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 between them, but if the women of the group had married into different clans, there is no reason arising out of the principle of exogamy which should prevent marriage between their children or lead to the use of a term common to them and the children of brothers. Similarly, if the society were matrilineal, the children of two sisters would necessarily belong to the same social group, but this would not be the case with the children of brothers who might marry into different social groups. If, however, there be only two social groups, the case is very different. It would make no difference whether descent were patrilineal or matrilineal. In each case the children of two brothers or of two sisters must belong to the same moiety, while the children of brother and sister must belong to different moieties. The children of two brothers would be just as ineligible as consorts as the children of two sisters. Similarly, it would be a natural consequence of the dual organisation that the mother’s brother’s children should be classed with the father’s sister’s children, but this would not be necessary if there were more than two social groups. I should have liked, if there were time, to deal with other features of the classificatory system, but must be content with these examples. I hope to have succeeded in showing that the social causation of the terminology of relationship goes far beyond the mere dependence of features of the system on special forms of marriage, and that the character of the classificatory system as a whole has been determined by its origin in a specific form of social organisation. I propose now to leave the classificatory system for a moment and inquire whether another system of denoting and classifying relationships may not similarly be shown to be determined by social conditions. The system I shall consider is our own. Let us examine this system in its relation to the form of social organisation prevalent among ourselves.