any other members of the same race can possibly have with one another.
But nobody falls in love with his sister. A profound instinct has taught
even the lower races of men (for the most part) to avoid such union of
the all-but-identical. In the higher races the idea never so much as
occurs to us. Even cousins seldom fall in love--seldom, that is to say,
in comparison with the frequent opportunities of intercourse they enjoy,
relatively to the remainder of general society. When they do, and when
they carry out their perilous choice effectively by marriage, natural
selection soon avenges Nature upon the offspring by cutting off the
idiots, the consumptives, the weaklings, and the cripples, who often
result from such consanguineous marriages. In narrow communities, where
breeding in-and-in becomes almost inevitable, natural selection has
similarly to exert itself upon a crowd of _crétins_ and other hapless
incapables. But in wide and open champaign countries, where individual
choice has free room for exercise, men and women as a rule (if not
constrained by parents and moralists) marry for love, and marry on the
whole their natural complements. They prefer outsiders, fresh blood,
somebody who comes from beyond the community, to the people of their own
immediate surroundings. In many men the dislike to marrying among the
folk with whom they have been brought up amounts almost to a positive
instinct; they feel it as impossible to fall in love with a
fellow-townswoman as to fall in love with their own first cousins. Among
exogamous tribes such an instinct (aided, of course, by other extraneous
causes) has hardened into custom; and there is reason to believe (from
the universal traces among the higher civilisations of marriage by
capture) that all the leading races of the world are ultimately derived
from exogamous ancestors, possessing this healthy and excellent
sentiment.

In minor matters, it is of course universally admitted that short men,