4 EVOLUTION OF MAMMALIAN MOLAR TEETH 



This main principle of the tritubercular theory has been widely but 

 by no means universally accepted. Able comparative anatomists, chiefly 

 Ameghino, Rose, Forsyth-Major, have urged against it that a molar 

 tooth with a number of tubercles is still more primitive. According to 

 this rival view, commonly known as the polybuny theory (TroXvs, 

 many, ftowos, a hillock), the tritubercular type, even in the Cretaceous 

 period, was the result of a secondary suppression of some of the numerous 

 original tubercles. This opposing view deserves fair consideration and 

 a clear statement (see p. 205). 



This first generalization as to primitive trituberculy in the later 

 Mesozoic and Tertiary periods must be clearly distinguished from the 

 following second generalization as to the earlier Mesozoic origin of 

 trituberculy. 



II. SECOND PRINCIPLE. THE ORIGIN OF THE TRITUBERCULAR TYPE 

 FROM THE SINGLE REPTILIAN CONE. 



Professor Cope's active and inquiring mind did not, however, stop 

 at this point. He asked himself, " If the oldest Tertiary mammals 

 exhibited a common tritubercular type, what do the mammals which 

 lived during the Mesozoic period, or age of reptiles, show as to the 

 origin of the tritubercular type ? " He had neither the time nor the 

 material to enter thoroughly into this inquiry ; but he looked into 

 Owen's memoir on Mesozoic mammals far enough to clearly perceive 

 that the tritubercular type developed during the long Mesozoic period, 

 from an ancestral reptilian type of tooth consisting only of a single 

 cone or cusp. Therefore he made a second generalization : that the 

 tritubercular type sprang from a single conical type by the addition of lateral 

 denticles. " I have already shown," he says, 1 " that the greater number 

 of the types of this series have derived the characters of their molar 

 teeth from the stages of the following succession. First, a simple cone 

 or reptilian crown, alternating with that of the other jaw. Second, 

 a cone with lateral denticles. Third, the denticles to the inner side of 

 the crown forming a three- sided prism, with tritubercular apex, which 

 alternates with that of the opposite jaw." 



III. THIRD PRINCIPLE. CUSP ADDITION OR DIFFERENTIATION. 



In the second generalization is, however, involved a third, which has 

 also been the subject of wide difference of opinion, namely, the successive 

 addition of new denticles, cuspules or smaller cones on the sides of the 

 original reptilian cone ; this may be simply known as the cusp addition 



1 Oriyin of the Fittest, p. 347. 



