372 MECHANICAL EVOLUTION". 



length is but little. The harder the ground, the larger the pro- 

 portion of impact it will receive. 



Tlie fact that the Perissodactyla did not develop the solid un- 

 gulate or equine foot until a late geological period, or, in other 

 words, that the orders so long retained the digital formula 4 — 3, 

 would indicate that it did not adopt a habitat which required 

 great speed as a condition of safety, so early as the Artiodactyla. 



