2 9 8 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



of foot preceded the diplarthrous in time. Besides the 

 alternation mentioned, it is quite general in both types 

 for the metapodial bones to possess a facet for con- 

 tact with that element of the carpus and tarsus next 

 exterior to the one to which they have their principal 

 articulation. From these facts it is evident that the 

 bones of the second carpal and tarsal rows have, in 



Fig. 77. Cervus elaphus ; A, B, C, humero-radial articulation ; A and B, 

 with the radius in position ; C, with radius twisted ; D. E, metatarsophalan- 

 geal articulation ; D, front ; E, distal view, twisted. 



the process of evolution, assumed a position interior 

 to their primitive position, with reference to the first 

 row proximad to them, and the metapodials distad to 

 them. The cause of this shifting of position is to be 

 found in the movements of the limbs in progression, 

 and especially in rapid progression (Fig. 78). 



If we observe the movements of the limbs in a 



