PALEONTOLOGY 327 



thus marking a distinct advance from the condition in Mesohippus. 

 The entire weight of the body was borne upon the middle (third) toe. 

 Quite as important as this reduction of the lateral toes was the change in 

 the teeth. The permanent molar teeth had moderately high crowns, 

 and the upper surface was worn down to a flat grinding surface marked 

 by sharp ridges of enamel set among dentine and cement (Fig. 222). 

 Merychippus was evidently a grazing animal, whereas its predecessors 

 must have fed upon succulent herbage which was crushed, not ground. 

 The milk molars of Merychippus, unlike the permanent ones, were short- 

 crowned and had little cement. The two sets of teeth thus showed in 

 the lifetime of one individual changes of the same kind as were taking 



Fig. 225. — Skull of Merychippus, about 3i.Q natural size. (From model prepared by 



Ward's Natural Science Establishment.) 



place in the race of horses, and recall the so-called Biogenetic Law 

 (Chapter X) according to which the development of an individual 

 repeats the evolutionary stages of the race to which the individual 

 belongs. The skull was enlarged (Fig, 225), and the lower jaw was 

 heavier in evident relation to the change of the teeth. The orbit of the 

 eye occupied a more posterior position relative to the teeth, making the 

 face relatively longer. The orbit was also completely closed behind 

 by a bar of bone which in the earlier forms was merely a process pro- 

 jecting down from above. The body had increased to a height of three 

 or four feet. 



Pliohippus. — This animal was not appreciably larger than the pre- 

 ceding member of the series, but differed from it in at least one striking 

 feature. The two lateral toes had disappeared (Fig. 221), except as 

 long splint bones. Pliohippus was thus the first one-toed horse. The 



