144 PROF. DR. L. BOLK 



I estimate that a persisting milk molar occurs once in about 400 

 individuals, i.e., in 0.25 per cent. As mentioned above, accord- 

 ing to de Terra the absence of the third milk molar occurs in 

 nearly 12 per cent. This agrees with my examination of the 

 voluminous material at my disposal, in which I found an absence 

 of this tooth in about 14 per cent. Therefore it is a very minute 

 chance that both anomalies, if wholly independent of each other, 

 occur simultaneously in the same individual. Regarding the 

 occurrence of both variations in one individual, the investigation 

 of my collection has brought to light the following facts. Among 

 thirty cases of a persisting milk molar, there were fourteen, or 

 nearly one-half, in which the third molar was wanting. And 

 when we consider that the latter generally is absent in only 12 

 or 14 per cent, the fact that it occurs in 50 per cent of the jaws 

 in which the milk molar persists, is a convincing proof that there 

 exists some relation between the two. This relation is also a 

 strong argument in favor of my opinion that man is on the way 

 to lose his hindmost molar, but at the same time a process of 

 compensation is commencing at the anterior end of his molar 

 region, in consequence of which the second milk molar is sub- 

 stituted for the second premolar. 



Following above we shall consider the dentition as a whole. 

 It seems to me that there is some difference between the modes 

 of progress in the lower and upper sets. In the lower set the third 

 molar diminishes by degrees until the tooth is not developed at 

 all. There is, therefore, not a gradual diminution in the size of 

 the tooth, from a well developed strong object to a scarcely visi- 

 ble form, but the least developed third molar is still invariably 

 of a fairly notable size. Therefore in the process of the dis- 

 appearance of the third lower molar two phases are distinguished : 

 in the first phase the object decreases regularly to a lower 

 limit of existence, suddenly becoming absent. The second pre- 

 molar of the lower set shows a somewhat identical behavior, 

 this tooth either being present in a fully dcA-eloped state, or 

 wholly absent and the second milk molar substituted for it. 

 This tooth is not subject to a gradual reduction in size. There- 

 fore the character of the changes taking place in the lower jaw, 



