The three forms of exogamy, accordingly, agree in prohibiting the marriage of brothers and sisters and, in so far as maternal descent may be regarded as the prevailing system, the marriage of a son with his mother. Both these prohibitions, doubtless, and especially the latter, reflect a feeling which was experienced by mankind at an early age. The aversion to the marriage of a son with his mother is greater than that to the marriage of brother and sister or even that of father and daughter. Consider the tragedy of Œdipus. It might, perhaps, be less horrible were it father and daughter instead of son and mother who were involved in the incestuous relation. Marriages between brothers and sisters have, of course, sometimes occurred. Thus, as has already been remarked, the Peruvian Incas ordained by law that a king must marry his sister. In the realm of the Ptolemies, likewise, the marriage of brother and sister served the purpose of maintaining purity of blood, and even to-day such marriages occur in some of the smaller despotic negro states. The custom is probably always the result of the subjugation of a people by a foreign line of rulers. Indeed, even the Greeks permitted marriage between half-brothers and half-sisters. Though these natural instincts were less potent in early times than in later culture, they may not have been entirely inoperative in the development from original endogamy to exogamy. Nevertheless, one would scarcely attempt to trace to the blind activity of such instincts those peculiar forms of exogamy that appear particularly among the Australian tribes. On the contrary, we would here also at once be inclined to maintain that the reverse is true, thus following a principle that has approved itself in so many other cases. The aversion to marriage with relatives has left its impress on our present-day legislation, not so much, indeed, in the positive form of exogamy, as in the negative form which forbids endogamy within certain limits. _This aversion, however, is not the source so much as it is the effect--at least in great measure--of the exogamous institutions of early culture._ All the more important is the question concerning the origin of these institutions. This question, in fact, has already received much attention on the part of ethnologists, particularly since the beginning of the present century, when it has become more and more possible to study the tribal organization of the Australians. Here, however, we must distinguish between the general theories that have been advanced concerning the causes of exogamy as such--theories which date back in part even to a fairly early period--and hypotheses concerning the origin of the various forms of exogamy.