This then, the primal law--avoidance between a brother and sister--with appalling conservatism has descended through the ages (in conservance of form, if not of ultimate purpose). It ordained in the dawn of time a barrier between mother and son, and brother and sister, and that ordinance is still binding on all mankind [but in Egypt and Peru, for example, the opposite of this rule, for special reasons, has prevailed]. Between these for ever, a bit was placed in the mouth of desire, and chains on the feet of lust. Their mutual relationship is one that has been held sacred from a sexual point of view, in most later ages. It only remains for us to repeat that it follows that this law, as applied in the group composed of a single family, is, as we pointed out, the parent of exogamy; continuance within the group necessarily and logically entailed marriage without; but, again as we said, it was itself the offspring of the early idea. For this idea, in its assumption that sovereignty in marital right was compatible with solitude alone, was shaken to its depths when a second presence threatened rivalry, and demanded remedy in the action of law, which it has seemed to us could only take the form we have tried to portray[2] in the primal law. NOTE TO CHAPTER IV To the Editor this theory seems worthy of the ingenuity of his old friend and kinsman. Granting that early man was a speechless jealous brute, dwelling in groups consisting of a patriarchal beast, and all the females whom he could catch, and all the females whom he could beget; granting that he drove all his adolescent sons out; and finally (under the circumstances described) kept one or a few sons at home, his rule would tabu all females of the group to these sons. Otherwise there would be a fight.