Generally speaking, the feeling that two persons are intimately connected in some way or other may, through an association of ideas, give rise to the notion that marriage or intercourse between them is incestuous. Hence the prohibitions of marriage between relations by alliance and by adoption. Hence, too, the prohibitions on the ground of what is called “spiritual relationship.” The Emperor Justinian passed a law forbidding any man to marry a woman for whom he had stood as godfather in baptism, the tie of the godfather and godchild being so analogous to that of the father and child as to make such a marriage appear improper.[1963] In the Roman Church sponsorship creates a bar to the marriage even of co-sponsors, and the restriction can be removed only by a dispensation.[1964] In Eastern Europe, the groomsman at a wedding comes under a set of rules which forbids intermarriage with the family of the bride to exactly the same extent as if he were naturally the brother of the bridegroom.[1965] A similar _cognatio spiritualis_, according to the old law-books of India, occurs between a pupil and his “guru,” that is, the teacher who instructs him in the Veda. The pupil lived in his guru’s house for several years, and regarded him almost as a father.[1966] Hence adultery with a guru’s wife was considered a mortal sin.[1967] But how, then, are we to explain the exceptions, apparent or real, to the rule that close living together inspires an aversion to intermarriage? How are we to explain the fact that, besides tribes that are exogamous, there are others that are endogamous, and that, besides peoples with very extensive laws against intermarriage, there are others among whom unions take place between very near relations, such as brothers and sisters, and even parents and children. In the next chapter we shall examine the psychological principle which underlies the endogamous marriage. For the present it is sufficient to say that endogamy never, except in cases of extreme isolation, seems to occur among peoples living in very small communities with close connections between their members. Concerning the Australians, Mr. Curr expressly states that those tribes which are endogamous are, as a rule, stronger in numbers than those in which exogamous marriage obtains.[1968]