Darwin thought of natural selection chiefly as the elimination of individuals by death. This is natural selection in the narrower sense. But it is better to avoid possible ambiguity by giving this kind of selection its distinctive name and separate treatment. It may appropriately be called lethal, that is death-bearing selection. Lethal selection, therefore, operates through the early elimination, or death, of relatively ill-adapted individuals. "Early" is here a relative term. Death operative by way of lethal selection occur either before physical maturity, or soon enough after to affect the amount of reproduction. Such death prevents the propagation of "unfit" characters.
Sexual selection depends on the advantage which certain individuals have over others of the same sex and species in respect of mating, and thus of reproduction. It is due to sexual preferences which favor the mating of certain individuals as against others of the same species, and so cause more reproduction of certain characters than of others; or, in another form, it is due to differences between individuals of the same species as regards power forcibly to appropriate mates. The first of these may well be called aesthetic, and the second, combative, sexual selection. Failure to mate, not failure to survive, is the mode of elimination in sexual selection. The individual must become adapted to the phenomena of sex within the species, as well as to outside "nature." "Selection in relation to sex" has an important part in Darwin's theory of organic evolution.
Among animals it is the relatively passive sex which exercises choice in æsthetic sexual selection, that is, usually the female. Hence the beauty and song of birds are male attributes. In combative sexual selection, on the other hand, the competition takes the form chiefly of actual fighting between rival members of the active sex. There is a difference between this struggle for mates and the "struggle for existence." "Nature, red in tooth and claw" is poetic license. The phrase gives no true notion of the workings of natural selection. The poet is apparently licensed to be inaccurate. The struggle for existence is chiefly a noiseless, inglorious effort to wrest from the environment sufficient food to maintain life. For the rest, some animals prey and others are preyed, upon. It is only in combative sexual selection, however, that bloody combat, which implies a degree of equality of prowess, is the regular thing. It is significant, likewise,
- He says, for example, natural selection "produces its effects by the life or death at all ages of the more or less successful individuals." "Descent of Man," last paragraph of the section entitled The Male Generally More Modified than the Female, Ch. VIII.
- These are Darwin's words, with the significant difference that he says "solely in respect of reproduction." See "Descent of Man," fourth paragraph of Ch. VIII. He thus fails to recognize what is called in this article reproductive selection, for his sexual selection is clearly a different thing.