Answering the last question first, it has been conclusively proved that there has been a general correspondence in the progress of vertebrate life in the two continents of the northern hemisphere. The differences, though numerous, are of minor importance. Some families of vertebrata have existed on the one continent, which were absent from the other, but the number of such is not large. Even the same genus occasionally existed on both continents.
The other questions must be answered by reference to the genealogies themselves, or phylogenies, as they are called.
In tracing back all the lines to which we have yet had access, the same kind of changes is found to have taken place in all of them. Let us take, for instance, the animals with hoofs. These embrace the cloven-footed and odd-toed orders, with their many species and families, which are represented by the ox, deer, camel, hog, and hippopotamus, for the cloven-footed; and the horse, tapir, and rhinoceros, for the odd-toed. Most of these creatures walk on their toes. Many of the first-named group have but two toes, more or less united together, while the horses, of the second group, have but one toe. The bones of the two rows which form both the palm and the sole alternate with each other; and the ankle-joint is a well-constructed tongue-and-groove arrangement. The teeth in many of them are highly complicated by the infolding of the enamel of the crowns of the molars, and this special development of the molars has been accompanied by a corresponding reduction in their number, and in the number of the incisors. In tracing the lines of these animals backward in time we have made the following discoveries: First, the infoldings of the enamel of the molars become shallower, and are finally represented by the valleys between four hills or tubercles, which stand to each other so as to be inclosed by a square figure. The number of the molar teeth increases. If incisor teeth were absent, they appear. The toes increase in number, becoming five on all the feet. The step becomes plantigrade or flat-footed, the heel reaching the ground. The tongue and groove disappear from the ankle-joint, which becomes flat. The bones of the two rows of the carpus and tarsus no longer alternate with each other, but rest, each one of the first on each one of the second row only. In 1874 I foretold that the ancestor of all the mammals above mentioned would prove to be a "pentadactyle, plantigrade bunodont"; that is, a five-toed sole-walker, with tubercular molar teeth. In 1881, seven years later, I obtained evidence that such a type of mammals abounded in North America during the early Eocene Tertiary period, and the prophecy was fulfilled. The best-known genus of this division has been called Phenacodus, and the figures of it will be found in the "American Naturalist" for 1884. In a still earlier formation of the Eocene, nearly all the hoofed mammalia were found to be of this type, showing conclusively that this group, which is known as the Condylarthra, was the ancestor of all hoofed mammals (Fig. 1).
