492 ESSENTIALS OF ZOOLOGY 



mammals in fullest outline, partly because they are relatively recent 

 and partly because the mammalian skeleton is readily fossilized. The 

 most complete pedigree in fossils has probably been worked out for 

 the horse, and a great deal of its material has been located right in 

 the southwestern part of the United States. 



The Rise of the Horse. — Primitive horselike animals are thought 

 probably to have arisen from an extinct group called Condylarthra, 

 which had five toes on each foot and a large part of the sole resting 

 on the ground. The first unquestionable horselike form found in 

 America is the small Eohippus which was about one foot tall and 

 the fossils of which came from the rocks of Eocene times. It had 

 the outer four digits complete on the forefoot, but no trace of the 

 thumb, while the hind foot had three complete digits with vestiges 

 or splints of the first and fifth. Following the foxlike Eohippus, 

 later in the Eocene period came the OroMppus with an enlarged 

 central digit in the forefoot and the loss of the splints in the hind 

 foot. Mesohippus, about the size of a large dog with a three-toed 

 foot both in front and behind but with the side toes much reduced 

 and a trace of the fifth digit in the forefoot, appeared during the 

 Oligocene epoch. Merychippus of Miocene times and Pliohippus of 

 the Pliocene epoch show a continuation of this reduction to a one- 

 toed type which leads to Equus cahallus of modern times. The 

 modern horse walks on the tip of the middle toe of each foot with 

 the vestiges of digits 2 and 4 persisting as splints. 



Arthur Dendy, in 1911, wrote that the horse is an example of the 

 adaptation of a lowland type to become a plains type, as the ex- 

 tensive, dry, grass-covered plains developed. The adaptation has 

 proceeded along two lines. The limbs have become elongated by 

 the elevation of the heel, thus putting the animal on tiptoe and 

 fitting it for rapid flight from its enemies on the grass-covered open 

 prairie. At the same time the neck and head became elongated to 

 enable the animal to graze the ground without bending the legs. 

 Along with this the teeth changed from a carnivorous form to a 

 complex, broad grinding type for feeding on grass. In addition to 

 these changes the brain advanced. 



These changes took place gradually through millions of years and 

 the intermediate forms give the paleontologist a graphic picture of 

 the history of this modern species. The sequence of these stages 

 seems to fit in exactly with the theory that each has been derived 



