PROBLEMS OF HUMAN DENTITION 101 



teeth belong together and seem to be one regidarly formed func- 

 tional entit3\ The variations of these two teeth always bear 

 the same character, in contradistinction to the second permanent 

 molar, w^hose variability is of quite another nature. This funda- 

 mental similarity between the second deciduous and first perma- 

 ment molar, easily demonstrated in each jaw bearing teeth of 

 normal structure, becomes yet more evident when the milk 

 molar is of a somewhat peculiar form. In such cases, regularly, 

 the first permanent molar repeats this peculiarity and we can 

 say that the second milk molar always follows the scheme after 

 which the first permanent molar is built up. In order to eluci- 

 date this relation I gi^'e in figures 1 to 4 some reproductions of 

 the upper jaw^s of children, with somewhat aberrant forms of 

 the crown of the second milk molar. In all cases the first per- 

 manent molar shows the same peculiarity as the milk molar. 

 The fundamental cause of this resemblance will not be discussed ; 

 for the present it is sufficient to have established this relation. 

 It is a strong argument in favor of my hypothesis and of great 

 value, since between the first and second permanent molars there 

 exists, as a rule, a dissimilarity in cusp-differentiation. 



The evolution of the third milk molar of our jjlatyrrhine 

 ancestral form, as expressed by my hypothesis, would have been 

 impossible had not the development of its changing tooth — the 

 third premolar — been suppressed. I firmly believe that this 

 event really took place and shall now advance some arguments 

 proving the justice of this opinion. 



It is a well-known fact that a tooth which was lost in an 

 earlier phase of phylogenetical e\olution, reappears some- 

 times as an indi\'idual variation of ata\-istic natiu'e. It would 

 strengthen my hypothesis if unquestionable examples could be 

 supplied of the reappearance of the lost third premolar of our 

 platyrrhine ancestor. This is not a simple matter. One might 

 collect from odontological literature the descriptions of several 

 cases in which the number of the premolars was increased to 

 three, even to four. But additional premolars do not always 

 have the same genetical significance. And to consider each case 

 of three premolars in man as an atavistic variation — a reminis- 



