domestic animals, on the other hand, mere survival is not enough. Where the breeder intervenes, propagation becomes the critical point. The breeder can use inferior cattle as draft animals. He favors some definite type for reproduction, but rejected individuals are not therefore destroyed. They may be put to some other use. Breeders selection has, as we shall come to see, the character of reproductive selection. What Darwin, for the most part, dealt with as natural selection, we shall find it better to call lethal selection.
The root-idea of natural selection, and of selection in general, is segregation into classes distinguished by differences as regards continued existence of the type. One type is better adapted and survives, another is eliminated. Selection means, etymologically, a picking out and setting apart.[1] It is isolation in breeding. One eminent biologist and evolutionist, Romanes, would substitute this, as the more general term, for natural selection, and would make the latter but a species of isolation. If a superior type is to be evolved and preserved, breeding must be confined to those possessing in high degree the characteristics of that type. The most direct and sure way to isolate the fit and to prevent the propagation of unfit types is to kill off the unfit individuals. This is just what "nature" does. But there are other ways of attaining the same goal.
Darwin never attempted a formal classification of the forms of selection. He does name, and treat at length, one other form besides natural selection, that is, sexual selection. Other kinds, which are of comparatively little importance in subhuman species, he either altogether fails to distinguish or touches only casually. By his use of the term sexual selection, which he contrasts with "ordinary "[2] or natural selection, he does imply that the word selection is, by destiny, if not by established usage, a generic term, to be qualified by an adjective in order to indicate the various species of selection.
The Four Kinds of Selection
We have now come to the distinctive purpose of this essay, that is a classification of the forms of selection having general applicability. I believe that adequate analysis—of course from the point of view of the sociologist, which is at the same time the most general point of view—gives us four species of selection, named as follows:
Lethal selection.
Sexual selection.
Re-productive selection.
Group selection.
These terms, some of which are already familiar, are now to be defined.
