So too with the forms of marriage. While polygyny has been claimed to be due entirely to economic causes, we have seen that these so-called economic causes have only been the opportunities for the polygynous instincts of man to assert themselves. These polygynous instincts of man have asserted themselves more or less under all conditions of society, but under certain conditions, when there was an accumulation of wealth, and especially with the institution of slavery, they had greater opportunity to assert themselves than elsewhere. Thus the basic cause of polygyny is not economic, but psychological; and given certain moral and economic conditions of society, these polygynous tendencies assert themselves. Monogamy, on the other hand, has in no sense been determined by economic conditions but is fundamentally determined by the biological fact of the numerical equality of the sexes. This is doubtless the main reason why monogamy has been the prevalent form of the family everywhere. Certain moral and psychological factors which go along with the development of higher types of culture have, however, powerfully reenforced monogamy. It is doubtful if economic conditions can to any extent be shown to have equally reenforced the monogamic life. Our conclusion must be, then, that while the form of the family and the form of industry are closely related, so closely that the form of industry continually affects more or less the family life, yet there is no reason for concluding that the form of the family is wholly or even chiefly determined by the form of industry. SELECT REFERENCES _For brief reading:_ WESTERMARCK, _History of Human Marriage_, Chaps. XX-XXII. _For more extended reading:_ MCLENNAN, _The Patriarchal Theory._ MORGAN, _Ancient Society._ PARSONS, _The Family._ WAKE, _The Development of Marriage and Kinship._ CHAPTER VI THE HISTORICAL DEVELOPMENT OF THE FAMILY