\ 



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 



tA^w>-»i\ 



Hypothetical Ancestors witl\ Five Toes on Each Foot 

 Teeth like Itiose of Jlonkeys 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 mfolding ot 

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



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



