It is thus that natural selection acts. Natural selection weeds out relentlessly all organisms which display unfavourable variations. It is thus obvious that many species may, and we believe do, exist which possess characters of no direct utility to them, or even slightly harmful ones. For this reason Wallace and his followers fail in their attempts to prove that every patch of colour in every organism is of direct utility. Natural selection has to take an animal as it finds it—the good with the bad. If an organism as a whole is not wanting—that is to say, if it is able to hold its own against other organisms, and is fitted to fill any place in nature—that organism will probably survive, although it may be defective in many respects. As its name implies, natural selection is a mere selecting agency. It has to choose from what is presented to it. It is not, as many seem to think, a manufacturer or inducer of variations. Natural selection can no more _make_ an animal vary in any given direction than the human breeder can. Its power is limited to the destroying of all variations which do not pass the test prescribed by it. CHAPTER VII SEXUAL DIMORPHISM Meaning of the term—Fatal to Wallaceism—Sexual Selection—The law of battle—Female preference—Mutual Selection—Finn’s experiments—Objections to the theory of Sexual Selection—Wallace’s explanation of sexual dimorphism stated and shown to be unsatisfactory—The explanation of Thomson and Geddes shown to be inadequate—Stolzmann’s theory stated and criticised—Neo-Lamarckian explanation of sexual dimorphism stated and criticised—Some features of sexual dimorphism—Dissimilarity of the sexes probably arises as a sudden mutation—The four kinds of mutations—Sexual dimorphism having shown itself, Natural Selection determines whether or not the organisms which display it shall survive. In some species the sexes are so similar in appearance that it is not possible to tell by mere outward inspection to which sex a given individual belongs. In other species the sexes differ so widely in external appearance that it is difficult to believe that the male and the female belong to the same species. Between these two extremes are a great number of species in which the sexes are more or less dissimilar. Those species in which the sexes differ in appearance are said to be sexually dimorphic. The phenomena of sexual dimorphism are fatal to that form of Neo-Darwinism which sees in natural selection an explanation of all the peculiarities of animal structure and colouration. It is not easy to understand how natural selection can have caused marked sexual dimorphism in a species where the habits of the sexes are the same, in the Paradise Flycatcher (_Terpsiphone paradisi_), for example, where the cock and the hen obtain their food in the same way, and share equally the duties of nest-building, incubation, and feeding the young.