The several forms of preferential mating, such as cross-cousin marriage, sororate and levirate, are well known and have been reported from all the ethnographic provinces of the world. Lately Lowie and Rivers have devoted special chapters in their books on social organization to the comparative study of these important institutions. Lowie has pointed out that there is strong evidence for the correlation of sororate and levirate. The later publication of Rivers hardly serves to make these matters any clearer than Lowie's work. Although the latter scholar, with Tylor and others, recognized the close connexion existing between sororate and levirate, the evidence at his disposal did not allow him to arrive at a similar conclusion with regard to the other forms of preferential marriage. Accordingly he had to treat them, for the time being, as institutions of independent origin.