562 THE MAMMALIA OF THE UINTA FOKMATION. 



to the ectal or ulnar side of the distal i-ow ; in the one order (Proboscidia) in which 

 the ulna is the larger bone, the proximal row is moved, if at all, to the ental or radial 



side. 



3. Relation of the size of the radius and ulna to displacement. It thus appears 

 probable that the transmission of the main body m"- e i g h t through 

 the ectal or ental side has caused displacement between 

 the first and second row of podial e 1 e m e n t s r e s po ct i vel y t o 

 the ental or ectal side. But this is clearly not the only ftictor ; if it were, 

 this displacement should be proportional in the Diplarthra to the enlargement of the 

 tibia and radius respectively ; this is certainly not the case either in the carpus or tarsus. 



In the early horse series (II>jracotJienum), in which the radius is enlarged and 

 the ulna reduced more rapidly than in any other, the ectal displacement of the lunar 

 is not so uniform nor so rapid as in the Rhinoceros series in which these bones retain 

 approximately their primitive proportions. Compare the Triplopus carpus, with the 

 scaphoid resting chiefly upon the magnum and the lunar resting wholly upon the 

 unciform, anteriorly, as a type of extreme displacement, with that of Hyracotheriam 

 or Epihippus, in which the displacement is slight. The latter presents much less 

 divergence from the taxeopod type, although the ulna is proportionately more reduced. 

 "We meet still gi'eater difficulties in the pes, where, as seen by the contrasts in the 

 members of the Equus, Hyracodon and Rhhioceros series, the extension of the astrag- 

 alus upon the cuboid is seen to be entii'ely independent of tibial enlargement. Nor 

 is this extension a resultant of weight, as proven by the diversity of the cubo- 

 astragalar articulation in Dendrohyrax and liyrax. We reach the conclusion that 

 while the di i-ec t i on of the displacement may be determined by 

 the trans m i s s i o n o f the m a i n weight through the ental or 

 ectal side, the degree of displacement is not proportionate 

 to this enlargement, but is modified by other causes. 



4. Growth and reduction take place by metatrophism, the separate elements of 

 the foot being hypertrophied or atrophied by increased oi- diminished use, resulting 

 in a transfer of nutrition, one element growing at the expense of another. We have 

 to deal with the adaptations to both vertical pressui'e and lateral strain, since the 

 motion of the limbs, of the foi'c limb especiallv, is not in a perfectly direct line. It 

 may be assumed that the parts grow most rapidly which are subjected to the maxi- 

 mum resultant of these vertical and oblique forces. As a strong confirmation of the 

 theoi'y of displacement here advanced, it may be mentioned that from a study of the 

 resistant facets the writer reached independently the same conclusions as to the mo- 

 tion of the fore feet as those drawn from the study of instantaneous photographs. 



