72 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
Then came in succession Orohippus, of the Upper Eocene, Mesohippus 
of the Lower Eocene, Pliohippus of the Upper Pliocene, and finally 
Equus: Qua- 
ternary and 
Recent. 
Pliohippus : 
Phocene. 
Protohippus : 
Lower Plio- 
cene, 
Miohippus : 
Miocene, 
Mesohippus: 
Lower Mio- 
cene. 
Fic. 2.—Feet and teeth in fossil pedigree of the horse. (After Marsh.) 
a, Bones of the fore foot; b, bones of the hind foot; c, radius and ulna; d, fibula 
and tibia; ¢, roots of a tooth; f and g, crowns of upper and lower teeth. 
Equus of the Quaternary and Recent. Other genera might be men- 
tioned, but the history of this series has been pictured in a classic 
