THE EVOLUTION OF TYPES 279 



ding of the most anterior members of the series. In Hyrax 

 there is to be observed the beginning of this forward move- 

 ment. At first the anterior cheek teeth are more crowded to- 

 gether so that the total extent of occlusal surface of the entire 

 series becomes less as the animal grows older. Finally the 

 crushed and worn anterior teeth are shed to give room for the 

 more recently erupted permanent ones. The same process on 

 a more pronounced scale is seen in the Kangaroo, the Manatee 

 and other Mammals. The Wart Hog is a conspicuous example. 

 When the permanent teeth first erupt the dentition is almost 

 the same as that of the Pig, certain important teeth being ab- 

 sent. The formula is: 



113 3 

 I^,Cy,P|.M|. total 34. 



The third molar equals in antero-posterior length the entire 

 series of cheek teeth in front of it. As they become worn 

 down, all these anterior teeth are shed one by one, the last 

 molar alone occupying their place and the formula becoming: 



I-|, C y, My. total lfi. 



We have already seen that molars undergo striking dif- 

 ferences in appearance in advance of equally extensive changes 

 in the temporo-mandibnlar joint. Further we have noted 

 that similar parallel or convergent types of tooth form develop 

 independently in many orders. On all these grounds, there- 

 fore, we must reject not only the hypothesis that teeth are 

 modified in response to changes in the mode of mastication 

 but also the whole mechanical theory of Kinetogenesis as ap- 

 plied to tooth forms. We cannot resist the conclusion that 

 modifications in dentition are in part the expression of herit- 

 able constitutional factors concerning which Ave know nothing 

 as yet. 



Now this brings us once more to the difference between 



