EQUID^E 



379 



FIG. 158.- 



Outer vi 



islands of the two softer substances, dentine and cement, separated 

 by variously reduplicated and contorted lines of intensely hard 

 enamel, resulted (Fig. 157, c). The crown continued lengthening 

 until in the modern Horses it has assumed the form called " hyps- 

 odont" (Fig. 158, b). Instead of contracting into a neck, and 

 forming roots, its sides continue parallel for a considerable depth in 

 the socket, and as the surface wears away, the whole 

 tooth slowly pushes up, and maintains the grinding 

 edge constantly at the same level above the alveolus, 

 much as in the perpetually growing Rodent's teeth. 

 But in existing Horses there is still a limit to the 

 growth of the molar. After a length is attained 

 which in normal conditions supplies sufficient grind- 

 ing surface for the lifetime of the animal, 

 a neck and roots are formed, and the 

 tooth is reduced to the condition of that fl/lfl 

 of the brachydont ancestor. It is per- jft^jg-l^ 

 fectly clear that this lengthening of the 

 crown adds greatly to the power of the 

 teeth as organs of mastication, and en- 

 ables the animals in which it has taken 

 place to find their sustenance among the l w er raolar tooth ? f 



. , . . . , ' (brachydont form) ; 6, corresponding 



comparatively dry and harsh herbage tooth of Horse (hypsodont form). 

 of the open plains, instead of being 



limited to the more succulent vegetable productions of the marshes 

 and forests in which their predecessors probably dwelt. 



The modifications of the limbs which took place pari passu with 

 those of the teeth must have been associated with increased speed, 

 especially over firm and unyielding ground. Short, stout legs, and 

 broad feet, with numerous toes, spreading apart from each other 

 when the weight of the creature is borne on them, are sufficiently 

 Avell adapted for plodding deliberately over marshy and yielding 

 surfaces, and the Tapirs and the Rhinoceroses, which in the 

 structure of the limbs have altered but little from the primitive 

 Eocene forms, still haunt the borders of streams and lakes and 

 the shady depths of the forests, as was probably the habit of 

 their ancient representatives, while the Horses are all inhabitants of 

 the open plains, for life in which their whole organisation is in 

 the most eminent degree adapted. The length and mobility of 

 the neck, position of the eye and ear, and great development of the 

 organ of smell, give them ample means of becoming aware of the 

 approach of enemies, while the length of their limbs, the angles 

 the different segments form with each other, and especially the 

 combination of firmness, stability, and lightness in the reduction of 

 all the toes to a single one, upon which the whole weight of the 

 body and all the muscular power are concentrated, give them speed 



