the influence of free intercrossing, in obliterating individual differences,
and thus giving uniformity of character to the individuals of the same race or
species; for if additional vigour and fertility be thus gained, the crossed
offspring will multiply and prevail, and the ultimate result will be far
greater than otherwise would have occurred. Lastly, the question is of high
interest, as bearing on mankind. I shall therefore discuss this subject at
full length. As the facts which prove the evil effects of close interbreeding
are more copious, though less decisive, than those on the good effects of
crossing, I shall, under each group of beings, begin with the former.

There is no difficulty in defining what is meant by a cross; but this is by no
means easy in regard to "breeding in and in" or "too close interbreeding,"
because, as we shall see, different species of animals are differently
affected by the same degree of interbreeding. The pairing of a father and
daughter, or mother and son, or brothers and sisters, if carried on during
several generations, is the closest possible form of interbreeding. But some
good judges, for instance Sir J. Sebright, believe that the pairing of a
brother and sister is much closer than that of parents and children; for when
the father is matched with his daughter he crosses, as is said, with only half
his own blood. The consequences of close interbreeding carried on for too long
a time, are, as is generally believed, loss of size, constitutional vigour,
and fertility, sometimes accompanied by a tendency to malformation. Manifest
evil does not usually follow from pairing the nearest relations for two,
three, or even four generations; but several causes interfere with our
detecting the evil--such as the deterioration being very gradual, and the
difficulty of distinguishing between such direct evil and the inevitable
augmentation of any morbid tendencies which may be latent or apparent in the
related parents. On the other hand, the benefit from a cross, even when there
has not been any very close interbreeding, is almost invariably at once
conspicuous. There is good reason to believe, and this was the opinion of that