The development of fore and hind limbs from fins, of lung from swim-bladder, and of instincts appropriate to the new medium, would strike all conservative fishes as highly immoral deviations from that biological tradition which had given stability to the glorious race of fishes. Nothing could seem more a-biological. Similarly with the development of fur and feathers and of the parental instinct, all of which were probably, in part at any rate, invested with survival value by the spell of cold which followed the Secondary period, and which perhaps conditioned the substitution of birds and mammals for the hitherto ubiquitous reptile as dominant vertebrates on the earth. From the point of view of the conservative reptile such changes would appear highly anti-biological. In moments of racial crisis, therefore, it is dangerous to generalize as to what is biologically good from past experience alone. In the past a high degree of fertility has been, for most species, a biologically valuable asset. It does not follow that it will continue to be so for the human race. In fact there are good reasons for supposing that it will not. The human race is now passing through a biological crisis unprecedented in the history of life. It has achieved a mastery over nature such that mankind is now economically unified throughout the world by the astounding feats of intercommunication and transport. But as yet the human race has achieved little ethical unification. It is directly in the interests of the race that such unification should take place, and all things which promote it may therefore be considered biologically good. And of those things a restriction of human fertility is one of the most important. What then is to be our biological criterion of racial fitness and our standard for judging of a nation’s merit? It is clear that our biological criterion must be racial rather than individual. Division of labour and differentiation of function are carried to such lengths in civilized societies that it does not seem possible to hold up any individual type as an ideal of biological fitness. Qualities which, to the solitary animal, would irrevocably spell extinction may for the gregarious animal have the highest survival value. Thus no attribute would be more irremediably fatal to a non-social animal than sterility. Yet the sterility of 999 out of 1000 female bees in the community of the hive has endowed the species with a vitality and a biological importance such that it has largely conditioned the appearance on the planet of many kinds of entomophilous flowers. Our biological criterion must therefore, with our standard of merit, be social rather than individual, and the following general outline is suggested.