THE ANIMALS OF THE PAST 



431 



study of these ancient forms also throws light on many 

 conditions of structure in modern animals otherwise diffi- 

 cult to understand. For example, while most of the ani- 

 mals closely related to the horse have five toes on each of 

 their feet, the horse has only one. We know that the leg 



FIG. 252. Saurian bird with jointed tail, claws on wings, and teeth in jaws (Archce- 

 opleryx lithographica), from the Jurassic rocks of Bavaria. After NICHOLSON, 

 from Owen. 



and foot of a horse are homologous with the leg and foot 

 of a dog, yet a dog's foot has five (on the hind foot usually 

 four) toes, while the horse's foot has never more than one 

 toe. But the study of the ancient horses makes all this 

 clear. The remains of over thirty different ancient horse- 

 like animals have been found in the rocks of the Tertiary 

 era. The Eohippus^ the earliest of these horse-like animals, 

 found in the oldest Tertiary rocks, was little larger than a 

 fox, and its fore feet had four hoofed toes, with the rudi- 

 ment of a fifth, while the hind feet had three hoofed toes 

 (Fig. 253). 



In later rocks is found the Orohippus, also small, 

 hut with the rudimentary fifth toe of the fore foot gone. 

 Still later appeared the Mesohippus and Miohippus, horses 

 about the size of sheep, with three hoofed toes only on both 



