338 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



root of the tooth. It appears that the posterior root 

 has been extended backwards, so as to occupy a posi- 

 tion below the middle of the superior molar, while the 

 tritubercular crown has been confined to the space be- 

 tween the crowns of the superior molars. This would 

 follow of necessity from the alternating action of the 

 crowns of the opposite series, in connection with a 

 general increase in size of the teeth. In the opening of 



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Fig. loi. — Series of inferior molar crowns representing the transition from 

 the simple (haplodont) to the quadritubercular. From Osborn. 



the jaws in a Creodont, the elevated portion of the in- 

 ferior crown shears by its posterior face against the 

 anterior face of the superior molar, thus restraining its 

 extension posteriorly. The stimulus of use, however, 

 develops a low extension posteriorly, or a heel, which 

 covers the posterior root, and opposes in mastication 

 the internal extremity or tubercle of the crown of the 

 superior molar above it. Thus a molar element in 



