ng interferes with the improvement of old races, and
especially with the formation of new ones. It is important as indirectly
bearing on Hybridism; and perhaps on the extinction of species, when any
form has become so rare that only a few individuals {115} remain within a
confined area. It bears in an important manner on 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. Hence I shall 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 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 reason to believe,