between men and women by body type alone.

In society, however, we are much more interested in the mental than the
purely physical qualities of the two types of bodies, especially since
the use of machines has so largely replaced brute strength with skill.
Most employments do not even require a muscular skill beyond that
possessed by ordinary individuals of both sexes.

Even this ignores the primary consideration in the sex problem in
society, the first of the following two parts into which the whole
problem may be divided: (1) _How to guarantee the survival of the group
through reproduction_ of a sufficient number of capable individuals; and
(2) How to make the most economical use of the remaining energies, first
in winning nutrition and protection from the environment, second in
pursuing the distinctly human values over and above survival. The sex
problem as a whole is concerned with adjusting two different general
types of individuals, male and female, to the complicated business of
such group life or society. The differences between these two sex-types
being fundamentally functional, the best way to get at them is to trace
the respective and unlike life cycles.

We have already shown in rude outline how a difference (apparently
chemical) between two fertilized eggs starts them along two different
lines of development in the embryonic stage. One develops the
characteristic male primary and secondary sex characters, the other the
female. Throughout the embryonic or intra-maternal stage this
differentiation goes on, becoming more and more fixed as it expresses
itself in physical structures. Childhood is only a continuation of this
development--physically separate from the mother after the period of
lactation. Until puberty, when sex ceases to be merely potential and
becomes functional (about 12-14 in girls and 14-16 boys), the
differences in metabolism are not very marked. Neither are they in old
age, after sex has ceased to be functional. It is during the period when
sex is functional (about 35 years in women and considerably longer in
men) that the gross physiological differences manifest themselves.

Before puberty in both sexes, calcium or lime salts are retained in the
tissues and go to build up the bony skeleton. (A mere sketch of calcium
metabolism is all that can be given here--for details consult such works
as 15 and 17 in bibliography; summary in 14; pp. 34f. & 161f.) Note that
puberty comes earlier in gi