and of the different steps and stages of this evolution. Monogeny therefore in this general sense of evolution from some primitive mammalian type, may be accepted as the present conclusion of science for man as it has come to be for the horse, dog, and so many other animals which are his constant companions. Their evolution can in many cases be traced up, through successive steps, to some more simple and generalized type in the Eocene; and it may be permitted to believe that if the whole geological record could be traced as far back as that of the horse, in the case of man and the other quadrumana, their pedigree would be as clearly made out. This, however, does not conclude the question, for it is quite permissible to contend that in the case of man, as in that of the horse, though the primary ancestral type in the Eocene may be one, the secondary types from which existing races are more immediately derived may be more than one, and may have been evolved in different localities. Thus in the case of the dog, it is almost certain that some of the existing races have been derived from wolves, and others from jackals and foxes; but this is quite consistent with the belief that all the canine genus have been evolved from the marsupial Carnivora of the Eocene, through the Arctocyon, who was a generalized type, half dog and half bear. In fact, we have the authority of Darwin himself, as quoted in the beginning of this chapter, for saying that this would be quite consistent with his view of the origin of species. Now the controversy between monogenists and polygenists has turned mainly on these comparatively recent developments of secondary types. It has been fought to a great extent before the immense antiquity of the human race had been established, and it had become almost certain that its original starting-point must be sought at least as far back as in the Eocene period. The main argument for monogeny has been that the different races of mankind are fertile among themselves. This is doubtless true to a great extent, and shows that these races have not diverged very far from their ancestral type. But the researches of Darwin and his successors have thrown a good deal of new light on the question of hybridity. Species can no longer be looked upon as separated from one another and from races by hard-and-fast lines, on one side of which is absolute sterility and on the other absolute fertility; but rather as blending into one another by insensible gradations from free intercrossing to sterility, according as the differences from the original type became more pronounced and more fixed by heredity.