Even greater is the difficulty presented by those species in which the males alone are provided with horns or antlers. “When,” writes Darwin (_Descent of Man_, p. 767), “the males are provided with weapons which in the females are absent, there can hardly be a doubt that these serve for fighting with other males; and that they were acquired through sexual selection, and were transmitted to the male sex alone. It is not probable, at least in most cases, that the females have been prevented from acquiring such weapons on account of their being useless, superfluous, or in some way injurious. On the contrary, as they are often used by the males for various purposes, more especially as a defence against their enemies, it is a surprising fact that they are so poorly developed, or quite absent, in the females of so many animals.” We have, we believe, demonstrated that Darwin’s theory of sexual selection is unable to account satisfactorily for all the phenomena of sexual dimorphism. But, as we have seen, it is quite possible that sexual selection is a real factor of evolution. We trust that what we have said will stimulate some leisured naturalist to study the question of male and female preference. We now pass on to consider briefly some of the other attempts that have been made to explain the phenomena of sexual dimorphism. Wallace’s Explanation of Sexual Dissimilarity Wallace does not accept the theory of sexual selection. He admits that the form of male rivalry, which Darwin calls “the law of battle,” is “a real power in nature,” and believes that “to it we must impute the development of the exceptional strength, size, and activity of the male, together with the possession of special offensive and defensive weapons, and of all other characters which arise from the development of these, or are correlated with them” (_Darwinism_, p. 283). But the view that the female selects the most beautiful of her suitors has always seemed to Wallace “to be unsupported by evidence, while it is also quite inadequate to account for the facts.” For example, the accessory plumes of birds “usually appear in a few definite parts of the body. We require some cause to initiate the development in one part rather than in another.”