“It will have been seen that I cannot follow Mr Wallace in the belief that dull colours, when confined to the females, have been in most cases specially gained for the sake of protection. There can, however, be no doubt, as formerly remarked, that both sexes of many birds have had their colours modified, so as to escape the notice of their enemies; or in some instances, so as to approach their prey unobserved, just as owls have had their plumage rendered soft, that their flight may not be overheard” (_The Descent of Man_, p. 745). The Theory of Thomson and Geddes Thomson and Geddes have attempted to explain sexual dimorphism on the hypothesis that males are essentially dissipators of energy, while females tend to conserve energy. They point out that the spermatozoon is a small intensely active body, which dissipates its energy in motion, while the ovum is a large inert body—the result of the female tendency to conserve energy and to build up material. The various ornaments and excrescences which appear in male organisms are the result of this male tendency to dissipate energy. In the spermatozoon the dissipated energy appears in the form of active movement; in the adult organism it takes the shape of plumes and other ornaments, of song and contests for the females. This theory, however, does not explain what we might call the haphazard nature of sexual dimorphism. If sexual dissimilarity is due to the tendency of the male to dissipate energy, why do we see very marked dimorphism in one species, and no dimorphism in a very nearly allied species? Why are the males larger than the females in some species, and smaller in other species? Again, how is it that in certain species of birds—the quails of the genus _Turnix_, the Painted Snipe (_Rhynchæa_), and the Phalaropes—it is the female who possesses the more showy plumage? Moreover, this theory, equally with that of Wallace, does not explain why the excrescences which characterise the male appear in various parts of the body in different species. Stolzmann’s Theory