On the negative side too, there is evidence to be gained from the nature of the taboos and institutions erected against incest. According to Frazer[99] the exogamous systems of the Australian aborigines seem to have originated in the first place as a means of preventing connections between brother and sister, the prohibition of marriage between other relatives having been brought about by subsequent developments and elaborations of the primitive two class system, instituted for the purpose of avoiding brother-sister marriages. The abhorrence of brother-sister incest is indeed very marked in many primitive communities, and that this abhorrence represents the repression of a genuine desire for incest of this kind is shown by the remarks of travellers that the "avoidances" and other methods of enforcing the prohibitions are often "very necessary"[100] and by the fact, already referred to, that as soon as the customary restrictions are relaxed, the otherwise forbidden connections are freely indulged in. To this evidence from anthropology there might be added the scarcely less convincing data from mythology and literature, which has been studied in such detail by Rank[101] and which perhaps, for this reason, we need not stop to dwell on here; it being sufficient to remind the reader in passing of such well known mythological cases as the unions of Zeus and Hera and of Osiris and Isis, or, as regards literature, to refer him to such recent examples as Artzibasheff's "Sanine" or d'Annunzio's "City of the Dead" where the existence of erotic feeling between brother and sister is treated in an open manner. [Sidenote: Displacement of parent--regarding tendencies on to more distant relatives] As a further stage of development the original parent love may be displaced, not on to a brother or sister, but on to some more distant relative, such as a cousin (a brother or sister substitute) or an uncle or aunt (more directly parent substitutes)[102]. Cousin marriage is, among ourselves, passing through the stage of being legally permissible though still regarded with some degree of moral disapproval or suspicion. In other times and places it has, like brother-sister marriage, been the object both of sternest prohibition[103] and of warm approval[104]. Any kind of sexual relationship between nephews and aunts or between nieces and uncles seems to have been, too, reminiscent of the repressed tendencies to parent-incest to have received sanction either legally or morally, but unions of this kind have nevertheless sometimes been found among primitive peoples[105], and are not infrequently present as objects of desire in the unconscious mind of those who live in civilised communities to-day. [Sidenote: Relatives by marriage]