RELATIVE STABILITY OF LIVING MATTER 299 



1. Hyracotherium. Skull only. Found in London clay of 

 the Lower Eocene (earliest mammals). Specimen in British 

 Museum. 



2. Eohippus. Much better known, coming from the Lower 

 Eocene of Wyoming and New Mexico. Teeth like the former ; 

 four toes on the front foot, with a splint of the fifth ; three toes 

 behind, with a splint of another, showing that considerable 

 departure had already taken place from its evident five-toed 

 ancestry. Height of animal 12 to 16 inches. 



3. Protorohippus (Wyoming, 1880). Four complete toes in 

 front and three behind ; no splints ; skeleton of the size of a 

 small dog. Described by Cope as " the four-toed horse." 



4. Orohippus. Only parts of jaws and teeth, but these show 

 some advance. Specimen at Yale University. 



5. Epihippus (Upper Eocene, New World). Only incomplete 

 specimens found, but much time has elapsed and considerable 

 development is noted, especially in the teeth. The toes are 

 still four and three, but the central toe is " becoming much larger 

 than the side toes." 



Collateral branches of the same period in the Old World 

 (Paleotherium and Paloplotherium) had three toes of nearly 

 equal size on each foot. They were very abundant at this time 

 but seem to have become extinct, a fate that overtook most 

 of the branches of this fertile and progressive stem. 



6. Mesohippus (White River Formation). Three toes on each 

 foot, the middle much the largest, the side toes bearing little 

 weight. By this time the animal ranges in size from the coyote 

 to the sheep, and the molar teeth are well developed. All parts 

 of the skeleton known. Fifth toe represented by a splint. 



7. Anchitherium (Lower Miocene). Found both in Europe 

 and in America. Much like its predecessor, but larger, with 

 better-developed teeth. May be one of the "side branches" 

 rather than in the direct line of the modern horse. 



8. Hypokippus and Parahippus. A complete skeleton of Hypo- 

 hippus was found, 1901, by the Whitney Expedition near 

 Pawnee Butte, Colorado. In the forefoot small rudiments still 

 represent the first and fifth toes, but the splints are gone; 

 the second and fourth digits still touch the ground, though 



