TIME IN EVOLUTION—ZEUNER 249 
{. TENTATIVE TIME i. RANGE ii. GENERA SPACED 
SCALE IN MILLION OF GENERA ACCORDING TO EXTENT OF 
YEARS STRUCTURAL DIFFERENCES 
| PLEISTOCENE EQUUS EQuUS 
t 7 
5 tL 
ra PLIOHIPPUS 
Y PLIOHIPPUS 
e) 
x 11 
10 
MERYCHIPPUS 
15 
MERYCHIPPUS 
us 18 
20-4 
VY 
Q 
>= 
25 PARAHIPPUS 
PARAHIPPUS . 
MIOHIPPUS 
30 t 5 
wy MIOHIPPUS MESOHIPPUS 
O 
3544 O ‘ 
2 16 
re) MESOHIPPUS 
40 | 
45 | 9 
a aii OROHIPPUS 
50 
I 
HYRACOTHERIUM 
55 HYRACOTHERIUM 
Ficure 1.—The evolution of the horse in North America correlated with the 
absolute time scale. (Based on the work of E. D. Cope, W. D. Matthew, W. B. 
Scott, G. G. Simpson, R. A. Stirton, and others.) 
