292 THE DENTAL SYSTEM OF THE PRIMATES [SECT. A 



Hypothesis no cusp is internal to the protocone. Professor Osborn 

 realizes this and points out that the premolar " deutero-cone " (first 

 accessory cusp) is not homologous with any molar cusp. And the 

 same conclusion applies to the remaining premolar cusps. On the 

 whole therefore, the study of the upper premolar teeth introduces 

 a further element of uncertainty in the Theory of Trituberculy as 

 originally stated. And these considerations fortify the position of 

 those who believe that the premolars give the key to the history 

 of the molar teeth ; that the molars originally evolved as the pre- 

 molars did at a later stage; and that the lack of agreement described 

 above is to be regarded as militating against the Tritubercular 

 Theory, and not (as Professor Osborn believes) as merely indicative 

 that the upper premolars have evolved differently from the molars. 



The view here taken is that the ultimate solution of the problem 

 of molar and premolar evolution will owe much to the Theory of 

 Trituberculy, but that theory will require even more modification 

 than it has yet experienced before it can claim to hold the field to 

 the exclusion of its rivals, and before all the existing objections 

 have been dismissed. 



Of the rival theories, three will be considered briefly. A. The 

 " Premolar-analogy " theory of cusp-development. B. The theory 

 of Multituberculism. C. The theory of Concrescence. 



A. The " Premolar-analogy " theory is thus the first rival of 

 the Tritubercular Theory to be enumerated. It is based largely 

 on the work of Scott (quoted above), and it postulates the origin 

 of cusps from the cingulum. In its favour are the difficulties 

 (just described) in the application of the Tritubercular Theory, 

 while the actual appearances of the post-canine series in the 

 Eocene mammals Dissacus and Mesonyx (figured by Wortman) 

 provide substantial and impressive evidence in its favour. Pro- 

 fessor Osborn admits this 1 , yet he endeavours to rebut it not 

 directly, but by adducing evidence to shew the " internal " position 

 of the primitive cusp in an early mammal (Dryolestes). The pre- 

 molar analogy theory demands a more thorough refutation than 

 this. 



B. The theory of Multituberculism or Polybuny. Some of 

 the earlier opponents of Cope's theory are ranged on the side of 



1 Evolution of Mammalian Molar Teeth, 1907, pp. 217 et seq. 



