PKOBLEM8 OF HUMAN DENTITION 143 



tary factors, was of a progressive nature with regard to the 

 development of his dentition, his dental lamina produced a 

 second milk molar able to function during a period of life much 

 longer than normal and to accomplish this the appearance of the 

 germ of a second jiremolar was suppressed. 



Perhaps, among the readers of this article, there is some one 

 whose opinion differs as to the relation between the absence 

 of Po and the persisting of m2, and who inclines to a more 

 mechanical explanation. From this standpoint the absence of 

 the germ of P2, should be considered the primary event and the 

 persisting of nio only the necessary consequence of it, because a 

 succeeding tooth, pushing out the milk tooth, is lacking. I 

 cannot concur with this o])inion. I think the persisting of nio 

 and the absence of P2 are due to a common biological, aetiologi- 

 cal factor, this factor being the tendency of our dentition to 

 substitute the second milk molar for the second premolar. This 

 factor exerts a simultaneous influence on the dental germ of 

 the second milk molar and that of the second premolar, stimu- 

 lating the first and suppressing the development of the latter. 

 From this standpoint the persistence of the milk molar is not 

 a secondary consequence, but an occurrence as primaiy as the 

 absence of the second premolar. But I admit that the relation- 

 ship between the two occurrences may be viewed also from the 

 mere mechanical standi)oint. But this is not my view. 



Figures 24 and 25 show a point of difference. In the first 

 figure the individual possessed, with his persisting milk molar, 

 the three normal molars also. And this person, during his 

 adult life, was actually in possession of four molars in the lower 

 jaw. The individual from whom the photograph in figure 25 

 was taken, was on a higher level of evolution, for in this case 

 there occur two progressive variations, viz. : persisting of the 

 second milk molar and absence of the third molar. 



Although exceptional in people of today, such cases will be 

 the normal condition in the human race of future ages. 



It may be remarked, that the coincidence of the two varia- 

 tions in the case of figure 25 is a merely casual one. That 

 this is not the case can be demonstrated in the following; manner. 



