X 



H.iQO Charles Darwin. 



these successive genera are the following: ist, in- 

 crease in size, from Orohippus, as large as a fox, to 

 the modern horse ; 2d, increase in speed through 

 concentration of the limb-bones ; 3d, elongation of 

 the head and neck and modification of the skull. 

 The increase of speed was a direct result of a gradual 

 and striking modification of the limbs. These were 

 slowly concentrated by the reduction of their lateral 

 elements and enlargement of the axial one, until the 

 force exerted by each limb came to act directly 

 through its axis in the line of motion. This concen- 

 tration is well shown in the fore-limb. There was, 

 1st, a change in the scapula and humerus, especially 

 in the latter, which facilitated motion in one plane 

 only ; 2d, an expansion of the radius and reduction 

 of the ulna, until the former alone remained entire 

 and effective ; 3d, a shortening of all the carpal 

 bones and enlargement of the median ones, ensuring 

 a firm wrist ; 4th, an increase in size of the third 

 digit at the expense of those on each side, until the 

 former alone supported the limb. The latter change 

 is clearly seen in the accompanying diagram, which 

 represents the forefeet of four typical genera in the 

 equine series, taken in succession from each of the 

 geological periods in which this group of mammals 

 is known to have lived. The line of descent ap- 

 pears to have been direct, and the remains now 

 known supply every important intermediate form. 

 Considering the remarkable development of the group 

 throughout the entire Tertiary period, and its exist- 

 ence even later, it seems very strange that none of 

 the species should have survived, and that we are 

 indebted for our present horse to the Old World." 



