been productive of healthy offspring, and successive generations
of offspring of incestuous connection are not unknown; but, although
statistics are lacking, it seems to be very often true that children
of such unions are degenerate. It may be that the reason for this is
that with the laws and social sentiments now prevailing in all
civilized communities, only degenerates ever contract incestuous
alliances. Desirable as it may be from a social point of view that
this strong sentiment against incest should continue, it is not yet
_proven_ that even the closest blood relationship between the parents
is directly injurious to the offspring. The "instinctive horror of
incest" is a myth, for although a horror of incest does very properly
exist in civilized, and in some tribal societies, it is purely a
matter of custom and education, and not at all a universal law.

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 s