by that abhorrence of incest
which is usually, though not always so clearly marked among the different
races of mankind.[184] It was once commonly stated, as by Morgan and by
Maine, that this abhorrence was the result of experience; the marriages of
closely related persons were found to be injurious to offspring and were
therefore avoided. This theory, however, is baseless because the marriages
of closely related persons are not injurious to the offspring.
Consanguineous marriages, so closely as they can be investigated on a
large scale,--that is to say, marriages between cousins,--as Huth was the
first to show, develop no tendency to the production of offspring of
impaired quality provided the parents are sound; they are only injurious
in this respect in so far as they may lead to the union of couples who are
both defective in the same direction. According to another theory, that of
Westermarck, who has very fully and ably discussed the whole
question,[185] "there is an innate aversion to sexual intercourse between
persons living very closely together from early youth, and, as such
persons are in most cases related, this feeling displays itself chiefly
as a horror of intercourse between near kin." Westermarck points out very
truly that the prohibition of incest could not be founded on experience
even if (as he is himself inclined to believe) consanguineous marriages
are injurious to the offspring; incest is prevented "neither by laws, nor
by customs, nor by education, but by an _instinct_ which under normal
circumstances makes sexual love between the nearest kin a psychic
impossibility." There is, however, a very radical objection to this
theory. It assumes the existence of a kind of instinct which can with
difficulty be accepted. An instinct is fundamentally a more or less
complicated series of reflexes set in action by a definite stimulus. An
innate tendency at once so specific and so merely negative, involving at
the same time deliberate intellectual processes, can only with a certain
force be introduced into the accepted class of instincts. It is as awkward
and artificial an instinct as would be, let us say, an instinct to avoid
eating the apples that grew in one's own yard.[186]

The explanation of the abhorrence to incest is really, however,
exceedingly simple. Any reader who has followed the discussion of sexual
selection in the present volume and is also familiar with the "Analysis of
the Sexual Impulse" set fo