TERTIARY ANIMALS. 541 / 



metacarpal (splint), and possibly a corresponding rudimentary fifth toe 

 (the thumb), like a dew-claw. Also, the two bones of the leg and fore- 

 arm were yet entirely distinct. This animal was no larger than a fox. 

 Next, in the Middle Eocene (Bridger beds), came the OvoMppus of 

 Marsh, an animal of similar size, and having similar structure, except 

 that the rudimentary thumb or dew-claw is dropped, leaving only four 

 toes on the fore-foot. Next came, in the Loiuer Miocene, the MesoMp- 

 pus, in which the fourth toe has become a rudimentary and useless 

 splint. Next came, still in the Miocene, the Miohippus of the United 

 States and nearly-allied Anchithere of Europe, more horse-like than the 

 preceding. The rudimentary fourth splint is now almost gone, and the 

 middle hoof has become larger ; nevertheless, the two side-hoofs are 

 still serviceable. The two bones of the leg have also become united, 

 though still quite distinct. This animal was about the size of a sheep. 

 Next came, in the Upper Miocene, and Lower Pliocene, the Protohip- 

 pus of the United States and allied Hipparion of Europe, an animal 

 still more horse-like than the preceding, both in structure and size. 

 Every remnant of the fourth splint is now gone ; the middle hoof has 

 become still larger, and the two side-hoofs smaller and shorter, and no 

 longer serviceable, except in marshy ground. It was about the size of 

 the ass. Next came, in the Pliocene, the Pliohippus, almost a complete 

 horse. The hoofs are reduced to one, but the splints of the two side- 

 toes remain to attest the line of descent. It differs from the true horse 

 in the skull, shape of the hoof, the less length of the molars, and some 

 other less important details. Last comes, in the Quaternary, the mod- 

 ern horse — Equus. The hoof becomes rounder, the splint-bones shorter, 

 the molars longer, the second bone of the leg more rudimentary, and 

 the evolutionary change is complete. 



Similar gradual changes, becoming more and more horse-like, may 

 be traced in the shape of the head and neck, and especially in the grad- 

 ually-increasing length and complexity of structure of the grinding- 

 teeth. All these changes are shown in Fig. 922, for which we are in- 

 debted to the kindness of Prof. Marsh. The Eohippus is omitted, as 

 no figures of this have yet been published. 



There can be no doubt that if we could trace the line of descent still 

 further back we would find a perfect five-toed ancestor. From this 

 normal number of five, the toes have been successively dropped, ac- 

 cording to a regular law. In the Perissodactyl line first the thumb, 

 No. 1, was dropped ; then the little finger, No. 5 ; then the first and 

 ring-fingers, Nos. 2 and 4, were shortened up more and more and finally 

 disappeared, and only the middle finger, No. 3, remained in the modern 

 horse. In the Artiodactyl line, after the dropping of No. 1, then Nos. 

 2 and 5 of the four-toed foot were shortened and gradually disappeared, 

 and Nos. 3 and 4 remained in the ruminants. 



