(9) Finally, the tendency to incestuous direction of affection, having once been brought into existence, has no doubt been strengthened and consolidated by the actual practice of incest that has pretty certainly occurred on a wide scale among certain races and at certain levels of development[229]. [Sidenote: The occurrence of incest may also be inferred from certain practices and institutions] Apart from the actual observation of incestuous practices at the present day, the previous occurrence of incest on a wide scale may (as we have already to some extent indicated in earlier chapters) frequently be inferred with some degree of certainty from the nature of practices, customs, observances and institutions which seem to be remnants or vestiges of a one-time general prevalence of incest. We have already referred to the practice of brother-sister marriage among certain lines of monarchs (p. 91), to the customs of the levirate and sororate (p. 93) and of group marriage (p. 90), the _droit de seigneur_ (p. 143) and the licence frequently permitted at certain festivals such as initiation (p. 89). Evidence for the previous existence of incest is also forthcoming from the measures and prohibitions erected to prevent it. The "avoidances" practised by a large number of savage peoples are very numerous and have reference to all the principal relationships, both those of blood and those acquired by marriage. These "avoidances" are unhesitatingly regarded by most authorities as customs adopted as a precaution against incest. [Sidenote: Especially from Exogamy] The most striking institution of this kind is however undoubtedly that of Exogamy. There is as yet no complete consensus of opinion as to the causes that have led to the origin and development of exogamy, but the majority of the eminent investigators who have devoted themselves to the subject agree that the avoidance of incest is the principal factor that has led to the creation of the system. The various stages of exogamic development, as seen in Australia, appear to constitute so many fresh encroachments upon the liberty of incest[230], the later and more complex four class system prohibiting certain unions between relatives that the earlier and simpler two class system has permitted, while the eight class system in turn prevents those that are not excluded under the four class system, though the actual relationships prohibited differ somewhat according to whether descent is traced in the male or female line. [Sidenote: Exogamy was probably preceded by Endogamy]