Tertiary Age of Mammals. 95 



add to the height of the body, various tribes have taken to walking on 

 their tip-toes ; that is, have become digitigrade. In the case of the 

 horse, he has, in addition, discontinued the use of one toe after an- 

 other, thereby losing the use of them, till only one toe is left. The 

 radius and ulna of the forearm were gradually coalesced into one bone 

 in the course of development, and the fibula of the hind leg was by de> 

 grees aborted, the tibia acquiring greater size and doing the work of both. 

 The most ancient horse in America is from the Eocene of the Green 

 river basin in southwestern Colorado, and was named Eohippus b] 

 Marsh, who made the discovery. This horse was no bigger than a fox. 

 The bones of the leg and forearm were entirely distinct, as in man. 

 On the fore foot he had four serviceable toes and a rudimentary 01 

 partly aborted fifth one. On the hind feet were three toes. Another 

 horse, same size, in the Middle Eocene, had quite dropped the rudimentary 

 toe of the fore feet. In other respects he resembled the first. He 

 is named Orohippus. In the Lower Miocene another horse much the 

 same as the first, had dropped the use of the fourth toe, which was 

 reduced to a rudimentary splint. That is the Mesohippus. The Mio- 

 hippus is found in the Miocene also. He was about the size of a sheep. 

 In him the ulna and radius are pretty well consolidated and the fibula 

 reduced to a rudiment. He has three toes on each foot, beside a faint 

 rudiment of the fourth on the fore feet. In the Lower Pliocene is found 

 the Protohippus of America and its equivalent, the Hipparion of 

 Europe. It was about as large as an ass. Three toes all around and no 

 rudiment of the fourth left. 



Higher in the Pliocene is another, the Pliohippus, who has but one 

 toe on each foot, the other two appearing on either side as rudimentary 

 splints. The consolidation of the two bones of the fore arm is nearly 

 complete, and the rudiment of the fibula diminished. 



The Equus, or horse, of the Quartenary and the present is an improve- 

 ment on this animal in the shape of the head, size of the brain, propor- 

 tions of the body and limbs, style of the teeth and various details, 

 which together make a vast difference in the functional value of the 

 animal. 8 The Eohippus named here is certainly not the first of the 

 horse famil} T . He was undoubtedly an immigrant to this country. 

 His ancestors in Asia, or wherever they began to diverge from the 

 Carnivorous and Omnivorous tribes were five-toed. 



The history of the Camel is similar to that of the horse. He came to 

 this country in Miocene times, and during that and the Pliocene 

 became modified into the modern camel form. 

 8 See Le Conte GeoloRy, 610. 



