ORDER UNGULATA. 



36; 



digits of the foot (fig. 1236, b) have become larger and the middle digit 

 smaller ; traces of the fifth metacarpal being retained in the American 

 species. The next step is probably made by some form allied to Anchi- 

 lophus or Pachynolophus of the Upper Eocene ; and from such a type 

 the transition is easy to the Lower Eocene Hyracotherium and Systemo- 

 do?t, in which all the species are very small, the dentition is of the simple 

 Lophodont type, with a large third lobe to the last lower true molar, and 

 the fore-foot (fig. 1236, a) has four complete digits, which are of sub- 

 equal size ; while there may be {Eohippus) a rudimental metacarpal of 

 the pollex. Finally, the earliest stage of this series is formed by Phenac- 

 odus of the Lowest or Puerco Eocene of North America, in which there 

 are five digits to each foot, and of which the structure will be more 

 fully noticed under the head of the suborder Condylarthra. It should 

 be observed, however, that Professor Cope would introduce an inter- 



Fig. 1236.— Bones of the left manus of Hyracotherium (a), Anchitherium (b), 

 Hipparion (c), and Equus (d). 



mediate stage between Systemodon and Phenacodus ; and that the same 

 authority also introduces the genus Palceotherium between Hyraco- 

 therium and A7ichitherium, although most writers regard that genus as 

 quite off the line. 



In this connection may be noticed the remarkable circumstance that 

 in the line of evolution culminating in the modern Horse a parallel 

 series of generically identical or closely allied forms occurs in the Ter- 

 tiaries of both Europe and North America, from which it has been sug- 

 gested that in both Continents a parallel development of the same genera 

 has simultaneously taken place — i.e., that in both regions Anchitherium 

 has given rise to Hipparion, and Hipparion or an allied type to Equus. 

 Now, seeing it is evident that in the case of species of a single genus 

 the evolution has taken place in separate lines, — that is to say, that the 

 existing Indian species of Cam's are probably derived directly from the 

 Pliocene forms of the same region, and the Brazilian species of that 

 genus have their predecessors of the cave-epoch of that country, — there 

 appears no logical reason for refusing to admit an analogous parallel 

 evolution in the case of genera, and there is accordingly a considerable 

 probability that the hypothesis in question may be a true one. Pro- 

 fessor Cope considers that in one country Protohippus, and in the other 

 Hipparion, was the immediate ancestor of Equus. 



