62 ELEMENTS OF MAMMALIAN ANATOMY. 
scendants of a five-toed ancestor of Eocene times. The 
evolution of the horse’s limb and the reduction in the num- 
ber 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 prob- 
ably more than 5,000,000 years ago, when that country was 
more or less marshy, and it was necessary that the mam- 
mals should possess a spreading foot which would not per- 
mit 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. Con- 
firmation of this ancestral 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 
phalangeal 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 com- 
plete and correspondingly as large as in Orohippus. 
As to the method of walking, mammals are spoken of 
as plantigrade, digitigrade, and unguligrade. The first 
mode of progression is exemplified by the bear, which places 
its metacarpals and phalanges flat on the ground in walk- 
ing. The cat is digitigrade, walking on its toes. The horse 
and cow are unguligrade, as they walk upon the hoof. In 
some cases, as in the Cetacea, the forefoot is adapted for 
swimming instead of walking, and the number of phalanges 
to each digit is more than three. 
PRACTICAL QUESTIONS AND SUGGESTIONS. 
1. What bones compose the shoulder girdle? 
2. How is the thoracic limb attached to the trunk? 
