imilarity 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.]

Professor Ward's idea of "difference of potential," or contrast, as
essential to the highest vigor of the race as well as to that of the
individual offspring, offers an alternative explanation of the
observed results of consanguineous marriages, and one which does not
necessarily conflict with the explanation already given. All the
phenomena of intensification are simply due to a resemblance between
husband and wife in particular characteristics, such as a common
tendency toward deafness or toward mental weakness. This resemblance,
which may or may not be the result of a common descent, renders more
probable the appearance of the trait in the offspring. If the parents
closely resembled each other in many respects they would be more
likely to "breed true" and the children would resemble one another in
their inherited traits, thus accounting for the high average of
deaf-mutes to the family, observed in the Irish statistics.[99]

[Footnote 99: _Cf. supra_, p. 66.]

The theory of contrast and resemblance supplements that of intensified
heredity where the resemblance is general, rather than in particular
traits or characteristics. In such a case the absence of the
stimulating effects of contrast might result in a lowering of
vitality, which in turn would react upon the youthful death-rate.

Where then related persons differ greatly in mental