The most important fact about this reproductive specialization is that beyond fertilization it is exclusive in the female.  Since the males cannot furnish the intra-parental environment for the young, the entire burden must fall on half the group.  If this aggregation is to even hold its own numerically, its women must have, on an average, two children each, plus about one more for unavoidable waste—­death in infancy or childhood, sterility, obvious unfitness for reproduction, etc., i.e., three in all.  If one woman has less than her three children, then another must have more than three, or the group number will decrease. Group survival is the fundamental postulate in a problem of this kind.

The above figure is for civilized society.  In primitive groups, the terrific wastage makes a much higher birth-rate necessary, several times as high in many cases.  If we suppose such a group, where child mortality, lack of sanitation, etc., necessitates an average of eight children per woman (instead of three), the biological origin of the division of labour between the sexes is much more clearly seen than it is in civilized societies.

If men are better hunters or fighters than women, the latter could nevertheless hunt and fight—­it is a question of superior or inferior adaptation to particular activities.  But it is more than that. Only the women are biologically specialized to the chief reproductive burden (intra-parental environment and lactation).  If half the women should withdraw from child-bearing, the remainder would be obliged to average sixteen apiece.  But even this is not all.  Unfortunately, the half of the women who would be found best adapted to hunting and fighting would be the more vigorous half.  The new generation would thus be born from the leftovers, and would be poor quality.  Such a division of labour within a group would be fatally foolish and entirely uncalled for—­since there are plenty of men adapted to hunting and fighting, but entirely unspecialized to child-bearing and nursing.

Group survival being the fundamental thing, the group is obliged to develop a division of labour which directs the activities of the individuals composing it to providing for its necessities, regardless of any interference with their own desires.  That is, if group survival requires that woman use her specialization to child-bearing instead of any adaptation she may possess in other directions, one of two things inevitably result:  (1) Either the group finds or evolves some social control machinery which meets the necessity, or (2) it must give way to some other group which can do so.  In either case, the result is a division of labour, which we see more clearly in primitive peoples.  The less efficient group is not necessarily exterminated, but if it loses out in the competition until some other group is able to conquer it and impose its division of labour the result is of course the extinction