298 



CAUSES OF VARIATION 



SECTION II , EVIDENCE FROM MUTABILITY 

 OF SPECIES 



Species do not, however, always remain unchanged. On the 

 contrary, they frequently exhibit a progressive development 

 truly marvelous. Horses, for example, are traceable backward 

 by easy stages and well-defined connecting links to a time far 

 beyond the appearance of man upon the earth, the line ending 



Head 



Fore Foot 



HindFoot 



Teeth 



OneJToe 



Splints of 



2nd and 4th 



digits 



OneToe 



Splints of 



2nd and 4th 



digits 



Protohippus 



Mesohippus 



ThreeToes 



Side toes 



not touching 



the ground 



ThreeToes 



Side toes 



not touching 



the ground 



Long- 

 Crowned, 

 Cement- 

 covered 



Three Toes 



Side toes 



touching the 



ground} 

 Splint of Sthdigit 



Protorohippus 



ThreeToes 



Side toes 



touching the 



ground 



Four Toes 



Short - 

 Crowned, 

 without 

 Cement 



Hyracothermm 

 (Eohippus} 



FourToes 

 Splint of 

 1st digit 



ThreeToes 

 Splint of 

 5th digit 



FIG. 31. Comparative drawings of skulls, feet, and teeth of prehistoric horses, 

 showing evolutionary development. Reproduced by permission from 

 Origin and History of the Horse by H. F. Osborn 



in one of the earliest mammals, a little five-toed creature not 

 much larger than the domestic cat. 



No less than twelve stages in this evolution are well known, 

 and represented by specimens more or less complete : l 



1 William D. Matthew, of the American Museum of Natural History, article 

 " Horse, the Evolution of," in Encyclopaedia Americana. This is one of the best 

 and one of the newest accounts of the development of the horse, and is chosen be- 

 cause of its accessibility and reliability. The outline given, while not marked by 

 quotations, is practically an abstract of the reference. 



