any kind in the structure or colouring of the feathers in the male appears to have been admired by the female. The fact of the feathers in widely distinct groups having been modified in an analogous manner no doubt depends primarily on all the feathers having nearly the same structure and manner of development, and consequently tending to vary in the same manner. We often see a tendency to analogous variability in the plumage of our domestic breeds belonging to distinct species. Thus top-knots have appeared in several species.