STUDIES IN EUGENICS I ^
versally accepted in future times, but it is the best that has hith- erto been devised for the parties primarily concerned, for their children, for home life and for society. The degrees of kinship within which marriage is prohibited is, with one exception, quite in accordance with modern sentiment, the exception being the disallowal of marriage with the sister of a deceased wife, the propriety of which is greatly disputed and need not be discussed here. The marriage of a brother and sister would excite a feeling of loathing among us that seems implanted by nature, but which, further inquiry will show, has mainly arisen from tradition and custom.
We will begin by giving due weight to certain assigned motives, (i) Indifference, and even repugnance, between boys and girls, irrespectively of relationship, who have been reared in the same barbarian home. (2) Close likeness, as between the members of a thoroughbred stock, causes some sexual indif- ference; thus highly bred dogs lose much of their sexual desire for one another, but will rush to the arms of a mongrel. (3) Contrast is an element in sexual attraction which has not yet been discussed quantitatively. Great resemblance creates indif- ference, and great dissimilarity is repugnant. The maximum of attractiveness must lie somewhere between the two, at a point not yet ascertained. (4) The harm due to continued interbreed- ing has been considered, as I think, without sufficient warrant, to cause a presumed strong natural and instinctive repugnance to the marriage of near kin. The facts are that close and con- tinued interbreeding invariably does harm after a few genera- tions, but that a single cross with near kinsfolk is practically innocuous. Of course, a sense of repugnance might become correlated with any harmful practice, but there is no evidence that it is repugnance with which interbreeding is correlated, but only indifference, which is equally effective in preventing it, but quite another thing. ( 5 ) The strongest reason of all in civilized countries appears to be the earnest desire not to infringe the sanctity and freedom of the social relations of a family group, but this has nothing to do with instinctive sexual repugnance. Yet it is through the latter motive alone, so far as I can judge,
