104 PKOF. DR. L. BOLK 



last centuiy, there was none with a regularly formed, complete 

 fourth molar, with four or five cusps, but were several cases with 

 a rudimentary fourth molar. Among the African negro race, 

 however, and among apes this supernumerary element is of a 

 more frequent occurrence, and may reach a degree of full develop- 

 ment, regularly elongating the row of the three normal molars. 

 From the investigations of Zuckerkandl- we learn the very 

 important fact that the dental lamina in man nearly always 

 exhibits the tendency to form a fourth molar. 



The appearance of a fourth molar in man and apes (in the 

 following I shall confine myself to these two groups of Primates), 

 is a wholly incomprehensible fact. Generally one is inclined to 

 declare it an atavistic phenomenon, this variation finding its 

 inevitable explanation in inheritance from an ancestral stock 

 which normally possessed the larger number of molars. If this 

 opinion should be the right one, the human denture must neces- 

 sarily have evolved from an ancestral form with four molars. 

 But the difficulty arises that among the representatives of the 

 eocene Primate, as yet known, there is none with such an in- 

 creased number of molars, all possessing only three. Zuckerkandl 

 has emphasized this difficulty, showing the improbability that 

 the fourth molar is an atavistic element. He inclines to the 

 opinion that this extension of our dental arch is of a progressive 

 character. Selenka, after having examined some hundreds of 

 Orang skulls, found a supernumerary molar in nearly 20 per 

 cent of the skulls. This author, also, does not believe that 

 the variation may be judged as an atavistic one, because an 

 ancestor with four molars is entirely unknown. He agrees with 

 the \-iew of Zuckerkandl that the development of a fourth molar 

 in man and apes is a progressive variation, such a tooth not l)eing 

 a reappearance of an element lost at an earlier date of Primate 

 evolution, but having evolved as an entirely new element. This 

 opinion indicates that the distal end of the denture of the higher 

 Primates is in a progressive state of development. I do not 

 concur in that opinion. If the \'ariation were only met with in the 

 Orang, the possibility that Selenka' s opinon may be correct, must 



2 Sitzungsber. Kais. Akad. d. Wiss., Wion, Bd. 100. 1S91. 



