510 THE HORSE'S POSITION IN THE ANIMAL WORLD 



In 'dealing with this part of the subject two courses are open, either to 

 trace the horse from its present condition backwards to the first-discovered 

 hoofed mammal in the lower Eocene, or to begin at the beginning and follow 

 the changes in size and arrangement of his various organs in successive 

 generations of horse-like animals, each series becoming more and more like 

 the horse, until, in the recent deposits, the differences are almost obscured 

 by similarities, and finally vanish altogether. The latter course will pror 

 bably be the more interesting and intelligible. 



It has been well said that the horse is an animal the evolution of which 

 from the Eocene to the Pleistocene may be compared to a chain in which 

 there is scarcely a missing link. 



Starting with the earliest hoofed mammal yet discovered, which, though 

 not a direct ancestor of the horse, has certain special characteristics in 

 'Common with it — the animal known as the Phenacodus deserves notice. 

 'The first specimen was dug up by Professor Cope from the Eocene marl 

 lon Bear River, Wyoming, and the restored skeleton of the animal is 

 represented in Plate LXX. 



The lighter shaded portions of the figure indicate the places where 

 missing portions of bones have necessitated restorations. No important 

 bones are absent, although, as necessarily happens in fossil specimens, some 

 displacement of parts has occurred. 



A glance at the skeleton of the Phenacodus will show that it belongs 

 to the perissodactyle or odd-toed mammals, and that the third digit is 

 ■distinctly larger than the rest. It is not to be understood that the 

 animal here shown is to be taken for the primeval horse, but it has 

 several characteristics in common not only with the horse but also with 

 the rhinoceros and ta23ir, which lead us to conclude that these animals 

 are all descended from nearly allied ancestors, of which the Phenacodus 

 may be taken as a re23resentative. 



In the later Eocene and the formations overlying it the remains of 

 hoofed mammals are found exhibiting remarkable changes in their teeth 

 and in the arrangement of the bones of that part of the extremities which 

 is rightly called the foot, the bones below the joints called the carpus or 

 wrist and tarsus or ankle in man, the knee and hock of the horse (see 

 Plate LXXI). From the five-toed Phenacodus the change to four, three, 

 and then one (with rudimentary splint bones) is seen to have gone on 

 with remarkable regularity, as indicated in the illustrations. 



In the Plate the extremities of the limbs have all been drawn to the 

 same scale, so as to show their relative sizes, fig. 1 representing the fore- and 

 hind-feet of the Phenacodus already mentioned — an animal about as large 

 as a fox — whilst fig. 7 represents those of the horse of the present day. 



