Page:Popular Science Monthly Volume 80.djvu/481

Rh 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.

For example, preferential mating might lead in a highly social community to the rejection of consumptive mates, while assortative mating might through localization or community of habit, lead to considerable consumptive correlation. Thus sexual selection as a whole may influence in diverse ways the inheritance of the consumptive taint.

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