From the standpoint of biology, reproduction is not an individual but a group problem, however many problems of detail it may give rise to in individual lives.  Sex involves the division of the reproductive process, without the exercise of which any human group would perish very shortly, into two complementary, mutually necessary but unequal parts. (This statement applies only to the reproductive process, as obviously the male and female gametes contribute equally to the formation of the new individual).  Neither part (the male or the female) of this process is more necessary than the other, both being absolutely necessary.  But the female specialization for furnishing the intra-maternal environment makes her share more burdensome.

Biologically considered, not even two individuals (male and female), together with their offspring, can be more than an arbitrary “unit” as concerns sex, since inbreeding eventually impoverishes the stock.  Hence outcrosses are necessary.  To intelligibly consider the sex problem in the human species, then, we must always predicate a considerable group of people, with such organization and division of activities as to guarantee that all the processes necessary to survival will be carried on.  Sex is a group problem.  Considering the mutual interdependence and the diversity of activities in human society, to make the generalization that one sex is superior to the other is on a par with saying that roots and branches are superior to trunks and leaves.  It is sheer foolishness.  Yet oceans of ink have flowed in attempts to establish one or the other of two equally absurd propositions.

Since the specialization to furnish the intra-maternal environment for the young makes the female part of the reproductive process essentially and unavoidably more burdensome than the male, it results that an economical division of the extra-reproductive activities of any group must throw an unequal share upon the males.  This specialization to carry the young during the embryonic period is thus at the base of the division of labour between the sexes.  It is the chief factor involved in the problems of sex, and gives rise, directly or indirectly, to most of the others.

But the sex problem as a whole is one of adaptation as well as of specialization.  An incident of the female specialization is a type of body on the average smaller, weaker and less well adapted to some other activities than is the male body, even when reproduction is not undertaken.  A great complication is added by the fact that some women, and also some men, are better adapted than others to nonreproductive activities.  This is another way of saying that the type of body associated with either type of sex glands varies a good deal, for reasons and in respects already pointed out.