Evolution 331 



variations in all possible directions. Most of these lines died out while 

 only a few of the more successful ones survived. Ultimately these 

 latter ones gave rise to the modern horse. 



The Evolution of the Horse. — The evolution of the horse has 

 been more carefully worked out than that of any other comparable ver- 

 tebrate. Although no other animal has gone through exactly the same 

 sort of evolution, it does tend to indicate the means by which evolu- 

 tion works. 



In general, the evolution of the horse has resulted in increased size, 

 a reduction in the number of toes, and a change in the form of the teeth 

 from low ones to high, cement-covered teeth with complex folds of 

 enamel. These changes did not appear all at once; in fact there were 

 times when the animals barely changed from one form to another. 

 Nevertheless, the resulting over-all picture shows these tendencies. The 

 changes are best shown by a brief characterization of some of the better 

 known fossil forms. 



Eohippus. — The first known horse, Eohippiis {Hyracotherium), 

 lived during the lower Eocene. Although there are some fossils of 

 Eohippus in Europe, they are most commonly found in northern Wy- 

 oming and northwestern New Mexico. To judge from the number of 

 fossils, it would appear that Eohippus was a highly successful, abundant 

 form. It was a small animal, varying in size from about 10 to 20 inches 

 in height at the shoulder. The back was quite arched and flexible, 

 in contrast to the straight back of the modern horses. The front foot 

 had four toes, each of which terminated in a tiny hoof. Primitively 

 there must have been five toes, and the four present on Eohippus repre- 

 sented the second to fifth ones. The rear feet each had but three func- 

 tional toes, with the third being the largest, the second and fourth 

 smaller, and the first and fifth missing. These toes were provided with 

 doglike pads rather than hooves. The skull was quite unlike that of 

 modern horses, and the brain was very small. 



Eohippus had the primitive mammalian number of teeth, that is, 

 44. Interestingly enough it showed the tendency toward the double 

 type of teeth so familiar in modern horses, that is, the front ones for 

 nipping and the back ones for crushing and grinding. The rear teeth 

 were already showing some enlargement; thus, apparently, Eohippus 

 was herbivorous. However, the teeth were not so well equipped with 

 enamel that the animals could possibly have fed on grasses, rather they 

 fed on succulent leaves. 



