If now we study our case from the Banks Islands more closely and compare the social conditions in Merlav with those of other islands of the group, we find definite evidence, which it will not now be possible to consider in detail, showing that sexual relations were formerly allowed between a man and his wife’s sisters and his brothers’ wives, and that there is a definite association between the classing of these relatives with the sister and the cessation of such sexual relations. If such people as the Melanesians wish to emphasise in the strongest manner possible the impropriety of sexual relations between a man and the sisters of his wife, there is no way in which they can do it more effectually than by classing these relatives with a sister. To a Melanesian, as to other people of rude culture, the use of a term otherwise applied to a sister carries with it such deeply-seated associations as to put sexual relations absolutely out of the question. There is a large body of evidence from southern Melanesia which suggests strongly, if not conclusively, that the common nomenclature I am now considering has arisen out of the social need for emphasising the impropriety of relations which were once habitual among the people. The second feature of Melanesian terminology which I have mentioned helps us to understand how the common nomenclature has come about. In most of the Melanesian cases in which a wife’s sister is denoted by a term otherwise used for a sister, or a husband’s brother by a term otherwise used for a brother, the term employed is one which is normally used between those of the same sex. Thus, a man does not apply to his wife’s sister the term which he himself uses for his sister, but one which would be used by a woman of her sister. In other words, a man uses for his wife’s sister the term which is used for this relative by his wife. This shows us how the common nomenclature may have come into use. It suggests that as sexual relations with the wife’s sister became no longer orthodox, a man came to apply to this woman the word with which he was already familiar as a term for this relative from the mouth of his wife. The special feature of Melanesian nomenclature according to which terms of relationship vary with the sex of the speaker here helps us to understand how the common nomenclature arose. The process is one in which psychological factors evidently play an important part, but these psychological factors are themselves the outcome of a social process, viz., the change from a condition of sexual communism to one in which sexual relations are restricted to the partners of a marriage. Such psychological factors as come into action are only intermediate links in a chain of causation in which the two ends are definitely social processes or events, or, perhaps more correctly, psychological concomitants of intermediate links which are themselves social events. We should be shutting our eyes to obvious