372 MECHAmCAL EVOLUTION". 



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

 portion of impact it will receive. 



The fact that the Perissodadyla did not develojo the solid un- 

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

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

 would indicate that it did not adopt a habitat wdiich required 

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



