In the further reduction of the perissodactyl foot, the fifth digit, being shorter than the remaining three, next left the ground, and gradually disappeared. [Fig. 81 B.] Of the three remaining toes, the middle or axial one was the longest, and retaining its supremacy as greater strength and speed were required, finally assumed the chief support of the foot [Fig. 81 C], while the outer digits left the ground, ceased to be of use, and were lost, except as splint-bones [Fig. 81 D]. The feet of the existing horse shows the best example of this reduction in the Perissodactyls, as it is the most specialized known in the Ungulates [Fig. 81 D]. In the artiodactyl foot, the reduction resulted in the gradual diminution of the two outer of the four remaining toes, the third and fourth doing all the work, and thus increasing in size and power. The fifth digit, for the same reasons as in the perissodactyl foot, first left the ground and became smaller. Next, the second soon followed, and these two gradually ceased to be functional, [and eventually disappeared altogether, as shown in the accompanying drawing of the feet of still existing animals, Fig. 82 B, C, D]. The limb of the modern race-horse is a nearly perfect piece of machinery, especially adapted to great speed on dry, level ground. The limb of an antelope, or deer, is likewise well fitted for rapid motion on a plain, but the foot itself is adapted to rough mountain work as well, and it is to this advantage, in part, that the Artiodactyls owe their present supremacy. The plantigrade pentadactyl foot of the primitive Ungulate--and even the perissodactyl foot that succeeded it--both belong to the past humid period of the world's history. As the surface of the earth slowly dried up, in the gradual desiccation still in progress, new types of feet became a necessity, and the horse, antelope, and camel were gradually developed, to meet the altered conditions. The best instance of such progressive modifications in the case of perissodactyl feet is furnished by the fossil pedigree of the existing horse, because here, within the limits of the same continuous family line, we have presented the entire series of modifications.