HEREDITY 



177 



In the living species of horse, E quits, there is but a single toe, 

 with its basal -bones. On each side of the base bone of this toe is 

 a small bone known as a splint bone. The splint bones are 

 apparently useless to the horse, but in extinct species of horse 

 these bones are developed as digits, bearing small hoofs. Occa- 

 sionally even now colts are born in which these splint bones bear 

 rudimentary hoofs. In the museum of Stanford University is 

 the leg of a high-bred colt 

 from Milpitas, California, bear- 

 ing a small hoof on each of 

 the two splint bones. 



The remains (Fig. Ill) of 

 over thirty different ancient 

 horse-like animals have been 

 found in the rocks of the 

 Tertiary era. The Eohippus, 

 the earliest of these horselike 

 animals, found in the oldest 

 Tertiary rocks, was little larger 

 than a fox, and its forefeet 

 had four hoofed toes, with the 

 rudiment of a fifth, while the 

 hind feet had three hoofed 

 toes. In the later rocks is 

 found the Orohippus, also 

 small, but with the rudi- 

 mentary fifth toe of the fore- 

 foot gone. Still later appeared 

 the Mesohippus and Miohip- 

 pus, horses about the size of 



sheep, with three hoofed toes only, on both forefeet and hind 

 feet, but with the rudiment of the fourth toe in the forefeet, 

 of the same size in Mesohippus, smaller in Pliohippus. Also, 

 the middle toe and hoof of the three toes in each foot was 

 distinctly larger than the others in both Mesohippus and Mio- 

 hippus. Next came the Protohippus, a horse about the size of 

 a donkey, with three toes, but with the two side toes on each 

 foot reduced in size, and probably no longer of use in walking. 

 The middle toe and hoof carried all the weight. Still later in 

 the Tertiary era lived the Pliohippus, an "almost complete 

 horse." The side toes of Pliohippus are reduced to mere rudi- 



FIG. 111. Foot changes in evolution of 

 the horse: a, Equus, Quaternary (re- 

 cent) ; b, Pliohippus, Pliocene; c, Pro- 

 tohippus, Lower Pliocene; d, Miohip- 

 pus, Miocene; e, Mesohippus, Lower 

 Miocene; /, Orohippus, Eocene. (After 

 FIG. 254 of "Animal Studies.") 



