+427+. The effect of exogamous arrangements has been to prevent marriage between persons related in blood.[774] In totemic organizations, when the totem is inherited, a division into two exogamous groups makes marriage of brother to sister impossible, since all the children of one mother are in the same group; and if there are four such groups and children are assigned to a group different from that of the father and that of the mother, marriage between parent and child is impossible. When the totem is not inherited (as is the case among the Australian Arunta) similar results are secured by a further subdivision. +428+. The particular exogamic customs vary considerably among early tribes, the differences following, in general, differences of social organization. In some more settled savage communities (as, for example, the Kurnai of Southeast Australia), in which there are neither classes nor totemic clans, marriage is permitted only between members of certain districts.[775] Well-organized social life tends to promote individual freedom in marriage as in other things. Marriage with a half-sister was allowed by the old Hebrew law,[776] and Egyptian kings often married their sisters. +429+. _Theories of the origin of exogamy._ Exogamy has been referred to a supposed scarcity of women, which forced the young men to seek wives abroad.[777] On the assumption of early sexual promiscuity it has been regarded as a deliberate attempt to prevent the marriage of blood relations.[778] It has been supposed to result from the absence of sexual attraction between persons who have been brought up together.[779] An original human horde being assumed, it has been suggested that the patriarch, who had possession of all the women of the horde, would, from jealousy, drive the young men off to seek wives elsewhere.[780] From the point of view of the totem as divine ancestor, exogamy has been supposed to arise from religious respect for the clan blood, which is held to share the divinity of the totem, and would be polluted (with danger to the clan) by outside marriages.[781]