ORIGIN OF FOOT STRUCTURES OF THE UNGULATES. 371 



I suspect that the production of a ginglymus in the middle of 

 the tarsus has been due to the use of the posterior limb in soft 

 swampy ground. In the absence of this condition, as in a life on 

 harder ground than swamp, no ginglymus would be formed. 

 The action of an ungulate in walking through deep mud is very 

 suggestive. The posterior foot is bent on the leg, and the antero- 

 posterior strain of the weight or propulsive force is transverse to 

 its long axis. In progression on dry land, the impact is in the 

 direction of the length or axis of the foot. The obvious effect of 

 a cross strain is to produce by degrees greater and greater mobility 

 of some articulation. The one which has yielded is that between 

 the two tarsal rows. Another effect of walking in swampy ground 

 is to spread the digits apart. As the first digit 

 of both feet is always of reduced size, there are 

 practically but four digits to be considered. 

 The weight falling nearly medially on these, 

 would tend to spread them equally, two on 

 each side. Thus the same cause may have been 

 effective in producing both the artiodactyle 

 structures. The perissodactyle structure, so 

 soon as the lateral digits are much reduced, 

 ceases to be favorable for progression in soft 

 ground, owing to the liability of the lateral 

 digits to injury, in following the principal one 

 into the yielding material, filled with sticks 

 and other hard debris. 



The lowest existing forms of the Artiodac- 

 tyla, the Omnivora, are universally swamp 

 lovers and livers. So, we are told, are the lower 

 existing Perissodactyla, the tapirs and rhinoce- 

 roses. The higher types of both orders are ^"^^ ^'*^ ^^""^ "^ '■'^- 

 dwellers on plains and in forests. We do not ^.f^^^. ^eidy, from 

 know the habits of the Eocene Perissodactyla, Colorado, three fiftim 

 but I doubt their having inhabited muddy natural size. From 

 ground to the same extent as the hogs and hip- pj^^^-kT^ ^cror , iv, 

 popotami, the lowest of the Artiodadyla, Now, 

 in progression on dry land, any pre-existent inequality in the 

 length of the digits would tend to become exaggerated. Such an 

 inequality exists in the Amblypoda, the third digit being a little 

 the longer. In rapid movement on hard ground the longest toe 

 receives the greatest part of the impact, even if its excess of 



Fig. 74.— Left fore 



