132 PROF. DR. L. BOLK 



be either of an atavistic or a progressive nature. Besides these 

 a third group ought to be distinguished, containing the varia- 

 tions in the structure of our denture which have nothing to do 

 with its developmental history, either in the past or in the 

 future. These anomalies are the results of a deviation in the 

 ontogenetical evolution of one of our teeth, the cause of which 

 is not of a hereditary nature. Undoubtedly this kind of varia- 

 tion occurs in the denture of man as well as of other mammals. 

 It will be readily understood that a pathological process, occur- 

 ring in the neighborhood of a tooth during its development, may 

 have an influence upon this organ, when fully formed. And it 

 is believable that by such a pathological cause, even the number 

 of teeth may be influenced. Now there are authors who are 

 inclined to absolutely deny the occurrence of atavistic varia- 

 tions in human denture, simply because there is a group of 

 anomalies which definitely lack any developmental significance. 

 I do not agree with this opinion. I admit, it is often very 

 difficult to decide at first view whether a variation belongs to 

 this latter group or not. The fact that w^e still possess an incom- 

 plete knowledge of the developmental history of our denture 

 must certainly be considered as the principal cause of this diffi- 

 culty. But just this lack of sufficient knowledge must be a 

 stimulant to penetrate more deeply into the mystery of this 

 part of our phylogenetical development, and to try to discover 

 facts until now unknown, which will enable us to build up this 

 history in a more complete manner. To accomplish this w^e must 

 first undertake as completely as possible a systematic investi- 

 gation of all kinds of variations occurring in human denture. 

 In this manner only shall we be able to discern the atavistic 

 and progressive variations from those without any phylogenetical 

 significance. However this task is not an easj' one as may be 

 illustrated by the following simple example. 



One of the most common anomalies of our denture is the 

 appearance of a supernumerary incisor in the upper jaw. Now 

 it may be taken for granted that one of our ancestral forms 

 possessed three incisors as a normal condition, and, taking this 

 fact into consideration, one must incline to the view that a 



