136 PROF. DR. L. BOLK 



Besides the reduction in the incisor region of the upper jaw 

 there is still another, also well-known in the molar region of both 

 jaws, viz., the reduction of our so-called wisdom teeth. This 

 process is much more important, because it is a repetition of the 

 homologous process which had taken place in an earlier period 

 of evolution. 



The absence of the third molar is by no means uncommon. 

 According to de Terra it is failing in about 12 per cent of the 

 European races. Concerning this reduction my opinion differs 

 somewhat from that commonly accepted. I believe the reduc- 

 tion of our hindmost molar to be connected with another occur- 

 ence of quite a different nature which concerns our second milk 

 molar and its successor, the second premolar. It may happen, 

 for instance, that the second milk molar is not shed, but per- 

 sists during a long period of human life, while the second pre- 

 molar never appears at all. This phenomenon is, contrary to 

 the current opinion of practitioners, a physiological and not a 

 pathological one, being the expression of a normal evolutionary 

 tendency. The grounds upon which this statement is based, 

 will be given in the next pages. 



It is very interesting that, while the reduction of our third 

 molar is a fact of general note, the persistence of our second 

 milk molar has scarcely arrested the attention of morphologists. 

 So far as I know the question as to an eventuallj^ evolutionary 

 significance of this anomaly has never been asked. It is the 

 common opinion of practitioners that a persisting milk molar is 

 to be regarded as a case of irregular or wholly impossible tooth- 

 changing caused by an impediment, without any developmental 

 importance; otherwise it has been maintained that this per- 

 sistence is only due to lack of space for the permanent tooth. 

 This statement is entirely without foundation and contradicted 

 by the fact that the mesio-distal dimension of the second milk 

 molar is larger than that of the second premolar. 



In the odontological collection of the Anatomical Museum of 

 the University of Amsterdam, there are about sixty preparations 

 with a persisting second milk molar. Some general morpho- 

 logical remarks may precede our consideration of the evolu- 



