No innate aversion to marriage with near relations, p. 320.—Innate aversion to sexual intercourse between persons living very closely together from early youth, pp. 320-330.—Local exogamy, pp. 321-323.—Connection between the prohibited degrees and the more or less close living together, pp. 324-329.—Connection between the ‘classificatory system of relationship’ and exogamy, p. 329.—The one-sidedness of prohibitions due in part directly to local relationships, in part to the influence of names, pp. 330, _et seq._—The prohibitions of marriage between relations by alliance and by adoption due to an association of ideas, p. 331.—The prohibitions on the ground of ‘spiritual relationship’ due to the same cause, _ibid._—Endogamy seldom occurs in very small communities, p. 332.—Marriage between half-brothers and half-sisters not contrary to the principle here laid down, _ibid._—Incestuous unions due to pride of birth, to necessity, to extreme isolation, and to vitiated instincts, p. 333.—Incest among the lower animals, p. 334.—The effects of cross- and self-fertilization among plants, p. 335.—Evil effects of close interbreeding among animals, pp. 335-337.—A certain amount of differentiation favourable for the fertilisation or union of two organisms, pp. 337, _et seq._—Difficulty of adducing direct evidence for the evil effects of consanguineous marriages among men, pp. 338, _et seq._—Close intermarrying among the Veddahs, pp. 339, _et seq._—The effects of marriage between first cousins, pp. 340-343.—The experience of isolated communities does not prove consanguineous marriages to be harmless, pp. 343-345.—The bad consequences of self-fertilization and close interbreeding may almost fail to appear under favourable conditions of life, pp. 345, _et seq._—Consanguineous marriages more injurious in savage regions than in civilized society, p. 346.—Tendency of endogamous peoples to die out, pp. 346-350.—Peoples who ascribe evil results to close intermarriage, pp. 350-352.—The horror of incest due to natural selection, pp. 352, _et seq._—Exogamy arose when single families united in small hordes, p. 353.—Love excited by contrasts, pp. 353-355. CHAPTER XVI SEXUAL SELECTION AS INFLUENCED BY AFFECTION AND SYMPATHY, AND BY CALCULATION