Page:Popular Science Monthly Volume 71.djvu/79

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that the comparison of nature to a cock-pit uses phenomena, not of natural, but of sexual, selection.

Reproductive selection depends directly on difference in degree of fertility. If any quality is generally associated with a particularly high or low degree of fertility, it is at an advantage or disadvantage due to this form of selection. Reproductive selection is the case of influences bearing directly on propagation, apart from obstacles to mating, in a way relatively to diminish or increase the number of offspring from individuals possessing certain characteristics. The idea of reproductive selection is not developed by Darwin,[1] though it is fully in accord with his general theory and supported by his emphasis on propagation. It has little applicability to the lower animals, but for man it has very great importance.

Differences in ability to procure mates with resulting differences in number of offspring can be distinguished from differential results where the opportunity to mate and reproduce is equal. The former is sexual selection; the latter is reproductive selection. The two are related as pertaining to propagation exclusively, and are contrasted with lethal selection in that they do not involve the question of individual survival. Reproductive selection is a phenomenon of the diverse results of equal opportunities for sexual intercourse. Sexual selection is a matter of obstacles to mating, that is to getting opportunities for sexual intercourse at all. The former rests on differences between individuals as regards degree of reproductivity, granted mating. The latter turns on differences in degree of ability to obtain mates. Though an element of each form may be present in a particular case of selection, the distinction is important, especially in mankind.

In order that the individual shall be "selected" in the fullest sense, he must successfully run a threefold gauntlet. He must live to maturity and enjoy a long and vigorous prime. In obtaining a mate, or mates, he must be as successful as the "best" of his fellows. He must also, equally with the most favored of his species, possess and exercise the power to reproduce his kind and to hand down his characteristics to a numerous progeny. If he fails in the first particular, he is eliminated by lethal selection. If he fails at the second point, his kind is eliminated by sexual selection. If he fails in the third respect, his kind is eliminated by reproductive selection. In all these three particulars his failure need not be absolute, but may be a matter of degree, in which ease the elimination is gradual. He may survive to maturity, but perhaps little beyond that. He may leave offspring that are too few in number as compared with those of his fellows. The critical question

  1. The name and idea are contributions of Professor Karl Pearson. See his essay "Reproductive Selection" in his "Chances of Death and Other Studios in Evolution"; also "Contributions to the Mathematical Theory of Evolution," III., in the Philosophical Transactions of the Royal Society, Vol. 188, p. 253,