RELATIVE STABILITY OF LIVING MATTER 



3 OI 



lightly. The animal was of the 

 regarded as being "off the direct 

 line of descent." See Fig. 33. 

 The companion form, Parahip- 

 pus, is regarded as nearer the 

 line, having better teeth and 

 smaller side toes. 



9 and 10. Protohippus and 

 Pliohippus (Middle and Upper 

 Miocene). Teeth much im- 

 proved as grinders, the val- 

 leys being filled with cement, 

 all showing the appearance 

 of harder vegetation. The side 

 toes (n and iv) are still complete, 

 but do not touch the ground. 

 In some species of Pliohippus 

 they have almost disappeared. 



11. Hipparion (Pliocene). 

 Much like Protohippus, but 

 larger, with more complicated 

 teeth. Found both in Europe 

 and in America, but is probably 

 one of the "side branches." 



12. Equus (Pleistocene and 

 Recent). The modern horse, in 

 which digits I and v have en- 

 tirely disappeared and n and iv 

 are represented by splints. 

 This single remaining branch of 

 the horse family (including the 

 asses) has developed one of the 

 most specialized of animals. It 

 has left behind many unsuc- 

 cessful relatives, representing 

 departures that proved either 

 unprofitable or unfortunate and 

 coming thus to a more or less 



size of a Shetland pony, but is 



FIG. 33. Three-toed ancestor of the 

 horse, the Hypohippus : a complete 

 skeleton found in the Middle Mio- 

 cene, Colorado, showing the second 

 and fourth toes touching the ground 

 lightly. From specimens in the 

 American Museum of Natural His- 

 tory, New York. Courtesy of Direc- 

 tor H. F. Osborn 



