298 THE DENTAL SYSTEM OF THE PRIMATES [SECT. A 



ancestry of Man is traceable through Lemuroids and primitive 

 Insectivora 1 backwards to the Mesozoic Placentalia 2 , and Permian 

 forms of which the Cynodont fossils fortunately enable us to gain 

 an idea. In the later, i.e. the " post-Lemuroid," stages, forms 

 comparable to those of the Cebidae, Cercopithecidae and Simiidae 

 made their appearance. In regard to the two former families, 

 the evidence of the dentition is not so clear as in the case of 

 the latter, more especially the Chimpanzee. 



The problem of the causation of different dental forms in various 

 regions of one and the same jaw has been mentioned already. 

 The exponents of the Tritubercular theory have always laid stress 

 on " mechanical conditions " as accessory to functional adaptation 

 (cf. Cope, Primary Factors, pp. 319 et seq.). Certainly the 

 mechanical conditions will differ widely in different parts of the 

 jaw ; it is suggested that whereas the primary need was originally 

 prehension, and that the crocodilian or cetacean types of dentition 

 provide good examples of this ; it is urged that with the acqui- 

 sition and development of the habit of masticating food (whether 

 like a primitive insectivore in crushing the scaly coats of its prey, 

 or a primitive ungulate in reducing vegetable matter to pulp, or 

 again in the habit of shearing flesh from bone as shewn by Car- 

 nivora), specialization and adaptation were associated in that 

 section of the dental series at which power could be most advan- 

 tageously applied ; it is submitted that the comparative simplicity 

 of the anterior teeth is contingent upon the retention of the 

 prehensile function by this portion of the dental arcade. 



And finally, when the Hominidae are considered, the inter- 

 pretation thus advanced is found to apply to them just as to 

 other Eutherian mammals, with the special qualifications intro- 

 duced by the factor to which reference has been so often made, 

 viz. the peculiar degree of reduction of the human jaws and the 

 concomitant limitation of their functions. 



1 Cf. Matthew, Bulletin of the American Museum of Natural History, Vol. xxxi. 

 pp. 307—314. 



- In a private communication (March 28, 1914) Professor Gregory inclines to 

 the view " that the ancestral Placental mammals were not minute exclusively in- 

 sectivorous forms, but small sub-carnivorous animals, with stout zygomata, sharp 

 wedge-like upper molars, and with a talonid basin in the lower molars." 



