354 ANIMAL BIOLOGY 



to enable us to trace the gradual evolution of some of our 

 modern types. The history of the evolution of the horse 

 is especially complete. In our modern horses the digits 

 of the feet are reduced to a single one corresponding to 

 one middle digit, the hoof representing a greatly enlarged 

 and thickened nail. On either side of this middle digit 

 are two rudiments, the splint bones, representing the 

 basal part of the second and fourth digits. The first 

 representative of the series of horse-like animals was a 

 small creature, the Eohippus, about the size of a fox, 

 that lived during the earliest division of the tertiary 

 period. Its fore foot contained four toes with hoofs 

 and a rudimentary fifth, and its hind foot had three hoofed 

 toes. This animal was succeeded in more recent deposits 

 by a type with four toes on the fore foot and three on the 

 hind foot. Later appeared somewhat larger horses about 

 the size of a sheep, with three toes on both fore and hind 

 feet, and a rudiment of a fourth toe on the fore foot; 

 while still later forms had but three toes on fore and hind 

 feet, the lateral ones being much reduced in size, but still 

 bearing hoofs. Later these lateral hoofs and their digits 

 disappeared with the exception of the rudimentary splint 

 bones. As the horses increased in size, the middle digit 

 became larger and came to bear more and more of the 

 weight of the body, while the lateral ones became smaller, 

 and finally all but disappeared. From thirty odd species 

 of fossil horses that are known, we can select a series of 

 connecting links which afford the strongest evidence of 

 the descent of our modern horses from a five-toed ancestral 

 species. 



Only in relatively late deposits do we meet with any 

 fossil remains of man, and but very few of the oldest re- 

 mains have been preserved. The oldest of all represented 

 by the top part of the skull, a femur and a few minor 



