For the family among the lower animals in addition to Letourneau, Hellwald, and Westermarck, consult Brehm, _Tierleben_ (Leipzig and Vienna, 1891); his _Bird-Life_ (London, 1874); Herman Müller, _Am Neste_ (Berlin, 1881); Schäffle, _Bau und Leben des socialen Körpers_ (Tübingen, 1881); Espinas, _Des sociétés animales_ (2d ed., 1878); Groos, _Die Spiele der Thiere_ (Jena, 1896), or the English translation (New York, 1898); and Wagner, "Die Kulturzüchtung des Menschen gegenüber der Naturzüchtung im Tierreich," in _Kosmos_, 1886, I. In this connection read also Houzeau, _Études sur les facultés mentales des animaux_ (Mons, 1872); Vignoli, _Ueber das Fundamentalgesetz des Intelligenz im Tierreiche_ (Leipzig, 1879); and Salt, _Animals' Rights_ (New York, 1894). On the problems of sex and kinship mentioned in the text see Geddes and Thompson, _Evolution of Sex_ (New York, n. d.); Ellis, _Man and Woman_ (London, 1896); Finck, _Primitive Love_ (New York, 1899), vigorously attacking some of Westermarck's theories; his _Romantic Love and Personal Beauty_ (London, 1887); Duboc, _Psychologie der Liebe_ (Hannover, 1874); Mantegazza, _Physiologie der Liebe_ (30th ed., Berlin, 1897); Klebs, _Verhältniss des männ. und weibl. Geschlechts in der Natur_ (Jena, 1894); Schroeder, _Das Recht in der geschlechtl. Ordnung_ (Berlin, 1893); Thomas, "Relations of Sex to Primitive Social Control," and his "Difference in the Metabolism of the Sexes," both in _Am. Journal of Sociology_, III (1898); Sadler, _The Law of Population_ (London, 1830); Starkweather, _The Law of Sex_ (London, 1883); Hofacker and Notter, _Uber die Eigenschaften ... welche sich auf die Nachkommen vererben_ (Tübingen, 1827); Ploss, _Das Weib_ (Leipzig, 1895); also his _Ueber die das Geschlechtsverhältniss der Kinder bedingenden Ursachen_ (Berlin, 1859); Schenk, _Einfluss auf das Geschlechtsverhältniss_ (3d ed., Magdeburg and Vienna, 1898); the brilliant monograph of Düsing, _Die Regulierung des Geschlechtsverhältnisses_ (Jena, 1884); Huth, _Marriage of Near Kin_ (2d ed., 1887); Lewkowitsch, "Die Ehen zwischen Geschwisterkindern and ihre Folgen," in _ZFE._, VIII; and Mitchell, "Blood-Relationship in Marriage," in _Mem. of London Anth. Society_, 1865, II, 402 ff. Several important points are treated in Tylor's _Early History of Mankind_ (New York, 1878); and in his _Method of Investigating Institutions_. See also Kovalevsky, _Tableau des origines et de l'évolution de la famille_ (Stockholm, 1890); Swinderen, _De Polygynia_ (Groningae, 1795); and for a curiosity, read Premontval, _La monogamie_ (1751). In general, the literature cited in Bibliographical Note II has been used, and so need not here be described.] I. THE PROBLEM OF PROMISCUITY