ELEPHANTS, RHINOCEROSES, AND HORSES 273 



penetrating it nearly to the root. There are four pairs of 

 premolars in the upper, and three in the lower jaw, while on 

 each side of both jaws there are three molars. The molar teeth 

 of the modern horses are extremely long-crowned, or hypsodont. 

 The triple fangs of the root have, in course of time, become 

 quite inconspicuous, and the tuberculated, crater-like crown of 

 the old type (such, for instance, as is seen in the molar teeth of 

 a man) is lengthened and lengthened until it is nearly three times 

 as long as the roots, while its surface is folded into intricate 

 zigzag ridges of dentine bordered with enamel. The develop- 

 ment of the horse's teeth is illustrated in so many modern works 

 of zoology, coincidently with the gradual diminution of the toes 

 and of the secondary bones in the arm and leg, that it is not 

 necessary to go further into the question here. 



A three-toed horse (Hipparion] once inhabited England, but 

 became extinct before the arrival of man. When man came to 

 these islands these peninsulas of Europe, as they then were 

 he possibly found still lingering a somewhat primitive type of 

 horse, Equus stenonis^ the molar teeth of which offer some 

 approximation to those of the Hipparion, or three-toed horse. 

 But True Horses, more or less similar to those still found existing 

 in Tibet and perhaps in Central Asia, made their appearance in 

 Britain in the middle of the Pleistocene Epoch, and had soon 

 overrun England, Wales, Scotland, and Ireland. 



Equus caballus. THE TRUE HORSE 



The genus Equus, which includes Equus stenonis above 

 mentioned, seems to have originated in India or some contiguous 

 parts of Asia from a form like Trotohippus}- This early type 

 spread westward as far as Britain, and into North America. 

 Other one-toed horses had probably been developed in North 

 America, previously, from Protohippus, and had spread right down 

 into the extreme end of South America, where they only became 



1 The Protohippus with two small lateral toes was, as far as is known, 

 North American, but an allied form more equine than Hipparion may 

 well have inhabited Eastern Asia. 



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