In the museum of Yale University may be seen the fossil history of this splendid ungulate, from the time it was a clumsy little quadruped only 14 inches high, and with four or five toes on each foot, down to existing horses. The earliest known ancestor of the horse, the eohippus, lived at the beginning of the Eocene epoch. It had five toes, almost equal, on each front foot (four toes behind), and was about the size of a fox. The orohippus, which lived a little later, had four toes on each front-foot, and three behind. The mesohippus, found in the Miocene, had three toes and one rudimentary toe on each front-foot, and three toes behind. It was about the size of a sheep. The miohippus, which is found later, had three toes on each of its four feet, with the middle toe on each foot larger than the other two. The pliohippus, living in the Pliocene epoch, had one principal toe on each foot, and two secondary toes, the two secondary toes not reaching to the ground. It was about the size of a donkey. Existing horses have one toe on each foot—the digit corresponding to the big middle finger—and the ruins of two others in the form of splints on the back of each ankle. In the embryo of the horse these splints are segmented, each of them, into three phalanges. Fossil remains representing all stages in the development of the horse have been found in the regions about the upper waters of the Missouri River. It is an important fact that the types of structure forming any series grow more and more generalised as the distance from the present increases, and that different lines of development, when traced back into the past, often converge in types which combine the main characters of various existing groups. The horses, rhinoceroses, and tapirs, great as are the differences among them now, can be traced back step by step through fossil forms, their differences gradually becoming less marked, until ‘the lines ultimately blend together, if not in one common ancestor, at all events into forms so closely alike in all essentials that no reasonable doubt can be held as to their common origin.’ ‘The four chief orders of the higher mammals—the primates, ungulates, carnivora, and rodents—seem to be separated by profound gulfs, when we confine our attention to the representatives of to-day. But these gulfs are completely closed, and the sharp distinctions of the four orders are entirely lost, when we go back and compare their extinct predecessors of the Cenozoic period, who lived at least three million years ago. There we find the great sub-class of the placentals, which to-day comprises more than two thousand five hundred species, represented by only a small number of insignificant pro-placentals, in which the characters of the four divergent orders are so intermingled and toned down that we cannot in reason do other than consider them as the precursors of those features. The oldest primates, the oldest ungulates, the oldest carnivora, and the oldest