r. More work being thus thrown on the remaining
four, they were strengthened by interlocking with the wrist (or ankle)
bones above them, as just mentioned; and also by being brought closer
together.

[Illustration: FIG. 82.--Bones of the foot of four different forms
of the artiodactyl type, showing gradual reduction of the number of
digits, coupled with a greater consolidation of the bones above the
digits. The series reads from right to left. Drawn from nature
(_Brit. Mus._).]

The changes which followed I will render in the words of Professor
Marsh.

Two kinds of reduction began. One leading to the existing
perissodactyl foot, and the other, apparently later, resulting in
the artiodactyl type. In the former the axis of the foot remained
in the middle of the third digit, as in the pentadactyl foot. [See
Fig. 81.] In the latter, it shifted to the outer side of this
digit, or between the third and fourth toe. [See Fig. 82.]

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