This hypothesis, which has recently been discussed by Swanton,[38] was already advanced only to be proved inadequate by Morgan himself. Taking the Seneca for illustration, where descent is in the maternal line, Morgan shows that the children of two sisters would indeed be members of the same clan and hence clan brothers and sisters but that this explanation no longer holds for the children of two brothers. By the law of exogamy these would be required to marry into another clan and there is no reason why their wives should belong to the same clan. Hence the brothers’ children will not be clan brothers and sisters, yet, according to Seneca terminology, the offspring of brothers no less than of sisters are classed with own brothers and sisters. Accordingly, the clan system--though it has a definite place in Morgan’s scheme of evolution--is not regarded by him as the determining factor of the Seneca-Dakota principle.[39] But the objection vanishes if we accept the theory that the Dakota principle arose as a reflection not of a multiple clan system but of an organization with exogamous moieties. This theory, which to my knowledge was first developed by Tylor[40] and has since been advocated by Rivers,[41] has obvious advantages. Even on the simple clan hypothesis it is clear why the father’s brothers should be classed with the father and separated from the maternal uncles, since the latter by exogamy must belong to a different clan. The term which we translate ‘father’ would really be seen to mean ‘male member of the father’s clan and generation’. With the moiety theory the same facts are explained, but also in addition the designations for other relatives. To take again the Seneca instance, the sons of two brothers _must_ be members of the same social division because with a dual organization the brothers are restricted to the same division in the choice of a mate; hence it is quite natural that the sons of brothers should call one another brothers. Again, the difference between parallel cousins and cross-cousins is perfectly intelligible. The mother’s brother’s and the father’s sister’s son can never be of my moiety; if descent is matrilineal they belong to my father’s moiety, if patrilineal to my mother’s. Hence it is natural that they should not be classed with my brothers who in either case are my moiety-mates. This hypothesis also explains features not yet referred to, but often found in conjunction with those grouped under the heading of the Dakota principle, _e. g._, the frequent classification of the father’s sister’s husband with the maternal uncle. Given exogamous moieties, these relatives must belong to the same half of society, to my own moiety if descent is maternal, to my mother’s if it is patrilineal. The Tylor-Rivers theory thus explains very satisfactorily the rather numerous features that jointly constitute what I have called the Dakota principle; we can at once see that here is