THE EVOLUTION OF TYPES 277 



the various attempts made to explain how changes in dentition 

 have been brought about. Ryder, Cope and Osborn have suc- 

 cessively attacked this problem but the real significance of an 

 internal factor in the organism itself dawned only upon the last 

 mentioned. 



Ryder's view was that modifications in type of mastication 

 result in changes in tooth form. He pointed out that there are 

 different degrees of lateral movement in mastication in Ungu- 

 lates; none at all in the Pig, slight movement only in the 

 Wart Hog, movement progressively greater in the Tapir, Deer, 

 Camel and Ox in this order. Ryder also pointed out that in 

 Ruminants the food is masticated on the side of the mouth on 

 which the lower molars cross the uppers from within outward 

 (ental form), whereas in Horses and Rhinoceroses it is upon the 

 side on which the lowers cross the uppers from without inward 

 (ectal form). In the Rodentia the movement of the mandible 

 on the upper jaw is anteroposterior (proal form). Ryder be- 

 lieved that the selenodont forms of Ungulate teeth are mechan- 

 ically produced as the result of the direction and extent of 

 excursion of the mandible in mastication. 



Cope elaborated Ryder's interpretation and deduced certain 

 mechanical causes as operating to bring about dental modifica- 

 tions. The operation of these supposed mechanical causes with 

 their resultant effect upon the organism, Cope termed Kineto- 

 genesis. We have already seen that modifications of tooth 

 forms are apt to precede changes in the type of the temporo- 

 mandibular articulation so that we cannot accept Ryder's 

 interpretation. We shall see immediately that it is not possible 

 to agree with Cope's theory of Kinetogenesis either. 



In considering Cope's theory Osborn 's attention was drawn 

 to the fact that those areas of tooth crowns most subject to 

 wear are precisely those which in subsequent geological types 

 develop new cusps. Upon a purely mechanical basis this can 

 only mean an increasing resistance leading to actual growth of 

 tissue, the exact opposite of Cope's view according to which 

 there is diminished resistance leading to plastic moulding of the 



