268 MAMMALIAN DENTITION 



Some for example have brachyodont molars, others hypsodont, 

 yet each species- though living side by side with the other breeds 

 true. 



"We have noted the fact that different genera within a single 

 family show strikingly how completely dissimilar one dentition 

 may be from another. We have also seen that traced back to 

 their simplest or earliest representatives the dentitions of most 

 if not all mammalian orders indicate derivation from a com- 

 mon forerunner. In mentioning one external factor Avhieh 

 assists in bringing about modifications in the dentition, namely 

 diet, Ave have merely opened the question of the origin of modifi- 

 cations. We have made no suggestion of a possible force in- 

 herent within the organism tending to bring about alterations. 

 To the problem, unsolved as yet, of Iioav changes are produced 

 we have devoted no attention whatever. One cannot leave the 

 subject of teeth without at least glancing at these enigmas of 

 how and why. Many problems of modern biology have found 

 or will find their solution in experimental investigation, a field 

 closed to us in the study of teeth. So far as dentition is con- 

 cerned we can but observe the imperfect record of Nature's 

 experiments carried on over vast periods of geological time. 



Early in our inquiry it was noted that the conformation and 

 articulation of the jaws underwent great modification in the 

 transition from Reptile to Mammal and at the same time the 

 homodont dentition gaA^e place to the heterodont. It has been 

 suggested that the cheek teeth being close to the joint and to 

 areas of direct muscular pull are best situated for crushing or 

 shearing whereas the incisors and canines, further removed, 

 operate through a larger arc with greater velocity and are 

 hence in a position to execute cutting and piercing functions. 

 It is true that in many animals the transition betAveen the sim- 

 ple cutting incisiform and the larger crushing molariform 

 type is gradual and progressive through canine and premolars 

 but in the Horse and the Hyrax, to mention only tAvo examples, 

 the transition is abrupt. It cannot be doubted that such sud- 

 den transition is secondary in view of its non-occurrence in an- 



