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AN INTRODUCTION TO ZOOLOGY 



FIG. 156. Restoration of 

 four-toed horse, Eohip- 

 pus, Lower Eocene, North 

 America. After Lull. 



To return now to the horse, a long series of fossil forms have 

 been discovered, going back into the Eocene period, and these are 

 separated up into groups of allied species, from each of which we can 

 take as a representative a typical species. These are by no means all, 

 but they represent so far as we know the main stages in the evolution 

 of the horse. It is quite impossible to consider even superficially 

 all of these or to examine all the various points of the skeletons of 

 the forms selected. We shall consider then some of the characters 

 of a few of the best known and striking of these stages. 



Eocene. The first animal that can be undoubtedly considered 

 as on the ancestral line of the horse is Eohippus, which is found in 



the lower Eocene beds. It has been found 

 both in Europe and in North America, but 

 it is not known with certainty where it 

 originated or what it originated from. It 

 is interesting to note in this connection, 

 however, that the most primitive and 

 earliest known member of this group is 

 Hyracotherium, which was found in the 

 London clay, and is preserved in the British 

 Museum. If it originated in W. Europe, 

 then it must have migrated to North America, which was then 

 entirely forest clad, across what is now Bering Strait, and in 

 North America most of the succeeding stages were passed through. 

 Echippus was a small animal intermediate in size between, say 

 a cat and a fox, about n inches at the shoulder. 

 The fore limb had four well- developed toes with 

 hoofs, and the fifth was represented by a splint 

 bone, and the hind limb had three well- developed 

 digits with hoofs, and the fourth was represented 

 by a splint bone. The teeth indicate that it was a 

 vegetable feeder, although they are not nearly so 

 specialised as in the present-day horse. Its teeth 

 and limbs throw out hints of a still more remote 

 much less horse-like ancestor. You will see, how- 

 ever, that this animal does not resemble a horse at 

 all closely, and indeed, it is highly probable that if 

 no intermediate forms were known it would not 

 have been regarded as having close affinity with 

 the horse. Between Eohippus and Equus, however, 

 are a large number of intermediate forms that grade into one another 

 so completely that it becomes a matter of difficulty to draw a line 

 between one species and another. 



Later, in the middle Eocene, appeared Orohippus. This form 



FIG. 157. Hand 

 (A)andfoot(B) 

 of Eohippus. 

 One-fourth 

 natural size. 

 After Marsh, 

 from Lull. 



