516 THE THEORY OF EVOLUTION 



of origin, although the ancestors of EoMppus have not yet been 

 discovered. After the horse family had flourished for a consider- 

 able period in North America, some of its members migrated to 

 South America, if we may judge from the fossils, and spread widely 

 on that continent. This southern branch of the American family 

 became extinct in the Pleistocene, as did the North American 

 horses. As in the case of the disappearance of ammonites and 

 many other types that have flourished and declined, the causes of 

 this extinction have not been ascertained. A plausible theory 

 is that all the horses of the New World may have been swept away 

 by some disease-producing parasite, but this is wholly speculation. 

 We only know that they disappeared in the Americas, while a few 

 species survived on the continents of the Old World, probably 

 migrating back to this region as the ancestors of the original 

 American horse family (cf. Fig. 276) had migrated into the New 

 World from an unknown locahty. 



The probable course of evolution by which forms like Eohippus 

 became changed into horses of the modern genus Equus are indi- 

 cated by the succession of fossils (Fig. 276). The series begins 

 with an animal about the size of a fox, with relatively short neck 

 and limbs. The feet are also short and there are four functional 

 toes on the front foot and three on the hind ; but the hind foot has 

 splint-like remains of other digits, as though a man should have 

 three toes functional and the little toe and great toe rudimentary. 

 Since similar rudiments of a fifth toe occur on the fore feet, it 

 appears that the ancestors of Eohippus, and therefore of mod- 

 ern horses, were five-toed (cf. Fig. 277). The principal evolution- 

 ary changes, as shown by the succession of types, are the loss of side 

 toes until only one persists as the functional digit ; consolidation of 

 leg bones so that the radius and ulna and the tibia and fibula 

 become fused ; increasing complexity of the teeth in relation to the 

 functions of grazing with incisors and grinding with molars; in- 

 crease in size and complexity of the brain; and a great increase 

 in size of the whole body with changes of shape and proportions, 

 particularly in the head, neck, and back. In the modern horse 

 there is only one fully developed digit, but there are splint bones 

 on the fore limbs representing the rudiments of two others, and 

 in the early embryo there are five digits upon both fore and hind 

 limbs. 



The Elephants. — The succession of types that is believed to be 



