2 g6 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



the external part of the humeral condyles is converted 

 into a roller which is set off from the trochlear crest by 

 the abrupt contraction of its diameter ; while the cor- 

 responding part of the head of the radius projects to 

 fit it exactly. 



A probable explanation of the form of this roller 

 may be derived from a consideration of the almost 

 identical structure of the meta- 

 podio-phalangeal articulation of 

 the Artiodactyla. The internal 

 and external sides of the distal 

 metapodial condyles are not sim- 

 ilar, the external being more 

 strongly impressed than the in- 

 ternal (Fig. 77D). This is simply 

 due to the unequal pressure ex- 

 erted on the two extremities of 

 the condyle by the phalanges, 

 owing to the divergent direction 

 of the digits when serving as a 

 support. In the distal end of 

 } the humerus the same effect is 



Fig. ys.-Efbow-oim of seen > the external part of the con- 



eiaphus (red deer) dyle nearly resembling the corre- 

 sponding part of the metapodial 



bones. This is traceable to the same cause, viz. : the 

 divergent position assumed by the fore arm on the hu- 

 merus, when the weight is supported on one fore leg 

 only. This brings the line of pressure through the ex- 

 ternal part of both the head of the radius and the hu- 

 meral condyle (Fig. 77 A). That the higher ungulates 

 are "knock-elbowed" may be readily observed by 

 watching their gaits (Fig. 76). 



