So far we have seen no luminous and consistent account of how mankind became exogamous, if they began by being promiscuous. The theories rest on the idea that man, dwelling in an "undivided horde" (except so far as it was divided into co-operative magical societies), bisected it into two exogamous intermarrying moieties. Durkheim has put forward a theory which is not at all points easily understood. He supposes that, "at the beginning of societies of men, incest was not prohibited ... before each horde (_peuplade_) divided itself into two primitive 'clans' at least" (_L'Année sociologique_, i. pp. 62, 63). Each of the two "clans" claimed descent from a different animal, which was its totem, and its "god." The two clans were exogamous,--out of respect to the blood of their totem (with which every member of the clan is mystically one), and, being hostile, the two clans raided each other for women. Each clan threw off colonies, which took new totems, new "gods," though still owning some regard to their original clan, from which they had seceded, while abandoning its "god." When the two "primary clans" made alliance and _connubium_, they became the phratries in the local tribe, and their colonies became the totem kins within the phratries.
We are not told why the original horde was disrupted into two hostile and intermarrying "clans": we especially wonder why the horde, if it wanted an animal god, did not choose one animal for the whole community; and we may suspect that a difference of taste in animal "gods" caused the hostility of the two clans. Nor do we see why, if things occurred thus, the totem kins should not represent twenty or thirty differences of religious taste, in the original horde, as to the choice of animal gods. If the horde was going to vary in opinion, it is unlikely that only _two_ factions put forward animal candidates for divinity. Again, a "clan" (a totem kin, with exogamy and descent derived through mothers) cannot overflow its territorial area and be therefore obliged to send out colonies, for such a clan (as Durkheim himself remarks) has no territorial area to overflow. It is not a _local_ institution at all.
While these objections cannot but occur, Durkheim does provide a valid reason for the existence of exogamy. When once the groups (however they got them) had totems, with the usual taboos on any sort of use of the totem by his human kinsfolk, the women of the kin would be tabooed to the men of the same kin. In marrying a maiden of his own totem, a man inevitably violates the sanctity of the blood of the totem (_L'Année sociologique_, i. pp. 47-57. Cf. Reinach, _Cultes, mythes et religions_, vol. i. pp. 162-166).