A HISTORY OF SCIENCE 



cessive equine genera are as follows : First, increase in 

 size; second, increase in speed, through concentration 

 of limb bones ; third, elongation of head and neck, and 

 modifications of skull. The eocene Orohippus was the 

 size of a fox. Miohippus and Anchitherium, from the 

 miocene, were about as large as a sheep. Hipparion 

 and Pliohippus, of the pliocene, equalled the ass in 

 height; while the size of the quaternary Equus was 

 fully up to that of a modern horse. 



" The increase of speed was equally well marked, and 

 was a direct result of the gradual formation of the 

 limbs. The latter were slowly concentrated by the 

 reduction of their lateral elements and enlargement 

 of the axial bone, until the force exerted by each 

 limb came to act directly through its axis in the 

 line of motion. This concentration is well seen e. g., 

 in the fore -limb. There was, first, a change in the 

 scapula and humerus, especially in the latter, which 

 facilitated motion in one line only; second, an expan- 

 sion of the radius and reduction of the ulna, until the 

 former alone remained entire and effective; third, a 

 shortening of all the carpal bones and enlargement of 

 the median ones, insuring a firmer wrist ; fourth, an in- 

 crease of size of the third digit, at the expense of those 

 of each side, until the former alone supported the 

 limb. 



"Such is, in brief, a general outline of the more 

 marked changes that seemed to have produced in 

 America the highly specialized modern Equus from his 

 diminutive four-toed predecessor, the eocene Orohip- 

 pus. The line of descent appears to have been direct, 

 and the remains now known supply every important 



no 



