KINE TOGENESIS. 



373 



duced by vermiform movements which utilize points of 

 resistance on the earth as aids to progression, while the 

 plane articulation has probably resulted from the per- 

 sistence of the fixed relation which is appropriate to a 

 body which should be relieved by the legs of all share 

 in movements necessary to progression. That this 

 position is correct is sustained by the fact that the cer- 

 vical vertebrae of various reptiles and mammals which 

 have plane dorsal vertebrae have the ball-and-socket 

 structure. This is probably due to the constant flex- 

 ures to which that part of the column has been sub- 

 jected, as compared with the fixity of the dorsal re- 

 gion. 



Owing to the comparatively advanced state of our 

 knowledge of the phylogeny of the Mammalia (Chap- 

 ter II.), this class furnishes especial opportunities for 

 the study of kinetogenetic evolution. But our knowl- 

 edge is not yet sufficiently complete to enable us to 

 account on mechanical grounds for the origin of all 

 the characters which distinguish all its subdivisions. 

 This being the case, I have not presented the subject 

 in taxonomic order, but have contented myself with 

 offering it in the order of evidential value. I have first 

 described certain cases where the action of kinetogen- 

 esis is self-evident. This has been followed by the 

 presentation of cases where the evidence amounts to a 

 high degree of probability. 



Referring now to the table of definitions of the or- 

 ders of Mammalia on pages 127-128, I will go over the 

 characters seriatim, and show how far our knowledge 

 warrants us in giving a kinetogenetic explanation of « 

 their origin. No mechanical cause can be at present 

 assigned for the loss of the coracoid and episternal 

 bones in the Eutheria. In the Eutheria the presence 



