It is time to abandon the notion that biology prescribes in detail how we shall run society.  True, this foundation has never received 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 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.