188 Darwin, and after Darwin. 
first to leave the ground, to dwindle, and finally to 
disappear. More work being thus thrown on the 
remaining four, they were strengthened by inter- 
locking with the wrist (or ankle) bones above them, as 
just mentioned; and also by being brought closer 
together. 
The changes which followed I will render in the 
words of Professor Marsh. 
Two kinds of reduction began. One leading to the existing 
perissodacty] foot, and the othcr, apparentiy 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 Peris- 
sodactyls, as it is the most specialized known in the Ungulates 
[Fig. 81 D]. 
In the artiodacty] 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 perisso- 
dacty! 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 
