308 OUTLINES OF EVOLUTIONARY BIOLOGY 



FIG. 153. Outlines of Horses of different Geological Periods, showing their 

 Eelative Sizes. (From Lull.) 



a, Protorohippus (Eocene) ; b, Orohippus (Eocene) ; c, Mesohippus (Oligocene) ; d, Meryc- 

 hippus (Miocene) ; e, Pliohippus (Pliocene) ; /, Equus (Recent). 



