ANCESTRY OF THE HORSES.--The LOPHIODONTIDAE and the PALAEOTHERIIDAE are two of the most interesting extinct families of Perissodactyles; for among them we find what would appear to be the ancestral forms of both the existing Tapirs and Horses. The Rhinoceroses also would seem to be derivable from the Palaeotheriidae. The very vagueness of the characters of these creatures, considered from a classificatory point of view, has led to much diversity in their placing. This though gratifying to the evolutionist is tiresome to the writer who wishes to give a methodical account of their various characters. It will be best perhaps not to attempt an accurate placing or to reconcile conflicting opinion, but to give some salient features of osteology which lead to the belief in their relationship to existing groups of Perissodactyles. A book upon the history of mammals would be incomplete without some account of that well-ascertained series of forms which seem to connect these primitive Perissodactyles with the modern Horse. _Equus_, in fact, is not only the "show horse" of the doctrine of evolution, but also the "stalking horse." In the Eocene of both Europe and America are met with a number of forms from which we may start. _Hyracotherium_, regarded on the one hand as the type of a sub-family of the Equidae themselves, and on the other as a member of the family Lophiodontidae, was a small-sized animal, three feet or so in length; it possesses the complete Eutherian dentition with a slight diastema. The orbits are not separated from the temporal fossa; the fore-limbs were four-toed, the hind three-toed, with moderately long metapodia, especially on the hind-feet. The shoulder blade {248} has a well-marked coracoid process. The radius and ulna are separate; so too are the tibia and fibula. _Eohippus_, belonging to the same sub-family, is slightly more primitive; for the hind-feet have a rudiment of digit I. _Orohippus_ is a little nearer to the Horses in that the molar teeth have acquired a little further advance towards the equine type. Instead of the tubercles of the teeth remaining for the most part separate, they have fused into a set of ridges, of which, however, the pattern is less complex than in the modern Horses. In other respects _Orohippus_ is much like _Hyracotherium_. _Pachynolophus_ seems to be but a synonym. The next stage is shown by _Mesohippus_, a Lower Miocene form, usually referred to the neighbourhood of _Palaeotherium_. It has nearly lost one of the toes of the fore-foot, a rudiment only remaining; the metapodials, at any rate of the fore-feet, seem to be slightly increased in length. The orbit is not encircled by bone, but there is a strong process from the frontal, which nearly meets the zygomatic arch.