EVOLUTION, VARIATION AND HEREDITY 453 



shows a distinct increase in size, and was about 14 inches at the 

 shoulder, and in size and proportions resembed a whippet. The 

 fore foot had lost all trace of the first digit, and the hind foot all trace 

 of both the first and the fifth digit. In addition to this, certain 

 alterations are also observable in the teeth. 



Oligoeene. In the lower formations of this period appeared 



FIG. 158. -Restoration of four-toed 

 horse, Orohippus, Middle Eocene, 

 Wyoming. After Lull. 



FIG. 159. Restoration of three- toed 

 horse, Mesohippus, Middle Oligoeene, 

 North America. -After Lull. 



r 



Mesohippus, which was a slightly larger animal, about 18 inches 



high at the shoulder. This is known as the three-toed horse, because 



in the fore foot the fifth digit was present only 



as a splint bone and therefore quite rudimentary, 



while the hind foot remained much as before. 



Thus both fore and hind feet possesses three 



toes with hoofs. 



Miohippus was a still later three-toed horse, 

 from the Upper Oligoeene beds. It represents 

 an advance in size on Mesohippus, being 24 

 inches high at the shoulder, i.e. about as large 

 as a sheep. In its skeleton it is not much more 

 advanced save that the splint of the fifth digit 

 is in a still more rudimentary condition. Both 

 types had the middle digit larger than the 

 lateral digits, indicating that it was the most 

 important in locomotion, and in both also the 

 teeth show some advance in complexity. 



Miocene. This period also furnishes a group 

 of horse-like forms. Miohippus continues over 

 into the lower formations, while in the middle formations we 

 find Pamhippus and Hypohippus. These are types which, while 

 showing certain advances in size, etc., also indicate specialisation 



LG. 160. Hand 

 (A) and foot (B) of 

 Mesohippus. One- 

 fourth natural size. 

 After Marsh. 



