One of the most remarkable extensions of the scope of taboo is the _taboo which rests on relations by marriage._ The history of exogamy, whose earliest stages are represented by the totemic marriage laws of the Australians, clearly teaches that the aversion to marriage between blood relations was not the cause but at most, to a great extent, the effect of exogamous customs that everywhere reach back into a distant past. But there is a second class of marriage prohibitions, and this likewise has found a place even in present-day legislation--the prohibition of unions between relations by marriage. Such prohibitions are from the very beginning outside the pale of exogamous laws. Indeed, it is clear that all unions of this sort--such, for example, as are forbidden by our present laws--were permitted by the totem and clan exogamy of the Australians and that of the American Indians. In the case of maternal descent, the group from which a man must select his wife included his mother-in-law as well as his wife. Similarly, in the case of paternal descent, the husband and father-in-law were totem associates. There is another set of customs, however, which is generally connected with even the earliest forms of exogamy, and which fills out in a very remarkable way the gap that appears in the original totemic exogamy when this is compared with present-day legislation. These customs are no other than the laws of taboo. One of the earliest and most common of these regulations is the _taboo of the mother-in-law_. Corresponding to it, not so common and yet obviously a parallel phenomenon occasionally connected with it, is the _taboo of the father-in-law_. The relative distribution of the two taboos is analogous to that of maternal and paternal descent in the primitive condition of society, for it is maternal descent that is dominant. This is not at all meant to imply that there is any casual[1] relation between these phenomena. Rather is it true, probably, that they are based upon similar motives, and that these motives, just as in the case of marriage between relations, are more potent in the case of the mother than in that of the father. In general, however, the taboo of parents-in-law signifies that the husband must so far as possible avoid meeting his mother-in-law, and the wife, her father-in-law. Now, it is evident that in so far as this avoidance excludes the possibility of marriage, the custom is, in a way, supplementary to exogamy. Wherever maternal descent prevails, no one may marry his mother; and, where taboo of the mother-in-law exists, no one may marry his mother-in-law. The same holds of father and daughter, and of father and daughter-in-law, in the case of paternal descent. This analogy may possibly indicate the correct clue to the interpretation of the phenomena. It would certainly be erroneous to regard the taboo of the mother-in-law as a regulation intentionally formulated to prevent unions between direct relations by marriage.