Page:Popular Science Monthly Volume 71.djvu/81

Rh treat it as on the same level with lethal, sexual and reproductive selection, constituting a fourth species.

It is repeating to say that successful reproduction of kind is the essential fact in selection. But the importance of the point is great enough to hear such a repetition. It is significant that Darwin got his idea from the practise of breeders of domestic animals, which is based upon the principle of reproductive selection. Lethal selection is more radical and more incisive in its methods, but death itself operates as a selective agency only through preventing reproduction. Elimination by death after the reproductive period is passed is not selectional. It merely makes more room for the new generation. Lethal selection comes through early death. It is probable that most animals die either when very young and immature or else after considerable reproduction. Survivorship tables for man exhibit the same general phenomenon, that is low mortality at the prime of life. Though we can not know all the possibilities of selection until we distinguish the four modes, they are not independent explanatory principles. All are reducible to effective propagation of kind, to success in leaving offspring. The fate of the individual as such, counts for nothing. For selection, the continuation or destruction of the line of descent is the thing. An individual is important only as belonging to or representing such a line of descent. The "struggle for existence" is only an incident, or a method, in selection. Selective propagation is what is essential.

The classification above presented is made with reference to the needs and point of view of the sociologist. One might well doubt whether the careful discrimination of reproductive selection, which has been attempted, would be at all justifiedly the little scope of application it finds among the lower animals. We know that sexual selection also has but limited applicability, and only to higher forms of life. In strictness, reproductive selection has been the factor that has, on occasion, adaptively increased the fertility of a species, no matter how low in the scale; while natural selection must have been the means of adaptively decreasing such fertility. But this is a minor point. Sexual selection seems to be the nearest that nature comes to admitting reproductive selection as an important factor. As regards domestic animals, also, what the breeder controls is mating rather than strictly and directly reproduction. This case well illustrates the difficulty of sharply discriminating reproductive selection. In man, however, fertility is extremely variable, by nature and through artificial means, so that we must, in man, take account of sheer natality, apart from other selective factors. It is significant that the point of view of