Page:Darwinism by Alfred Wallace 1889.djvu/313

 development of tegumentary appendages. Among birds the most brilliant colours are possessed by those which have developed frills, crests, and elongated tails like the humming-birds; immense tail-coverts like the peacock; enormously expanded wing-feathers, as in the argus-pheasant; or magnificent plumes from the region of the coracoids in many of the birds of paradise. It is to be noted, also, that all these accessory plumes spring from parts of the body which, in other species, are distinguished by patches of colour; so that we may probably impute the development of colour and of accessory plumage to the same fundamental cause.

Among insects, the most brilliant and varied coloration occurs in the butterflies and moths, groups in which the wing-membranes have received their greatest expansion, and whose specialisation has been carried furthest in the marvellous scaly covering which is the seat of the colour. It is suggestive, that the only other group in which functional wings are much coloured is that of the dragonflies, where the membrane is exceedingly expanded. In like manner, the colours of beetles, though greatly inferior to those of the lepidoptera, occur in a group in which the anterior pair of wings has been thickened and modified in order to protect the vital parts, and in which these wing-covers (elytra), in the course of development in the different groups, must have undergone great changes, and have been the seat of very active growth.

Mr. Darwin supposes, that these have in almost every case been developed by the preference of female birds for such males as possessed them in a higher degree than others; but this theory does not account for the fact that these plumes usually appear in a few definite parts of the body. We require some cause to initiate the development in one part rather than in another. Now, the view that colour has arisen over surfaces where muscular and nervous development is considerable, and the fact that it appears especially upon the accessory or highly developed plumes, leads us to inquire whether the same cause has not primarily determined the development of these plumes. The immense tuft of golden plumage in the best known birds of paradise (Paradisea apoda and P. minor)