5. Varieties or races of the same species are fully reproductive with each other, which is not the case with true species. Mutual sterility of varieties of the same species is an exceptional peculiarity, if it ever truly exist; and, on the other hand, the cross-fertilization of varieties of the same species, whether in animals or plants, tends to vigorous life, and also to return to the primitive or average type. On the other hand, intermixture of distinct species rarely, if ever, occurs freely in nature. It is generally a result of artificial contrivance. Again, hybrids produced from species known to be distinct are either wholly barren, or barren _inter se_, reproducing only with one of the original stocks, and rapidly returning to it; or if ever fertile _inter se_, which is somewhat doubtful, rapidly run out. It has been maintained by Pallas and others, and Darwin leans to this idea, that there is still another possibility, namely, that of the perfect and continued fertility of such mixed races, especially after long domestication; but their proofs are derived principally from the intermixture of the races of dogs and of poultry, which are cases actually in dispute at present, as to the original unity or diversity of the so-called species. If we apply these considerations to man, our conclusion must be that, even in his bodily frame, he is not merely specifically but ordinally distinct from other animals, and that the differences between races of men are varietal rather than specific. This view is confirmed by the following facts: