EVOLUTION, VARIATION AND HEREDITY 455 



Pleistocene. Finally, in the lower Pleistocene appeared the 

 genus Equus, the relatives of our modern horse, and represented by 

 numerous skeletons of E. Scotti. It is, as is known, one of the best- 

 adapted animals for locomotion by means of running that we know, 

 and is marked off from all other living genera by the fact that it 

 possesses but a single toe on fore and hind feet. The relics of the 

 next two lateral bones are to be found as a pair of splint bones, 

 remains of metacarpals. At present there are three main types of 

 the horse family found wild in the world, known as horses, asses, and 

 zebras. They are mainly to be distinguished by external characters, 

 and their skeletons are remarkably alike. 



The slow development of the horse from the " dawn horse " 

 Eohippus of the early Eocene up to the appearance of the modern 

 genus Equus was contemporaneous with the disappearance of much 

 of the forest land in North America and its gradual replacement by 

 the plains and prairies we now find there. From a wood dweller the 

 horse gradually become fitted for a life on grassy plains, from an 

 animal with teeth for browsing on herbs and shrubs to one with 

 teeth for grazing on grass. The specialisation not only affected 

 the feet and size of the body, but the whole structure of the horse 

 including the head. This alteration is particularly noticeable in 

 the Miocene times, when much of North America became grass land 

 and several types suited to grazing appeared and also became 

 extinct. 



Throughout the Pleistocene times horses of the genus Equus 

 are to be found in North America, where the genus was apparently 

 widespread and numerous. Every now and again from Oligocene 

 times horses reinvaded Europe, as the discovery of their fossil remains 

 testify, but they do not appear to have flourished there as they did 

 in North America. Strangely enough, however, during the Pleisto- 

 cene period the invasion was successful, and the genus Equus became 

 very widespread over the plains of Eurasia. For some reason at 

 present unknown, horses died out completely at a subsequent period 

 in America, which became isolated from Asia by the breakdown of 

 the land connection and the formation of Bering Strait. All the 

 herds of horses, the mustangs, found all over the prairies of North 

 America by the early colonists are due to horses re-introduced in his- 

 toric times. They are the descendants of the horses that escaped 

 from the old Spanish explorers. The disappearance of the original 

 species of Equus in North America is all the more remarkable since 

 on the re-introduction of the same genus they flourished so well. 



This past history of the horse at which we have only briefly 

 glanced is undoubtedly one of the most complete, but it is paralleled 

 by that of other groups, e.g. the elephant, the rhinoceros, the camel, 



