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.