282 THE DENTAL SYSTEM OF THE PRIMATES [SECT. A 



These considerations have carried us away from the dentition 

 of the Primates, yet on returning to consider their dental formula, 

 we find that they retain the four great orders of teeth found in 

 the Triassic reptiles. The differentiation of those orders (incisors, 

 etc.) must have taken place at an earlier period, and must be 

 ascribed partly to mechanical influences, such as the size and 

 arrangement of the muscles acting on the mandible 1 . Not less 

 effective however were physiological influences, with which modifi- 

 cations of diet are connected. Alteration in the size and form of 

 the jaw was determined by these and to some extent also by the 

 increasing size of the brain. Of such factors, some were more 

 active than others at particular periods of the evolutionary history. 

 In the Primates, that factor which we denote generally by brain- 

 growth persisted and at length predominated. Coincidently the 

 jaws became reduced in antero-posterior extent, and with this 

 reduction the suppression of teeth was brought about so that the 

 dental formula was at length established as we now know it. 



VI. Theories of cusp-development and the origin and fate 



of cusps. 



In the incisor and canine series the change from a (? original) 

 " haplodont " form has been comparatively slight, though whether 

 the result has been determined by natural selection, or " use- 

 inheritance," would lead to a discussion upon which we cannot 

 enter here. 



But in the premolar and molar series the difficulties are greater: 

 for the problem is to determine the origin of complex many- 

 cusped teeth with multiple roots, from conical teeth with single 

 roots. 



The explanations which have been offered may be grouped 

 under three chief heads : viz. the hypotheses of Trituberculy, of 

 the Polybunodont origin of the teeth, and the hypothesis advanced 

 by Marett Tims, with which the Concrescence theory will be 

 considered. 



It must be at once stated that no one hypothesis covers all 



1 Cf. Gregory, Bulletins of the American Museum of Natural History, Vol. xxvu. 

 p. 111. 



