It will be exceedingly instructive to trace briefly the progressive specialization of the horse family through the Tertiary period. A wonderful series representing this family has been found in the American Tertiaries. First of all, the =Eohippus= is found in the earliest Eocene rocks. This little animal was about the size of a fox and had four perfect toes (hoofed) and a fifth, much smaller, imperfect one (splint) on the fore-feet; the hind-feet had three perfect hoofed toes. Later on, in the middle Eocene rocks, was found the =Orohippus= (Fig. 14), with three toes behind (b), and four in front (a)—the fifth imperfect toe (splint) being lost. This animal was also about the size of a fox. Still later on were found, in the Miocene rocks, the =Mesohippus= (c, d) and the =Miohippus= (e, f), with three toes behind (d, f) and three toes and a splint in front (c, e). The splint in the Miohippus is much smaller than in the Mesohippus, and also in the former the two side toes have become smaller and farther removed from the ground. Both of these animals were about the size of sheep. Further on, in the early Pliocene rocks, appears the =Protohippus= (g, h), with three toes on all the feet, but the middle toe considerably larger and longer than the side toes. This animal was about the size of an ass. Then, finally, came the =Pliohippus= (k, l) and =Equus= (m, n) in the latest Pliocene rocks. Here the middle toe is greatly enlarged and the side toes are reduced to useless splints. Thus by degrees, through slight modifications not only of the feet, but also of the skull, teeth, brain, etc., and by adaptations from age to age, was formed the modern horse. In like manner, by gradual changes and adaptation to surroundings, all the early generalized mammals pass into the higher specialized ones. =Quaternary Period.= The mammals of this age differ from those of the Tertiary period and from living species. Never before this period, and never since, have mammals been so large. They culminate in this period and then decline. The great mastodon, the great cave-bear, and saber-toothed tiger, the Irish elk, and gigantic sloths and armadillos lived in this age. Most interesting of all, the remains of _man_ now first appear associated with these extinct animals. Man must have been at first an apparently insignificant creature among the mighty mammals that surrounded him, and must for a time have contended more or less doubtfully with them for mastery. But as he increased in numbers and intelligence he became more and more the ruler of the brute creation; so that this age in which he rules may fittingly be spoken of as the Psychozoic Era, or =Age of Man=. [Illustration: FIG. 15.—A snake and two lizards; a, snake; c, Bipes (lizard); b, Cheirotes (lizard). From Shaler’s First Book in Geology. By courtesy of the publishers, D. C. Heath & Co.]