374 MECHANICAL EVOLUTIOK 



proximal ridges of the phalanges are very weak, and the concavities 

 in the extremity of the radius can not be called trochlear, as they 

 are adaptations to the carpal hones. 



I. The reduction in the number of toes is supposed to be due 

 to the elongation of those which slightly exceeded the others in 

 length, in consequence of the greater number of strains and im- 

 pacts received by them in rapid progression, and the complement- 

 ary loss of material available for the growth of the smaller ones. 

 This is rendered probable from the fact that the types with re- 

 duced digits are dwellers on dry land in both orders, and those 

 that have more numerous digits are inhabitants of swamps and 

 mud. In geological history it is supposed that the Perissodactyles 

 (Figs. 72, 73) originated from the AmUypoda, or primitive Ungu- 

 lata (Figs. 69, 70), which first assumed terrestrial habits, while 

 the Artiodadyla (Fig. 74), originating from the same order, long 

 continued as mud dwellers ; as witness the hippopotami and hogs 

 of to-day. The mechanical effect of walking in the mud is 

 to spread the toes equally on opposite sides of the middle line. 

 This would encourage the equal development of the digits on each 

 side of the middle line, as in the cloven-footed types. In progres- 

 sion on hard ground, the longest toe (the third) will receive the 

 greatest amount of shock from contact with the earth. There is 

 every reason to believe that shocks, if not excessive, encourage 

 growth in the direction of the force ap]")lied. This is strongly 

 suggested by the relations between the length of the legs and the 

 rate of speed of animals ; and the lengths of the teeth and their 

 long-continued use. Certain it is that the lengths of the bones of 

 the feet of the Ungulate orders have a direct relation to the dry- 

 ness of the ground they inhabit, and the possibility of speed which 

 their habitat permits them, or necessarily imposes on them. 



II. The hinge between the first and second series of tarsal 

 bones in the Artiodactyla may be accounted for by reference to 

 the habits which are supposed to have caused the cloven-footed 

 character. Observation on an animal of this order walking in 

 mud, shows that there is a great strain antero-posteriorly trans- 

 verse to the long axis of the foot, which would readily cause a 

 gradual loosening of an articulation like that connecting the two 

 series of tarsals in the extinct Ambhjpoda. Any one who has ex- 

 amined this part of Corypliodon will see that a little additional 

 mobility at this point would soon resemble the second tarsal joint 

 of the hogs. In the case of animals which progress on hard 



