TRITUBERCULAR EVOLUTION IN MAMMALS 47 



are several degenerate types among the Puerco and Wasatch Creodonts. 

 such' as Dissacus and J/c.s(* ////./. But there is a striking proof of the 

 superiority of the tritubercular molar in the fact that, according to our 

 present knowledge at least, the Jurassic mammals possessing aberrant or 

 degenerate molar types did not persist into the Puerco, nor did such types 

 in the Puerco persist into the Bridger. There is some doubt as to the 

 persistence of the sub-tritubercular stages ; the writer formerly considered 

 the T/t ///<//// ii* molars as triconodont ; but Mr. Lydekker has called 

 attention to the probability that the metaconid has disappeared and been 

 replaced by a heel as in the sectorial teeth of the Carnivora. The dis- 

 appearance of the degenerate types may be attributed to the general 

 principle that rapid ' specialization and loss of parts leads ultimately t<> 

 extinction, by depriving the animal of the means of adaptation to new 

 conditions or surroundings. The mechanical superiority of the trituber- 

 cular type over every other has been repeatedly demonstrated in its 

 plastic capacity of adaptation to the most extreme trenchant and crushing 

 functions. 



