276 DARWINISM chap. 



than half of the known sj^ecies. It is this class, therefore, that 

 "will affoi'd us the best materials for a discussion of the problem, 

 and that may perhaps lead us to a satisfactory explanation of 

 the causes to which sexual colour is due. 



The most fundamental characteristic of birds, from our 

 present point of view, is a greater intensity of colour in the 

 male. This is the case in hawks and falcons ; in many 

 thrushes, warblers, and finches ; in pigeons, partridges, rails, 

 plovers, and many others. When the plumage is highly 

 protective or of dull uniform tints, as in many of the 

 thrushes and warblers, the sexes are almost or quite identical 

 in colour ; but when any rich markings or bright tints are 

 acquired, they are almost always wanting or much fainter in 

 the female, as we see in the black -cap among warblers, and 

 the chaffinch among finches. 



It is in tropical regions, where from a variety of causes 

 colour has been developed to its fullest extent, that Ave find 

 the most remarkable examples of sexual divergence of colour. 

 The most gorgeously coloured l)irds knoAvn are the birds 

 of paradise, the chatterers, the tanagers, the humming-birds, 

 and the pheasant- tribe, including the peacocks. In all these 

 the females are much less brilliant, and, in the great majority 

 of cases, exceptionally plain and dull coloured birds. Not 

 only are the remarkable i:)lumes, crests, and gorgets of the 

 birds of paradise entirely wanting in the females, but these 

 latter arc usually without any bright colour at all, and rank no 

 higher than our thrushes in ornamental jilumage. Of the 

 humming-birds the same may be said, except that the females 

 are often green, and sometimes slightly metallic, Ijut from 

 their small size and uniform tints are never conspicuous. 

 The glorious blues and purples, the pure Avhites and intense 

 crimsons of the male chatterers are represented in the females 

 by olive -greens or dull browns, as are the infinitely A'aried 

 tints of the male tanagers. And in pheasants, the splendour 

 of plumage Avhich characterises the males is entirely absent 

 in the females, which, though often ornamental, have always 

 comparatively sober and protective tints. The same thing 

 occurs Avith many other groups. In the Eastern tropics 

 are many brilliant birds belonging to the families of the 

 Avarblers, flycatchers, shrikes, etc., but the female is ahvays 



