ved a surplus
of intelligent consideration. Sometimes human societies have built so
foolishly upon it that the result has been collapse. Somebody is always
digging around it in quest of evidence of some vanished idyllic state of
things which, having had and discarded, we should return to. This little
excursion into biology is made in the full consciousness that social
mandates are not to be found there. Human projects are the primary
material of social science. It is indispensable to check these against
biological fact, in order to ascertain which are feasible and which are
not. The biological basis may _help_ in explaining old social structures
or in planning new ones; but much wild social theory has been born of a
failure to appreciate the limitations of such material.

All the so-called higher animals, mammals and others, are divided into
two sexes, male and female. Besides the differentiation of germ-cells
there are rather obvious differences in the bodies of the two sexes. In
common with many other mammals, the human male has a larger and stronger
body, on an average, than has the human female. This is true also of the
anthropoid apes, the species which most resemble man physically and are
commonly supposed to be his nearest blood relatives in the animal
kingdom. It has been true of man himself as far back as we have any
records.

Such differences are only superficial--the real ones go deeper. We are
not so much interested in how they originated in the world as in how
they _do_ come about in the individual. At least, we can come a good
deal nearer ascertaining the latter than the former. In either case, our
real purpose is to determine as nearly as possible what the unlikeness
really consists of and so help people to sensibly make up their minds
what can be done about it.

To define sex with rigid accuracy as the term applies to human beings,
it is necessary to tell what it is in mammals, since man is a mammal.
The presence of distinct body-cells is not peculiar to mammals, but
there is one respect in which these latter are quite different from
non-mammals: A mammalian individual, beginning like a non-mammal with a
fertilized egg, has a period of intra-maternal development which a
non-mammal has not. That is, a non-mammalian is a fertilized egg _plus_
its parental (or extra-parental) environment; but a mammalian individual
is a fertilized egg, _plus its intra-maternal environment_, plus its
non-parental environm