from the sexual excitement of
novelty or other cause, which led to the former rather than to the latter
unions, would be augmented through natural selection, and thus might become
instinctive; for those individuals which had an innate preference of this
kind would increase in number. It seems more probable, that degraded
savages should {124} thus unconsciously have acquired their dislike and
even abhorrence of incestuous marriages, rather than that they should have
discovered by reasoning and observation the evil results. The abhorrence
occasionally failing is no valid argument against the feeling being
instinctive, for any instinct may occasionally fail or become vitiated, as
sometimes occurs with parental love and the social sympathies. In the case
of man, the question whether evil follows from close interbreeding will
probably never be answered by direct evidence, as he propagates his kind so
slowly and cannot be subjected to experiment; but the almost universal
practice of all races at all times of avoiding closely-related marriages is
an argument of considerable weight; and whatever conclusion we arrive at in
regard to the higher animals may be safely extended to man.

Turning now to Birds: in the case of the _Fowl_ a whole array of
authorities could be given against too close interbreeding. Sir J.
Sebright positively asserts that he made many trials, and that his
fowls, when thus treated, became long in the legs, small in the body,
and bad breeders.[269] He produced the famous Sebright Bantams by
complicated crosses, and by breeding in-and-in; and since his time
there has been much close interbreeding with these Bantams; and they
are now notoriously bad breeders. I have seen Silver Bantams, directly
descended from his stock, which had become almost as barren as hybrids;
for not a single chicken had been that year hatched from two full nests
of eggs. Mr. Hewitt says that with these Bantams the sterility of the
male stands, with rare exceptions, in the closest relation with their
loss of certain secondary male characters: he adds, "I have noticed, as
a general rule, that even the slightest deviation from feminine
character in the tail of the male Sebright--say the elongation by only
half an inch of the two principal tail-feathers--brings with it
improved probability of increased fertility."[270]

Mr. Wright states[271] that Mr. Clark, "whose figh