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.