732 PERISSODACTYLS xxx. 7-9 



association with those of forest dwellers. The attraction of speculating 

 about these creatures has not diminished with the demonstration of 

 its dangers. 



8. Palaeotheres 



These animals, from the later Eocene and early Oligocene of 

 Europe, were an early offshoot that paralleled in many ways the 

 evolution in North America. For example, they developed three-toed 

 feet, and the premolars became molarized. The teeth developed ridges 

 on a similar plan to the horses, but differing in details. Some forms 

 became hypsodont. Several lines of descent are included in the group. 

 *Palaeotherium became large, though not so large as the gravi- 

 portal brontotheres and rhinoceroses. The shortness of its nasal bones 

 suggests that it had a proboscis like a tapir. Palaeotheres, like horses, 

 have probably been derived from a *Hyracotherium-\ike stock. They 

 illustrate the importance of parallelism in evolution, and serve to warn 

 us against the easy assumption that a character that is shown by two 

 animals must have been present in their common ancestor. 



9. Horses 



The horse, besides its special interest as one of our oldest and most 

 useful commensals, has provided a rather complete and convincing 

 record of its origin. We shall therefore first describe its present 

 structure and then analyse the fossil record to discover exactly what 

 can be demonstrated about the evolution. Existing horses, asses, and 

 zebras, all referred to the genus Equns (Figs. 486-88), are highly 

 specialized for swift movement and eating grasses (p. 697). Only the 

 third digits are developed and covered with hoofs. These are ela- 

 borately organized pads, including several sorts of keratin, harder in 

 front, more elastic behind. The metapodials of digits II and IV are 

 present as small splint-bones. There is a horny callosity on the inner 

 side of the fore-limb in all species (also on the hind-limbs in E. cabal- 

 lus, the domestic horse), representing the vestigial hoofs of lateral 

 digits. 



There are three incisors in each jaw, usually one small canine (the 

 'tush', absent in females). The first premolar is vestigial in each jaw 

 ('wolf-tooth'); the remaining three resemble the three molars. All the 

 cheek teeth are hypsodont, square in cross-section, with ectoloph and 

 transverse protoloph and metaloph, joined by longitudinal ridges that 

 give the tooth a certain resemblance to the selenodont molars of artio- 

 dactyls, hence 'selenolophodont'. The skull is modified to allow space 



