Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/74

lii bone. The navicular and the cuboid unite, and the distal end of the fibula is anchylosed with the tibia.

In Cainotherium and Dichobune the upper incisors are fully developed. There are seven grinders; the teeth form a continuous series without a diastema. The metatarsals, the navicular and cuboid, and the distal end of the fibula remain free. In the Cainotherium, also, the second metacarpal, is developed, but is much shorter than the third, while the fifth is absent or rudimentary. In this respect it resembles Anoplotherium secundarium. This circumstance, and the peculiar pattern of the upper molars in Cainotherium, lead me to hesitate in considering it as the actual ancestor of the modern Tragulidæ. If Dichobune has a four-toed fore foot (though I am inclined to suspect it resembles Cainotherium) it will be a better representative of the oldest forms of the Traguline series; but Dichobune occurs in the Middle Eocene and is, in fact, the oldest known artiodactyle mammal. Where, then, must we look for its five-toed ancestor?

If we follow down other lines of recent and tertiary Ungulata, the same question presents itself. The Pigs are traceable back through the Miocene epoch to the Upper Eocene, where they appear in the two well-marked forms of Hyopotamus and Chœropotamus; but Hyopotamus appears to have had only two toes.

Again, all the great groups of the Ruminants, the Bovidæ, Antilopidæ, Camelopardalidæ, and Cervidæ, are represented in the Miocene epoch, and so are the Camels. The Upper-Eocene Anoplotherium, which is intercalary between the Pigs and the Tragulidæ, has only two or, at most, three toes. Among the scanty mammals of the Lower Eocene formation we have the perissodactyle Ungulata represented by Coryphodon, Hyracotherium, and Pliolophus. Suppose for a moment, for the sake of following out the argument, that Pliolophus represents the primary stock of the Perissodactyles, and Dichobune that of the Artiodactyles (though I am far from saying that such is the case), then we find in the earliest fauna of the Eocene epoch to which our investigations carry us the two divisions of the Ungulata completely differentiated, and no trace of any common stock of both or five-toed predecessors to either. With the case of the Horses before us, justifying a belief in the production of new animal forms by modification of old ones, I see no escape from the necessity of seeking for these ancestors of the Ungulata beyond the limits of the Tertiary formations.

I could as soon admit special creation, at once, as suppose that the Perissodactyles and Artiodactyles had no five-toed ancestors. And when we consider how large a portion of the Tertiary period elapsed before Anchitherium was converted into Equus, it is difficult to escape the conclusion that a large proportion of time anterior to the Tertiary must have been expended in converting the common stock of the Ungulata into Perissodactyles and Artiodactyles.

The same moral is inculcated by the study of every other order of Tertiary monodelphous Mammalia. Each of these orders is represented in the Miocene epoch: the Eocene formation, as I have already said, contains Chiroptera, Insectivora, Rodentia, Ungulata,