are the more remarkable, because, in the American
instances, there can be no doubt that it is the common hog which has
assumed these abnormal forms.

5. All varieties or races of men intermix freely, in a manner which
strongly indicates specific unity. We hold here, as already stated,
that no good case of a permanent race arising from intermixture of
distinct species of the lower animals has been adduced; but there is
another fact in relation to this subject which the advocates of
specific diversity would do well to study. Even in varieties of those
domestic animals which are certainly specifically identical, as the
hog, the sheep, the ox--although crosses between the varieties may
easily be produced--they are not readily maintained, and sometimes
tend to die out. What are called good crosses lead to improved energy,
and continual breeding in and in of the same variety leads to
degeneracy and decay; but, on the other hand, crosses of certain
varieties are proved by experience to be of weakly and unproductive
quality; and every practical book on cattle contains remarks on the
difficulty of keeping up crosses without intermixture with one of the
pure breeds. It would thus appear that very unlike varieties of the
same species display in this respect, in an imperfect manner, the
peculiarities of distinct species. It is on this principle that I
would in part account for some of the exceptional facts which occur in
mixed races of men.

What, then, are the facts in the case of man? In producing crosses of
distinct species, as in the case of the horse and ass, breeders are
obliged to resort to expedients to overcome the natural repugnance to
such intermixture. In the case of even the most extreme varieties of
man, if such repugnance exists, it is voluntarily overcome, as the
slave population of America testifies abundantly. By far the greater
part of the intermixtures of races of men tend to increase of vital
energy and vigor, as in the case of judicious crosses of some domestic
animals. Where a different result occurs, we usually find sufficient
secondary causes to account for it. I shall refer to but one such
case--that of the half-breed American Indian. In so far as I have had
opportunities of observation or inquiry, these people are prolific,
much more so than the unmixed Indian. They are also energetic, and
often highly intellectual; but they are of delicate constitution,
especially liable to scrofulous diseases, and th