The causes of exogamy are explained in the same way, by the fact that members of the same clan often live together in close intimacy. It is the small clans, formed of thirty or fifty individuals of a few families living together, which have the most severe laws against incest or endogamy. Where the families live in separate homes, such prohibitions do not exist. The Maoris, who are endogamous, inhabit villages which are widely separated, and marriage between relations is allowed. Endogamy generally exists where the clan life is little developed, and where relatives know and see little of each other. The aversion to marriage between persons living together has thus created prohibition of marriage between relations as well as that of marriage between members of the same clan. It is the same reason which has led to the prohibition of marriage between brothers-and sisters-in-law, between brothers and adopted sisters, etc. In people living in small communities, endogamy does not appear to have ever existed. Incest between relatives living together appears to have everywhere the same natural cause--the scarcity of women in isolated families living in remote districts. There is also a psycho-pathological form of incest associated with morbid appetites in the families of degenerates. In animals living alone and whose families break up very rapidly (cats for example) incestuous unions, between parents and young, for instance, are quite common. Let us now consider the scientific side of the question. We see everywhere that sexual union between quite distinct animal species gives no result. At the most, certain closely allied species, such as the ass and the horse, the rabbit and the hare, give progeny which are themselves sterile (mules, etc.). The feebleness and sterility of hybrids derived from widely separated races or nearly allied different species proves the deficiency in vital force of the offspring of fundamentally dissimilar procreators. But, on the other hand, the dangers of continuous consanguineous reproduction are no less evident. Perpetual unions between brothers and sisters for several generations, lead to degeneration of the race. For example, the still-births will be 25 per cent. instead of 8 per cent., which is the figure in ordinary crossings. The prejudice against consanguineous unions may, however, depend on the accumulation of certain pathological defects. Westermark admits that it is difficult to show clearly that consanguineous marriages are prejudicial in man. The consanguinity which causes evil effects in animals concerns long-continued unions between parents and children or brothers and sisters. But this never occurs in man. Animals and plants may be perpetuated for many years in the closest consanguinity without degeneration resulting. Among the Persians and Egyptians, intimate unions have existed for a long time without producing degeneration.