+423+. _Exogamy._[770] All over the savage world the general rule prevails (though not without exceptions) that a man must not marry a woman of his own clan; though the family proper (husband, wife, and children) exists, the clan is the fundamental social unit. When a tribe contains several clans it is commonly divided into groups (phratries), each phratry including certain clans, and the rule then is that a man shall not marry a woman of his phratry. Usually the number of phratries is two, but in some cases (as among the Australian Arunta and adjoining tribes) these are divided so that there are four or eight exogamous groups (subphratries). When the totem is hereditary the totemic clans are exogamous; otherwise (as among the Arunta) marriage between persons of the same clan is permitted. +424+. Whether the clan or the phratry preceded in time it is hardly possible to determine--clans may have united to form a larger group, or an original group may have been divided into clans. But in the latter case this original group was practically a clan, so that the question of precedence in time is not important. Where clan exogamy exists without phratries it is possible that these also formerly existed and have been dropped in the interests of freedom--that is, they limited the choice of a wife to an extent that proved inconvenient.[771] +425+. An almost universal feature of the marriage rules of low tribes is the classificatory system of relationship. According to this system, the community being divided into groups, terms of relationship indicate not kinship in blood but tribal status in respect of marriageability; thus, the same term is used for a child's real father and for every man who might legally have become the husband of his mother, and the same term for the real mother and for every woman whom the father might have married; the children of such possible fathers and mothers are the child's brothers and sisters; all possible spouses are called a man's "wives" or a woman's "husbands"; and similarly with all relationships.[772] +426+. The system has many varieties of form, and gives way in time to the formal recognition of blood kinship. It has been held to point to an earlier system of "group marriage," in which all the men of one group had marital relations with all the women of another group, and further to a primitive custom of sexual promiscuity.[773] In the nature of the case these hypotheses do not admit of proof or disproof. All that is certain is that the classificatory system has been and is an accompaniment of one stage of social and religious development.