Among such differences are the size and shape of bones and other body structures, the more plentiful hæmoglobin 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. What are the outstandingly significant sex differences which application of the above criterion leaves? (1) A less active and more uneven metabolism of woman; (2) Associated with this, less physical strength on the average--hence an inferior adaptability to some kinds of work, resulting in a narrower range of choice of occupation, disadvantageous in competitive society; (3) But the one fundamental difference, to which all the others are as nothing, is the specialization of the mammalian female body and metabolism to furnish the intra-maternal environment (approximately nine months in the human species) for the early development of the young and lactation for some months afterward. This last may be said to include the former two, which were arbitrarily placed first because they are always in evidence, whether reproduction is undertaken or not. This takes us out of cell and endocrine biology and into the general problem in group adjustment to environment which that specialization entails. BIBLIOGRAPHY FOR CHAPTER III 1. Goldschmidt, R. Experimental Intersexuality and the Sex Problem. Amer. Naturalist, 1916. Vol. 50, pp. 705f.