200 



adaptation for the chewing 

 material is so abrasive that 



FIG. 10.4. Molar teeth of 

 modern horse and of Hyra- 

 cotherium. A, teeth in lateral 

 view. 6, Crowns of the teeth. 

 (After Osborn; reprinted by 

 permission from Texfbook of 

 Geology, Part II, Historical 

 Geology, by Louis V. Pirsson 

 and Charles Schuchert, pub- 

 lished by John Wiley & Sons, 

 Inc., 1915, p. 932.) 



INTRODUCTION TO EVOLUTION 



of harsh grasses, containing silica. This food 

 it wears away teeth that chew it, and it must 

 be eaten in large quantities to provide the re- 

 quired nourishment. The problem is solved by 

 developing teeth which continue to grow as fast 

 as they are worn away. As shown in Fig. 10.4, 

 each individual grinding tooth is long or high- 

 crowned (hypsodont). Until the horse becomes 

 rather old the molars do not develop pronged 

 roots, as do our own molar teeth, but continue 

 to grow as the surface is worn away. This sur- 

 face has exposed on it a complicated pattern of 

 lines of hard enamel (Fig. 10.4). The enamel 

 pattern is set in a matrix of softer dentine and 

 cement. The latter wear away more rapidly than 

 does the enamel, with the result that the surface 

 is continually maintained in a roughened condi- 

 tion reminiscent of the surface of a millstone, 

 the ridges of hard enamel protruding above the 

 dentine and cement. In this manner the horse 

 is provided with a self-sharpening, self-renewing 

 grinding mechanism for use on the harsh mate- 

 rial comprising its diet. 



If space permitted, other adaptations of the 

 horse might be enumerated, but our purpose 

 will be served by concentrating attention on 

 those just discussed: (1) enlargement and 

 elongation of digit III, with loss of other digits 

 and development of a spring mechanism; (2) 

 elongation of the preorbital portion of the 

 skull; (3) development of premolars and mo- 

 lars into high-crowned, continuously grow- 

 ing grinders. To these should be added the 

 large size characteristic of most varieties of 

 horses. 



Hyracotherium 



Having reviewed the characteristics of the modern horse we turn our 

 attention to the characteristics of the first horse of which we have any 



