Among the Weddas, marriage between an elder brother and his younger sister is considered normal, while that between a younger brother and his elder sister, or between a nephew and his aunt, is regarded as unnatural. The latter simply shows that unions between young men and old women are not natural. Unions between brothers and sisters, and especially between half-brothers and half-sisters were licit among the Persians, Egyptians, Syrians, Athenians and ancient Jews. Those between uncles and nieces (more rarely between aunts and nephews) are sometimes permitted, sometimes prohibited. With the exception of Spain and Russia marriages between first cousins are allowed in Europe. =Exogamy and Endogamy.=--Among many savages the prohibition of consanguineous marriage may be extended to relationship of the third degree. Marriage may even be prohibited among all members of the same tribe or clan, even when they are not related. This is called _exogamous_ marriage, and reaches its extreme development among the Australians, who are only allowed to marry into remote clans. We thus see that the great majority of savages extend their idea of incest much further than we do. The reason of this has been much discussed. It was formerly said that consanguineous marriage was contrary to the commandments of God; that it offended the natural sentiment of modesty; that it obscures relationship, etc. Nowadays, it is said to be injurious to posterity. Ethnography teaches us, however, that these statements are of little value. Along with the exogamy of many tribes there is among other savages a system of _endogamy_, described by MacLennan; this is the prohibition of marriage between different clans. Spencer and MacLennan have different explanations of this custom which seem hardly natural. Westermark appears to be nearer the truth in remarking as follows: The sexual appetite, especially in man, is excited by new impressions and cooled by habit. It is not the fact of a man and woman being related, but intimate companionship since youth, which produces in them a repugnance to sexual union. We find the same repugnance between adopted brothers and sisters and between friends who have been intimate since childhood. When, on the contrary, brothers and sisters or near relatives have been separated from each other since an early age, they often fall in love with each other when they meet later on. There is, therefore, no innate or instinctive repugnance to incest in itself, but only against sexual union between individuals who have lived together since childhood. As it is parents and their children who are usually in this situation, everything is explained simply and clearly.