No such five-toed "atavus," however, has yet made its appearance among the few middle and older Eocene Mammalia which are known.
Another series of closely affiliated forms, though the evidence they afford is perhaps less complete than that of the Equine series, is presented to us by the Dichobune of the Eocene epoch, the Cainotherium of the Miocene, and the Tragulidoe, or so-called "Musk- deer," of the present day.
The Tragulidoe; have no incisors in the upper jaw, and only six grinding-teeth on each side of each jaw; while the canine is moved up to the outer incisor, and there is a diastema in the lower jaw. There are four complete toes on the hind foot, but the middle metatarsals usually become, sooner or later, ankylosed into a cannon bone. The navicular and the cuboid unite, and the distal end of the fibula is ankylosed 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 Tragulidoe. If Dichobune has a fore-toed fore foot (though I am inclined to suspect that 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 Hyopopotamus and Choeropotamus; but Hyopotamus appears to have had only two toes.
Again, all the great groups of the Ruminants, the Bovidoe, Antilopidoe, Camelopardalidoe, and Cervidoe, are represented in the Miocene epoch, and so are the Camels. The Upper Eocene Anoplotherium, which is intercalary between the Pigs and the Tragulidoe, 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 of 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 period 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 Cheiroptera, Insectivora, Rodentia, Ungulata, Carnivora, and Cetacea. But the Cheiroptera are extreme modifications of the Insectivora, just as the Cetacea are extreme modifications of the Carnivorous type; and therefore it is to my mind incredible that monodelphous Insectivora and Carnivora should not have been abundantly developed, along with Ungulata, in the Mesozoic epoch. But if this be the case, how much further back must we go to find the common stock of the monodelphous Mammalia? As to the Didelphia, if we may trust the evidence which seems to be afforded by their very scanty remains, a Hypsiprymnoid form existed at the epoch of the Trias, contemporaneously with a Carnivorous form. At the epoch of the Trias, therefore, the Marsupialia must have already existed long enough to have become differentiated into carnivorous and herbivorous forms. But the Monotremata are lower forms than the Didelphia which last are intercalary between the Ornithodelphia and the Monodelphia. To what point of the Palaeozoic epoch, then, must we, upon any rational estimate, relegate the origin of the Monotremata?
The investigation of the occurrence of the classes and of the orders of the Sauropsida in time points in exactly the same direction. If, as there is great reason to believe, true Birds existed in the Triassic epoch, the ornithoscelidous forms by which Reptiles passed into Birds must have preceded them. In fact there is, even at present, considerable ground for suspecting the existence of Dinosauria in the Permian formations; but, in that case, lizards must be of still earlier date. And if the very small differences which are observable between the Crocodilia of the older Mesozoic formations and those of the present day furnish any sort of approximation towards an estimate of the average rate of change among the Sauropsida, it is almost appalling to reflect how far back in Palaeozoic times we must go, before we can hope to arrive at that common stock from which the Crocodilia, Lacertilia, Ornithoscelida, and Plesiosauria, which had attained so great a development in the Triassic epoch, must have been derived.
The Amphibia and Pisces tell the same story. There is not a single class of vertebrated animals which, when it first appears, is represented by analogues of the lowest known members of the same class. Therefore, if there is any truth in the doctrine of evolution, every class must be vastly older than the first record of its appearance upon the surface of the globe. But if considerations of this kind compel us to place the origin of vertebrated animals at a period sufficiently distant from the Upper Silurian, in which the first Elasmobranchs and Ganoids occur, to allow of the evolution of such fishes as these from a Vertebrate as simple as the Amphioxus, I can only repeat that it is appalling to speculate upon the extent to which that origin must have preceded the epoch of the first recorded appearance of vertebrate life.