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 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