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 of the conquered group as an integral part of society. This is simply natural selection working on groups. Natural selection works chiefly in this manner on the human species, because that species lives in groups. Such group control of the component individuals as has been described has led to a division of labour between the sexes in every primitive society. All this means is that the group adopting such a division has greater survival value, and hence is more likely to be represented in later ages.
It must not be supposed that such systems of control were always logically thought out or deliberately planned. Even animals which live in herds or colonies have divisions of labour.
Through an infinite slaughter of the least fit, such groups arrive at some kind of instinctive adjustment to produce and protect the young. The crudest human intelligence must have eliminated much of the waste involved, by comprehending obvious cause-and-effect relations which animals have to arrive at through trial and error methods.
For example, an intelligence capable of employing artificial weapons is also able to see that the wielder of these for group defence cannot be encumbered with baggage or children when the group is in movement. Hence women became the burden bearers, and took care of the children, even after the nursing period. War parties could not generally be mixed, for the obvious reasons that such women as did not have young children would be pregnant a good deal of the time, or likely to become so. Moreover, a hunter and fighter must not have his courage, ferocity and physical initiative undermined by unsuitable employments and associations.
In a semi-settled group, the hunter and warrior cannot be relied upon to keep hearth-fires burning or tend crops, even though he may occasionally have time for such activities. These duties are therefore relegated to the women, whose child-bearing functions impose upon them a more sedentary existence. Women must reproduce practically up to their full capacity to fill up the gaps made by war, accident and disease as well as death from old age. To this biological service which they alone can perform are added those which lie nearest it and interfere least with carrying it out.
We must therefore keep in view all the activities of any group in which the sex problem is being studied. There is a certain tendency to disregard the female specialization to child-bearing, and to regard the sex question as one merely of adaptation to extra-biological services. In every group which has survived, some machinery—a "crust of custom," reinforced by more arbitrary laws or regulations—has sought to guarantee reproduction by keeping women out of lines of endeavour which might endanger that fundamental group necessity. Primitive societies which got stabilized within a given territory and found their birth-rate dangerously high could always keep it down by exposing or destroying some of the unfit children, or a certain per cent of the female children, or both.
In primitive groups, the individual was practically nil. But modern civilized society is able to survive without the rigid control of individual activities which the old economy entailed. Man comes to choose more and more for himself individually instead of for the group, uniformity weakens and individualism becomes more pronounced. As control of environment becomes more complete and easy, natural selection grows harder to detect. We turn our interests and activities toward the search for what we want and take survival largely for granted—something the savage cannot do. Natural selection becomes unreal to us, because the things we do to survive are so intricately mixed up with those we do for other reasons. Natural selection in gregarious animals operates upon groups rather than upon individuals. Arrangement of these groups is often very intricate. Some have territorial boundaries and some have not. Often they overlap, identical individuals belonging to several. Hence it is not strange that natural selection phenomena often escape attention.