Wallace considers that natural selection is able to explain all the phenomena of sexual dimorphism. He points out that, when the sexes are dissimilar among birds, it is almost invariably the female which is duller coloured. The reason for this is, he believes, that the hen birds, while sitting, “are exposed to observation and attack by the numerous devourers of eggs and birds, and it is of vital importance that they should be protectively coloured in all those parts of the body which are exposed during incubation. To secure this, all the bright colours and showy ornaments which decorate the male have not been acquired by the female, who often remains clothed in the sober hues which were probably once common to the whole order to which she belongs. The different amounts of colour acquired by the females have no doubt depended on peculiarities of habits and environment, and on the powers of defence and concealment possessed by the species.” In support of his contention, Wallace asserts that all species of birds, of which the hens are as conspicuously coloured as the cocks, nest in holes or build domed nests. The plumes and other ornaments, which the cocks of certain species display, Wallace would attribute to a surplus of strength, vitality, and growth power, which is able to expend itself in this way without injury. “If,” he writes, “we have found a _vera causa_ for the origin of ornamental appendages of birds and other animals in a surplus of vital energy, leading to abnormal growths in those parts of the integument where muscular and nervous action are greatest, the continuous development of these appendages will result from the ordinary action of natural selection in preserving the most healthy and vigorous individuals, and the still further selective agency of sexual struggle in giving to the very strongest and most energetic the parentage of the next generation.” (_Darwinism_, p. 293.) “Why,” he says, “in allied species the development of accessory plumes has taken different forms we are unable to say, except that it may be due to that individual variability which has served as the starting point for so much of what seems to us strange in form, or fantastic in colour, both in the animal and vegetable world.” Wallace’s Theory Criticised Wallace’s view that the dull plumage of the hen bird is due to her greater need of protection is based on the assumption that the hen bird alone takes part in incubation. Is this assumption a correct one?