the difference between the skull of the ordinary domestic hog and that of the wild boar is quite equal to that observed between the Negro and European skulls. Darwin shows that it is much greater, and illustrates this by an amusing pair of portraits. The breeds of swine even differ in directions altogether unparalleled in man. For instance, both in America and Europe solid-hoofed swine have originated and become a permanent variety; and there is said to be another variety with five toes.[172] These 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.