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72 ficatory system exactly such as would be the consequence of its origin in a social structure founded on the exogamous social group, but many details of these systems point in the same direction. Thus, the rigorous distinctions between father's brother and mother's brother, and between father's sister and mother's sister, which are characteristic of the usual forms of the classificatory system, are the obvious consequence of the principle of exogamy. If this principle be in action, these relatives must always belong to different social groups, so that it would be natural to distinguish them in nomenclature.

Further, there are certain features of the classificatory system which suggest its origin in a special form of exogamous social grouping, viz., that usually known as the dual system in which there are only two social groups or moieties. It is an almost universal feature of the classificatory system that the children of brothers are classed with the children of sisters. A man applies the same term to his mother's sister's children which he uses for his father's brother's children, and the use of this term, being the same as that used for a brother or sister, carries with it the most rigorous prohibition of marriage. Such a condition would not follow necessarily from a social state in which there were more than two social groups. If the society were patrilineal, the children of two brothers would necessarily belong to the same social group, so that the principle of exogamy would prevent marriage