1 88 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 

 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 Peris- 

 sodactyls, 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 perisso- 

 dactyl 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 



