THE SKELETON. 77 



except the Endentata and Ungulata possess five digits. 

 The tapir has four functional digits. The cow, deer, 

 sheep, and pig also have four digits, but only two are 

 functional, the second and the fifth being atrophied and 

 terminating in the small caudo-lateral hoofs which do not 

 touch the ground. The rhinoceros has three functional 

 digits, and the Equidae possess only one digit (Fig. 37), 

 the third, though the atrophied remnants of the second 

 and fourth metacarpals are present as splint bones which 

 do not support phalanges. 



As before stated, the Ungulates are undoubtedly de- 

 scendants of a five-toed ancestor of Eocene times. The 

 evolution of the horse's limb and the reduction in the 

 number of digits are shown in figure 35. Fossil remains 

 of the ancestors of the horse have been found in western 

 United States, Europe, and South America. Orohippus 

 lived in the region of Wyoming, Montana, and Idaho 

 probably more than 5,000,000 years ago, when that 

 country was more or less marshy, and it was necessary 

 that the mammals should possess a spreading foot which 

 would not permit them to sink too deep into the mud. 

 As the ground became firmer and preying Carnivora 

 more numerous, the foot of the horse adapted itself to 

 rapid flight over solid ground. Thus, through use and 

 natural selection the third digit was enlarged at the 

 expense of the other digits. Confirmation of this ances- 

 tral history of the horse is found by an examination of 

 the early embryonic stages. According to Ewart, a 

 horse embryo 35 cm. long possesses quite well-developed 

 second and fourth metacarpals terminating with pha- 

 langeal structures. The ulna and radius of an embryo 

 50 mm. long are strikingly similar to these same bones 

 in Mesohippus. In a still younger embryo the ulna is 

 complete and correspondingly as large as in Orohippus. 



