human races and animal species in a systematic point of view. In these various forms of humanity there is as much system as in anything else in nature, and by overlooking the thoughtful combinations expressed in them we place ourselves at once outside of the focus from which the whole may be correctly seen. In consequence of their constancy, these differences are so many limitations to prevent a complete melting of normal types into each other and consequent loss of their primitive features. That these different types are genetically foreign to one another, and do not run together by imperceptible, intermediate degrees, appears plain when their mixtures are compared. White and negro produce mulattoes, white and Indian produce mamelucos, negro and Indian produce cafuzos, and these three kinds of half-breeds are not connecting links between the pure races, but stand exactly in that relation to them in which all hybrids stand to their parents. The mameluco is as truly a half-breed between white and Indian, the cafuzo as truly a half-breed between negro and Indian, as is the mulatto, commonly so called, a half-breed between white and negro. They all share equally the peculiarities of both parents, and though more fertile than half-breeds in other families of the animal kingdom, there is in all a constant tendency to revert to the primary types in a country where three distinct races are constantly commingling, for they mix much more readily with the original stocks than with each other.[86] Children between mameluco and mameluco, or between cafuzo and cafuzo, or between mulatto and mulatto, are seldom met with where the pure races occur; while offspring of mulattoes with whites, Indians and negroes, or of mamelucos with whites, Indians, and negroes, or of cafuzos with whites, Indians, and negroes, form the bulk of these mixed populations. The natural result of an uninterrupted contact of half-breeds with one another is a class of men in which pure type fades away as completely as do all the good qualities, physical and moral, of the primitive races, engendering a mongrel crowd as repulsive as the mongrel dogs, which are apt to be their companions, and among which it is impossible to pick out a single specimen retaining the intelligence, the nobility, or the affectionateness of nature which makes the dog of pure type the favorite companion of civilized man. The question respecting the relation of the human races to each other is complicated by the want of precision in the definition of species. Naturalists differ greatly in their estimation of the characters by which species are to be distinguished, and of their natural limitations. I have published elsewhere my own views on this subject. I believe the boundaries of species to be precise and unvarying, based upon a category of characters quite distinct from those on which the other groups of the animal kingdom, as genera, families, orders, and classes, are founded.