Double heredity may account for all the observed ill effects of consanguineous marriage, including the high youthful death-rate, the higher percentage of idiocy, deafness and blindness, and probably also the scrofulous and other degenerate tendencies; nevertheless, there may be in some instances a lowering of vitality which this hypothesis does not fully explain.

The tendency of inbreeding in animals, it is well known, is to fix the type, the tendency of crossing, to variation.  Inbreeding then, tends to become simple repetition with no natural variations in any direction, a stagnation which in itself would indicate a comparatively low vitality.  Variation and consequent selection is necessary to progress.  “Sex,” according to Ward[96] “is a device for keeping up a difference of potential,” and its object is not primarily reproduction, but variation.[97]

[Footnote 96:  Pure Sociology, p. 232.]

[Footnote 97:  Pearson (Grammar of Science, p. 373) points out that variation does occur in asexual reproduction.  But that sex is at least a powerful stimulus to variation can hardly be questioned.]

It is organic differentiation, higher life, progress, evolution....  But difference of potential is a social as well as a physiological and physical principle, and perhaps we shall find the easiest transition from the physiological to the social in viewing the deteriorating effects of close inbreeding from the standpoint of the environment instead of from that of the organism.  A long-continued uniform environment is more deteriorating than similarity of blood.  Persons who remain for their whole lives, and their descendants after them, in the same spot, surrounded by precisely the same conditions, and intermarry with others doing the same, and who continue this for a series of generations, deteriorate mentally at least, and probably also physically, although there may not be any mixing of blood.  Their whole lives, physical, mental, and moral, become fixed and monotonous, and the partners chosen for continuing the race have nothing new to add to each other’s stock.  There is no variation of the social monotony, and the result is socially the same as close consanguineal interbreeding.  On the other hand, a case in which a man should, without knowing it, marry his own sister, after they had been long separated and living under widely different skies, would probably entail no special deterioration, and their different conditions of life would have produced practically the same effect as if they were not related.[98]

[Footnote 98:  Ward, op. cit., pp. 234-235.]