328 PRINCIPLES OF ANIMAL BIOLOGY 



teeth were moderately long-crowned and possessed grinding surfaces. 

 The body stood about 48 inches high. 



Equus. — The fossil horses of Pleistocene time were so nearly like the 

 living forms as to be included with the latter in the same genus. The 

 recent animals are 60 inches or more in height, and weigh many hundreds 

 of pounds. Each foot has but one toe (Fig. 221), on the tip of which 

 the animal stands like a ballet-dancer. Two lateral toes are evidenced 

 by splint bones, and in rare cases a reversionary horse is born with 

 externally visible digits articulated with one of these splints on each 

 fore foot. The teeth are long and columnar, and grow continuously 

 during early and middle life, during which time the wear at the upper 

 surface approximately equals the growth. The grinding surface is worn 

 flat, except that the enamel resists the abrasion more successfully than 

 do the dentine and cement, so that the enamel forms sharp cutting ridges. 

 The position of these ridges changes somewhat as the tooth is worn to 

 different levels and the pattern of the upper surface is indicative, in a 

 general way, of the age of the animal. Late in life growth of the teeth 

 practically ceases, and then the teeth may become quite short. The 

 skull is enlarged, particularly the front portion of it, probably in relation 

 to the battery of large teeth which it contains. The face is relatively 

 longer than in the ancestral forms, since the eye is set well back of the 

 teeth and the brain case has not been relatively enlarged. 



Relation to the Environment. — During Eocene time, whenEohippus 

 lived, North America is believed to have had a moist climate with an 

 abundance of soft or marshy ground. On such ground an animal with 

 four-toed or three-toed feet, especially if it rested partially on the soles 

 of these feet, could have walked more successfully than could an animal 

 with but one toe on each foot, particularly if it stood on the tips of these 

 toes. Later the climate became more arid, streams and lakes dried 

 up, and the hard plains lands were developed. One-toed horses could 

 occupy these plains successfully, and the lightening of the lower part of 

 the legs was in the interest of speed. If, as is not improbable, the vegeta- 

 tion of the earlier periods was tender or succulent, while later the harsh 

 grasses and sedges became prominent, the change in the teeth of the 

 horse-like animals from crushing structures to grinders also served to 

 fit the animals for the environment in which they lived. Whether the 

 increase in size is to be regarded as an advantage is uncertain. 



While the changes described above have in the main fitted the horses 

 to live in the environment which they occupy, it is not to be assumed 

 that the features of the environment caused the animals to change in 

 this way. The causes of such evolutionary modifications are discussed 

 in the following chapter. 



Evolution of the Camel. — -The camel family underw^-nt most of its 

 evolution in North America. The earliest genus recognized as ancestral 



