If Starcke's explanation of the origin of the dread of close intermarriage between kindred is too vague and ill supported by definite proof, his original suggestion that exogamy must take its rise in that horror is sustained and placed on a broader foundation by the singularly interesting researches of Westermarck[378]--a scholar who has rendered to social science a very important service by carrying the principles of organic evolution into the sphere of domestic institutions. He starts with the assertion that horror of incest is universal. Writers have, indeed, collected evidence which they believe points to a time when such an aversion did not exist. Thus marriage with a sister is permitted in Ceylon and Annam; in the royal families of Siam, Burma, and the Sandwich Islands; while the same custom prevailed, as is well known, among the Ptolemies of Egypt, and among the kings of ancient Persia.[379] But these unions are either "anomalous aberrations" from the general rule; or else they are allowed in order to preserve the purity of caste or the royal blood; or, in case of half-sisters, because relationship is traced in one line only;[380] while occasionally they may result from "extreme isolation" or from "vitiated instincts."[381] Everywhere prohibitions exist, though they vary greatly in the "degrees of kinship within which union is forbidden." As a rule, "among peoples unaffected by modern civilization the prohibited degrees are more numerous than in advanced communities, the prohibitions in a great many cases referring even to all the members of the tribe or clan." [378] WESTERMARCK, _op. cit._, chaps. xiv, xv, xvi, 290-382. These chapters should be read in the light of the results obtained in those on "Law of Similarity," the "Means of Attraction," "Sexual Selection," and the "Liberty of Choice." [379] For the evidence of incestuous marriages, see WESTERMARCK, _op. cit._, 292 ff., 331 ff.; STARCKE, _op. cit._, 44, 209 ff.; SPENCER, _Principles of Sociology_, I, 636; GIRAUD-TEULON, _Origines_, 60 ff. [380] This may perhaps explain why half-sisters and half-brothers may marry among the Todas where relationship is in the male line: MARSHALL, _A Phrenologist amongst the Todas_, 206, 221.