This tendency to mate within the tribe is a factor of first-rate biological and sociological importance, serving as it does to maintain racial boundaries. But if sexual selection be a real factor in the evolution of races—in the differentiation of groups as well as in their maintenance—its action must be sought within the race, an intra-racial sexual selection must be demonstrated.
Sexual selection in man may, as Pearson long ago pointed out, be of two kinds, preferential mating and assortative mating. By preferential mating one understands that certain classes of women are discriminated against by the average man, or by men as a class, or vice versa. By assortative mating one means that in the population of men and women who do marry, there is a definite tendency for certain classes of men to marry particular classes of women, and conversely. An almost prophetic quotation from Pearson may render the distinction clear.
Another illustration faces us in the problem of deaf mutism. Normal individuals discriminate against deaf mutes, for obvious reasons. There is a stringent segregation of the class, resulting from educational and social conditions, and as a result there is for the people, as a whole, a strong assortative mating, hearing people as a class marrying hearing, and deaf marrying deaf.
The scope of this review is limited to a discussion of the quantitative results which have been attained for assortative mating.
It is needless to say that a subject so fascinating to man as any-
- Pearson, K., Phil. Trans. Roy. Soc. Lond., A, Vol. 187, pp. 257-258, 1897; also "Grammar of Science," 2d ed., pp. 421-437, 1900.
- Thus in man sexual selection is a somewhat more complicated process than it has been assumed to be in the mass of writings on the lower animals.
- The quantitative data bearing directly on the problem of preferential mating are few. See K. Pearson, Phil. Trails. Roy. Soc. Lond., A, Vol. 187, p. 258, 1897; "Grammar of Science," 2d ed., pp. 425-428, 1900; Biometrika, Vol. 2, pp. 270-272, 1903.
the lower animals, it certainly plays a large part in influencing sexual choice among primitive men and more subtly among us in civilization. Just as soon as a social group recognizes the possession of certain physical traits peculiar to itself—that is, as soon as it evolves what Giddings has aptly termed a 'consciousness of kind'—its constant endeavor thenceforth is to afford the fullest expression to that ideal."
Westermarck ("History of Human Marriage," pp. 362-373, 1901) gives a terse summary of the social prejudices, tribal practises and laws concerning marriage between different castes, tribes or elans. Others are to be found in various sociological works.