332 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



I have there shown that the greater number of the 

 types of this series have derived the characters of their 

 molar teeth from the stages of the following succes- 

 sion. First a simple cone or reptilian crown, alternat- 

 ing with that of the other jaw (haplodont type). Sec- 

 ond, a cone with lateral denticles (the triconodont 

 type). Third, the denticles to the inner or outer side 

 of the crown, forming a three-sided prism, with tritu- 

 bercular apex, which alternates with that of the oppo- 

 site jaw (tritubercular type). Fourth, development of 

 a heel projecting from the posterior base of the lower 

 jaw, which meets the crown of the superior, forming a 

 tuberculosectorial inferior molar. From this stage 

 the carnivorous and sectorial dentition is derived, the 

 tritubercular type being retained. Fifth, the develop- 

 ment of a posterior inner cusp of the superior molar 

 and the elevation of the heel of the inferior molar, 

 with the loss of the anterior inner cusp. Thus the 

 molars become quadritubercular, and opposite. This 

 is the type of many of the Taxeopoda, including the 

 Quadrumana and Insectivora as well as the inferior 

 Diplarthra. The higher Taxeopoda (Hyracoidea) and 

 Diplarthra, add various complexities. Thus the tu- 

 bercles become flattened and then concave, so as to 

 form V's in the section produced by wearing ; or they 

 are joined by cross-folds, forming various patterns. In 

 the Proboscidia the latter become multiplied so as to 

 produce numerous cross-crests. 



d. Origin of tlie Carnivorous Dc7itition. 



The anterior cusplet of the triconodont crown is 

 (Fig. g8 A~), in the upper jaw, the paracone, and in the 

 lower jaw the paraconid ; and the posterior cusplet is 

 the metacone or metaconid, respectively. As the prin- 



