(1). _Temporary or perpetual monogamy_, or marriage between one individual of one sex and one of the other sex. This is the case with most birds and mammals and many races of man. (2). _Polygymy or polygamy_, or the marriage of one male with several females. This occurs in ruminants, stags, fowls, and other animals, as well as in some human beings; for example, the Islamites, negroes, American Indians, Mormons, etc. (3). _Polyandry_, or the marriage of one female with several males. This is met with chiefly in the ants, in which each female is generally fecundated successively by several males. In most of the higher animals, the jealousy of the males renders polyandry impossible. In man it is rare but exists among certain races. (4). _Marriage in groups_, or marriage between several males and several females. This singular custom is rare but exists in the Togas, a tribe of savages. I am not aware of its existence among animals. (5). _Promiscuity_, or free sexual intercourse between males and females. This occurs in many animals, especially in the lower animals in which the sexual instinct of the male is not associated with any regard for the female or the progeny. Promiscuity is still more natural when the female does not look after her young after she has laid her eggs. Nevertheless, in most animals the female limits herself to sexual intercourse before each brood, so that real promiscuity is not so frequent as would at first appear. In man, on the contrary, it attains its apogee in prostitution, which is the only absolutely complete form of promiscuity. But the result of prostitution as regards the preservation of the species, which is the proper object of all sexual union, is absolutely destructive. Polygamy or polygymy were licit among most ancient races, and is so still among most savages and among many civilized nations; but it has several varieties. In Mexico, Peru, Japan and China a man only possesses one legitimate wife, but has several concubines whose children are considered as legitimate as those of his wife. Polygamy existed legally among the Jews up to the Middle Ages. King Solomon possessed seven hundred wives and three hundred concubines. In Islamite countries the Jews are still polygamous. The Koran allows them four wives and as many concubines as they please. The latter do not enjoy the protection of their father, but apart from this they have the same rights as the legitimate wives. The Hindus and Persians are polygamous. The Romans were strictly monogamous, but they also had concubines. In Christian Europe, polygamy has occasionally been allowed or tolerated: St. Augustus did not condemn it. Luther allowed Philip of Hesse to marry two wives; and after the treaty of Westphalia bigamy was allowed because of the depopulation of Germany. The mistresses of the present princes are a relic of polygamy. Jesus having said nothing concerning polygamy, Luther did not prohibit it.