Pre-pubertal childhood and youth, on the contrary, has its biological contributions to society still before it.  The glandular activity of boys and girls is perhaps not so unlike as to justify society in giving them a different kind of education and preparation for group life.  The excuse for two sorts of training is that the two sexes will not do the same work after puberty.  Hence the question of youthful training is sociological almost entirely—­not biological—­or rather, it rests upon the biology, not of childhood but of the reproductive period, which society anticipates.

Instead of scattering attention over the whole history of the universe, then, or even over the general field of biology, in dealing with sex as a social problem, the emphasis must be upon the human life cycle during the functional-reproductive period.  Other biological data than that which concerns this period is merely introductory or explanatory.  The extent to which the sociological problem involved is linked up with general biological considerations like natural selection, adaptation and specialization will be summarized in a separate chapter.

Earlier female maturity and puberty, as well as lighter structure, have already been accounted for by the metabolism, especially of the calcium salts.  These have also been shown to be the key fact in the monthly periodicity of the mammalian female.  Nearly all of the anatomical and physiological sex differences catalogued by such pioneer workers as Ellis, Ploss, Thomas and Bucura are simply what we should expect from the less active and in some ways peculiar metabolism of woman.

Among such differences are the size and shape of bones and other body structures, the more plentiful haemoglobin in male blood during the reproductive period, and such blood peculiarities as the production of more carbonic acid or the higher specific gravity in the male.  The greater percentage of fat as compared with muscle in women[19], if it is generally true, is what we should expect from a lower metabolism and a tendency to store materials.  The long list of diseases which are more or less sex-limited [20; 14, pp.160f.; 18] are largely endocrine.  Even those which do not primarily concern the internal secretory system would be expected to work somewhat differently in the presence of unlike blood streams.  As to the greater average weight of the male brain, this is true of the whole body.  Brain weight, either absolute or relative to body weight, is not positively known to be in any way correlated (in normal people) with mental capacity.

A library might be stocked with the vast literature devoted to summarizing and cataloguing sex differences; and most of it would be useless from the standpoint of sociology.  Unaccompanied by the criticisms a biologist would have to make on the method of their ascertainment and validity, not to mention their significance, such lists can easily do—­and probably have done—­more harm than good.  One simple and reasonable criterion would reduce this catalogue to fairly modest proportions, so far as social science is concerned:  Which ones have an obvious or even probable social significance? Over and above that, while such contrasts may be of speculative interest, they lead imaginative people to argue from them by analogy and thus cloud the real issues.