Dynamic Theory. 

 our Figures of Horses' Left Feet, tt'-c. , Showing Dc- 



94 



Fi(i. 73. Twenty -f< 



vlopment from the Eocene to the Qunrternary ami Recent Periods. 



VI 



mal toes and fingers. 



I. Orohippus Middle Eocene. 



II. Mesohippus Lower Miocene. 

 Ill Miohippus Miocene. 



IV. Protohippus Lower Pliocene. 



V. PI iohippus Pliocene. 



VI. Eqnus Qnarternary and Recent. 

 <t -Fore foot. 



6 -Hi iid foot. 



(-Fore arm, Radius and Ulna separated 

 in the lowest but gradually fused together 

 as we ascend. 



cZ-Sl-ank bone or tibia. The fibula which 

 is fully formed in Orohippus, is gradually 

 reduced to a mere splint as we ascend. 



horns two heavy palmate ones in 

 the usual place and two short ones 

 pointing straight out from the fore- 

 head with the feet and legs of a 

 ruminant and probably a snout, 

 was a compromise between a rumi- 

 nant r.ml a pachyderm. There 

 were many other such combina- 

 tions too numerous to mention. 

 The general advance and speciali- 

 zation of the times in regard to 

 mammal life is shown in the brain 

 development of someof these larger 

 types, the later ones showing cere- 

 bral lobes of twice the bulk rela- 

 tively to the cerebellum, that the 

 corresponding earlier ones show. 

 . Some remarkable facts in this 

 direction constitute the history of 

 the horse. There were 35 or 

 40 species of the horse in the 

 United States ranging through 

 Tertiary times. The development 

 is in part a reverting one as 

 relates to the limbs at least, which 

 are improved for the use of the 

 animal by the addition of minus 

 or subtracti ve values. It is quite 

 certain that five is the typical or 

 original normal number of mam- 

 For the sake of speed or lightness of step, or to 



