CHORDATA — VERTEBRATA — MAMMALS 



393 



time to the four-toed Lower Eocene Eohippus of Europe 

 (Fig. 172). This tiny animal, less than a foot high, migrated 

 across Asia into western North America, where the evolution 

 (Figs. 172-175) of the modern horse (Equus) was completed, 

 giving an ascending series somewhat as follows : Eohip- 

 pus (Lower Eocene), Protorohippus (mid-Eocene), Orohippus 

 (Upper Eocene), Epihippus (topmost Eocene), all more or less 

 fully four-toed but increasing in size. The line is continued in 

 Mesohippus (Lower Oligocene), the two- foot-high Miohippus 

 (Upper Oligocene), both three-toed with the side toes touching 

 the ground, through the three-foot-high Merychippus (Miocene), 

 three-toed but side toes not touching the ground and hence 

 almost functionless, into the typical grazing horse, — the one- 

 toed Equus (Pliocene to present), with the side toes reduced 



Fore foot 



Hind foot Molar teeth 



Equus 



Pliocene 



Miocene 



fj~!T^ Protohippus 



One Toe 



Sptinl5 of 



2"^ioi 4'><l.jirj 



Three Toes 

 3idc toes 



nol (ouching (he (Jroiind 



One Toe 



Splinli ot 

 2"-"' and i'i-d.d.l. 



Three Toes 

 5ide (oes 



L nol toudiin^ the 6rxrun4 



u 



LonJ- 

 Crowned, 

 Cement 

 covered 



Oligocene 



Mesohippus 



Three Toes 



Stde Iocs 

 touching tilt ground; 

 3^ jplinl of i'id.^.l 



Three Toes 



5"de toes 

 toucJiing (he ground 



V^/ 



Four Toes 



Eocene 



Protorohippus 



H^racotherium 

 (Eohippus) 



Short- 

 CrownetJ 

 without 

 Cement 



n Four Toes 



!■% 5plinl of l'-dijil 



Three Toes 



5plrnl of 5'-di^il 



\^ 



Cretaceous 



Hypothetical Ancestors with Five Toes on Each Foot 

 and Teeth like those of Monkeys etc. 



Fig. 173. — Evolution of the horse. I. (Drawn to same scale.) Note the increase 

 in vertical diameter of the skull, the movement of eye backwards (hence increase 

 in length of face), the increase in height of teeth accompanied by an infolding of 

 cement between the enamel ridges of each tooth and the decrease in number of 

 toes upon both fore and hind feet. (After Matthew.) 



