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 environment.

Here in a nutshell is the biological basis of sex problem in human
society. Human individuals do wear out and have to be replaced by
reproduction. In the reproductive process, the female, as in mammals
generally, is specialized to provide an intra-maternal environment
(approximately nine months in the human species) for each new
individual, and lactation or suckling afterward. The biological phase of
the sex problem in society consists in studying the nature of that
specialization. From the purely sociological standpoint, the sex problem
concerns the customs and institutions which have grown up or may grow
up to meet the need of society for reproduction.