sifts, i. e., eliminates discriminatively, includes much more than internecine competition between fellows of the same kith and kin. As Darwin said, the phrase is to be used “in a large and metaphorical sense, including dependence of one being on another, and including (which is more important) not only the life of the individual, but success in leaving progeny.” It may be that the struggle is most severe between members of the same or nearly related species, though Darwin did not give many examples of this, but, in any case, we must not generalize the story of the black and brown rat into a theory of life. The struggle for existence is often manifested in an endeavor after well-being. It is the clash that occurs whenever organisms do in any way assert themselves against limitations and difficulties. The answers-back may be competitive or non-competitive, self-regarding or other-regarding, with teeth and claws, or with wits and kindness. In face of overwhelming difficulties and thwarting limitations, one creature sharpens its weapons, another thickens its armor, a third gives its offspring a better send-off on the journey of life, and a fourth makes some experiment in state-socialism. The modes of reaction are many, and one never to be forgotten is that evasive change of habit and habitat which we call parasitism—the door to which is always open. The struggle for existence includes all the endeavors of mate for mate, of parent for offspring, of kin for kin, as well as every degree of self-assertiveness from the young cuckoo ousting the rightful tenant of the nest to the cannibalism in the cradle that occurs in the egg-capsules of the whelk.
It is said, however, that in the long run what counts is that some members of a varying species are fitter for the conditions of life than their neighbors, and therefore survive. This is true, but the eliminating clash is not necessarily between the individuals, the pruning shears are often in the hands of the environment. The survivor in a plague-stricken family does not survive at the expense of his kin, nor compete with his kin; his phagocytes parry the microbe. In lining its nest with two thousand feathers the long-tailed tit unmistakably strengthens its own and its family’s foothold in the struggle for existence, but its reaction to environing difficulties does not hurt any other tit.
Two other points should be noted. The mode of the struggle for existence is not always competitive, and the result of the struggle for existence is not always the discriminate elimination of the relatively less fit to the conditions. Sometimes all that we can discern is a thinning—not a sifting—and that does not in itself make for evolution. The only result of the struggle for existence that necessarily makes for evolution—progressive or retrogressive—is discriminate selection, where the survivors survive in virtue of the possession of a particular character—which may be better weapons, stronger armor, swifter feet, greater
