THE ORIGIN OF SPECIES 361 



States and arranged them in the Yale Museum. The essen- 

 tial facts are these. Horse-like animals probably arose 

 from an extinct group known as the Condylarthra- which had 

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

 on the ground. However, the first unquestionably horse- 

 like form found in North America is a little animal less than 

 a foot in height, known as Eohippus, from rocks of the Eocene 

 age. The fore-foot of Eohippus has four complete toes 

 (digits 2, 3, 4, and 5) and a vestige of the first digit in the 

 form of a splint bone. The hind-foot has three toes (digits 

 2, 3, and 4) with a remnant of the fifth digit. Later in the 

 Eocene we find Protorohippus with the same functional 

 digits but lacking the vestiges. Coming to the Oligocene, 

 Mesohippus appears. This animal is about the size of a 

 sheep and still has three toes (digits 2, 3, and 4) on the hind- 

 foot, but only three complete toes (digits 2, 3, and 4) and the 

 vestige of a fourth (digit 5) on the fore-foot. Also the 

 middle toe (digit 3) is now much larger than the side toes, 

 which barely touch the ground. Then during the late 

 Miocene and early Pliocene we find Protohippus, an animal 

 about three feet tall, with three toes on each foot, but with 

 only one reaching the ground, and with no vestiges of other 

 digits. Finally, toward the end of the Pliocene, appears 

 the genus Equus which includes the modern horse, Equus 

 caballus, with one functional toe (digit 3) on each foot and 

 the remnants of two more (digits 2 and 4) in the splint bones. 

 (Fig. 189.) 



In this outline of what must be interpreted as the fossil 

 ancestors of the Horse of to-day, we have merely selected 

 several representative forms to emphasize changes in foot 

 structure. But the reader will realize that many other 

 equally significant changes were involved in the transforma- 

 tion of an Eohippus type into that of Equus. This much 



