560 THE MAMMALIA OF THE UINTA FORMATION. 



food and escaping enemies. Kowalevsky* and Riitinieyerf have treated these factors 

 very fully. Then there were the changes in the movement of the limbs with refer- 

 ence to the axis of the body ; for example, in the evolution of the horses Kowalevsky 

 has directed attention to the atrophy of muscles which rotated the fore limb, resulting 

 finally in a diiect fore and aft motion. Another factor was the transmission of the 

 weight unequally through the bones of the lower leg, as one or other of these ele- 

 ments o-rew or became reduced. Finally there were the influences which the reduc- 

 tion of the lateral digits in the artiodactyl and perissodactyl series exerted upon the 

 central digits and upon the podial elements by modifying the principal lines of "im- 

 pact and strain." ' 



The Manus. 



It has been shown that the modifications of the manus are more complex and yet 

 proceed along more definite lines than those in the pes, and in spite of the great 

 variety of influences exerted we can discover certain fixed laws of modification. 



1. Relations of growth, reduction and displacemeyit. As displacement changes 

 the vertical relations of the three rows of podial elements to each other, it follows 

 that there must be concomitant growth in some elements and reduction in others, 

 otherwise some of the lateral parts would be left without support. That the dis- 

 placement of an element, however, involves a principle distinct from mere growth 

 and is an actual shifting or "rotation" process, although not to the degree main-, 

 tained by Cope,J is shown by a number of facts. Pii-st, as to the chief factors in 

 each process. Growth is more directly brought about by vertical pressure, as seen in 

 the niaguum of the Equidte ; and displacement, by lateral strain, as seen in the shift- 

 ing of the metapodials to the ectal side of the carpus. Displacement is apparently 

 arrested in such a series as Ilyracotherium, where the fore limb acquired an orthal 

 movement and, the entire vertical pressure passing through the median toe, the 

 growth of the magnum counteracted the incipient displacement of the lunar upon the 

 unciform. We cannot explain the lateral facets of metacarpals ii and lu in the pen- 

 tadactyl Amblyjjoda and Proboscidia as due merely to growth of the elements dis- 

 placed, for the digits are strikingly equal in size ; they both extend to the ectal side, 

 and what mc. iii gains upon the unciform it apparently surrenders to mc. ii upon the 

 magnum. Finally, there is every reason to agree with Kowalevsky that the reduc- 



*Monogr. der G. Aiitlirucotbcruim, Paltuontograpliica, Bil. XXII, 1S7:\ jip. KU-T). 



t Op. cit., pp. 17-34. 



}Thc PcrissoiUiclyla. AnuTicaii Naliiralist, 1887, p. ',)8G. 



