58 THE GEOLOGICAL EVIDENCES 



the classic memoirs of the illustrious Cuvier. 

 The modifications here referred to are primarily 

 the greater or less differentiation of the elements 

 of the foot and leg (fore and aft). In the modern 

 horse there is but a single toe to each foot, which 

 is supported by a single metapodial (cannon-bone), 

 but in the more ancient horse-like animals the feet 

 were polydactyl in character, being furnished with f 

 three, four, or even five toes. This is seemingly 

 a broad difference, and it might reasonably be 

 supposed that there must exist strong grounds 

 for uniting animals that appear so widely removed 

 from one another. The rationale of our classifica- 

 tion is the fact that between the earliest horse-like 

 animals and the modern horse we have a series 

 of transitional forms which show almost every 

 grade of foot structure leading from polydactyl- 

 ism to monodactylism, the toes undergoing grad- 

 ual reduction in number, and (excepting the single 

 toe that is retained) growing smaller in size 

 as we proceed from the more ancient to the more 

 modern forms. This gradation is beautifully 



