302 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



and the astragalus behind, has been in the same direc- 

 tion. It has been shown by Dr. H. Allen that just 

 before the recover of the foot, the latter is directed 

 outwards from a line parallel with the axis of the body, 

 so that the weight falls on the inner part of the sole of 

 the former. This naturally causes the bones of the 

 foot to press inwards on the heads of the metapodials, 

 so that the latter tend to grow outwards on the second 

 tarsal row. In this way were produced the facets on 

 the external side of the heads of the metapodials. 

 Thus is accounted for, on simple mechanical princi- 

 ples, the phenomenon of carpal and tarsal displace- 

 ment exhibited in its highest development, by the Dip- 

 larthra. 



It is significant that diplarthrism has not appeared 

 in mammals which possess an elastic pad of connec- 

 tive tissue on the soles, as in Unguiculata generally, 

 and especially in the Carnivora. Diplarthrism is pres- 

 ent in the camels, which have a pad, but I have shown 

 that this pad did not appear until a comparatively late 

 geologic epoch, and long after diplarthrism had be- 

 come established in the camels' ancestors, the Poebro- 

 theriidae. 



The faceting of the head of the astragalus as the 

 result of impacts, is seen on comparison of the astra- 

 gali of Phenacodus and Hyracotherium in Ungulata 

 (Figs. 33-35), and of Dissacus and Mesonyx among 

 the Creodonta. In this last genus we have the only 

 faceted astragalus among carnivorous mammals, but 

 this genus is at the same time subungulate. 



b. The Forms of Vertebral Centra. 



The mutual articulations of the vertebral column 

 are those of the centra and of the zygapophyses. Many 



