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.