How could the splendid plumage of the humming-birds, of the pheasants, of the parrots, the wonderful colour-patterns of so many diurnal butterflies, be referred to the process of natural selection, since all these characters can have no significance for their possessors in the struggle for existence? Or of what use in the struggle for existence could the possession of its gorgeous dress of feathers be to the bird of Paradise; or of what service is the azure blue iridescence of the _Morpho_ of Brazil, which makes it conspicuous from a distance when it plays about the crowns of the palm-trees? We might indeed suppose that they are warning signs of unpalatableness, like those of the Heliconiides or of the gaily coloured caterpillars, but, in the first place, these gay creatures are by no means inedible, and are indeed much persecuted; and, secondly, the females have quite different and very much darker and simpler colours. The gleaming splendour of all these birds of Paradise and humming-birds, as well as that of many butterflies, is found in the male sex only. The females of the birds just mentioned are dark in colour and without the sparkling decorative feathers of the males; they are plain--just like the females of many butterflies. Alfred Russel Wallace has suggested that the explanation of this lies in the greater need of the females for protection, since, as is well known, they usually perform the labours of brooding, and are thus frequently exposed to the attacks of enemies. It is undoubtedly true that the dark and inconspicuous colouring of many birds and butterflies depends on this need for protection, but this does not explain the brilliant colours of the males of these species. Or can it be that these require no explanation further than that they are, so to speak, a chance secondary outcome of the structural relations of the feathers and wing-scales respectively, which brought with it some other advantage not known to us? Perhaps something in the same way as the red colour of the blood in all vertebrates, from fishes upwards, cannot be useful on the ground that it appears red to us, but because it is the expression of the chemical constitution of the hæmoglobin, a body which is indispensable to the metabolism, which here has the secondary and intrinsically quite unimportant peculiarity of reflecting the red rays of light.