CAUSES OF EXTINCTION 275 



crowned in all the genera of the merycoidodonts, except in the later terminal members. Brachy- 

 odont teeth could not withstand the wear caused by the harsher herbage of the late Oligocene and 

 Miocene. This probably was a potent contributory cause of extinction of Merycoidodon, Eporeodon, 

 Promerycochaems, and others. And yet, if hypsodonty had really been as necessary for survival as 

 it was in the horses, would not selection have developed long-crowned teeth earlier and more gener- 

 ally in this group? The brachyodont horses and camels also disappeared at approximately the same 

 time as these merycoidodonts. The harsher herbage consisted largely of grasses which had a high 

 silica content and also undoubtedly bore much dust and foreign matter. 



Another influence may have lain in the structure of the foot. The merycoidodonts never 

 became truly cursorial and never reduced the number of digits below four. In consequence they 

 could not flee rapidly from their enemies nor cover long distances to water or food with any degree of 



They were devoid of horns or other external means of defense, and this condition, coupled with 

 lack of speed, mitigated against a long racial survival. 



These animals never possessed more than a moderate-sized brain, and this fact may have 

 hampered them in resourcefulness in periods of stress, in finding new habitats, and in avoiding 

 dangers to which young and old were exposed. 



Another and probably important factor in partial extinction was the extreme specialization seen 

 in the Leptauchema-Cyclo-pidius stock and in the Merycochcems-Pronomotherium line. In such 

 groups, where only a single mode of living remains, any change would cause them to perish. 



