6o8 



HISTORICAL GEOLOGY 



America and southern Asia seems remarkable unless one remembers 

 that during the Tertiary tapirs ranged throughout the northern 

 hemisphere, making their way to South America late in the Pliocene. 1 

 Horses. — Few animals have a family history which goes so far 

 back into the past and is at the same time so well-known as that 

 of the horse. From an animal less than a foot in height, with a 

 skeleton more like that of a carnivore than a horse, the changes in 

 structure and size have been traced step by step to the present. 

 It should be borne in mind, however, that few of the so-called an- 

 cestors are truly in the direct line, but they show us rather what the 

 actual forebears were like. 



Theoretically, the history of the horse begins with a generalized, 

 five-toed animal which^walked with tire sole of the foot on the ground 

 7*yj) ( ?^q (plantigrade), or with the 

 heel but slightly raised ; with 

 the normal number of teeth 

 (44) ; with an arched back 

 somewhat like a carnivore's, 

 and with toes covered with 

 nails which were neither 

 hoofs nor claws; in other 

 words, an animal similar to 

 Phenacodus (p. 598). 



The earliest American horse 

 (Fig. 546) (Eohippus; Greek, 

 eos, dawn, and hippos, horse) 

 of which we have a record 

 lived in the early Eocene and 

 was a small and unhorselike 

 animal about the size of a fox. It still retained the normal number 

 of teeth (44), as did practically all of the animals of its time, the 

 teeth being simple with very short crowns, somewhat resembling 

 those of the pig and monkey, and very unlike the long, complicated 

 grinders of the horse of to-day. So generalized are the teeth of 

 this early horse that it is often a matter of great difficulty to dis- 

 tinguish them from those of the ancestors of what are now widely 

 removed orders of animals. There were four well-developed toes 



Fig. 546. — Model of the Eocene horse 

 Eohippus. (Restoration by C. R. Knight, 

 under the direction of Prof. H. F. Osborn.) 



1 If South America were raised in the centra] portion so as to permit the Amazon to deepen 

 it valley and drain its basin, the tapir would doubtless become extinct in the New World 

 The lesson is an important one when the extinction of other animals is considered. 



