The general rule among vertebrates, as regards colour, is, for the two sexes to be alike. This prevails, with only a few exceptions, in fishes, reptiles, and mammalia; but in birds diversity of sexual colouring is exceedingly frequent, and is, not improbably, present in a greater or less degree in more than half of the known species. It is this class, therefore, that will afford 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 we find the most remarkable examples of sexual divergence of colour. The most gorgeously coloured birds known 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 plumes, crests, and gorgets of the birds of paradise entirely wanting in the females, but these latter are usually without any bright colour at all, and rank no higher than our thrushes in ornamental plumage. Of the humming-birds the same may be said, except that the females are often green, and sometimes slightly metallic, but from their small size and uniform tints are never conspicuous. The glorious blues and purples, the pure whites and intense crimsons of the male chatterers are represented in the females by olive-greens or dull browns, as are the infinitely varied tints of the male tanagers. And in pheasants, the splendour of plumage which characterises the males is entirely absent in the females, which, though often ornamental, have always comparatively sober and protective tints. The same thing occurs with many other groups. In the Eastern tropics are many brilliant birds belonging to the families of the warblers, flycatchers, shrikes, etc., but the female is always much less brilliant than the male and often quite dull coloured. _Cause of Dull Colours of Female Birds._