]()() PROF, DR. L. BOLK 



]\Iy conception of the manner in which man's denture origi- 

 nated from a phityrrhinically constructed ancestral form, obvi- 

 ates all difficulties in a very simple and logical way. Our first 

 molar is the homologue of the third milk molar of our platyrrhine 

 ancestor, our second molar is identical with the first molar of 

 the platyrrhine Primates, our third molar with the second of 

 the latter, and consequently the occasional fourth molar of man 

 and apes, is nothing else than the equivalent of the third molar 

 of the American monkeys. 



The development of a fourth molar is also, in my opinion, an 

 atavistic phenomenon. But by my theory we are not obliged 

 to descend to a problematical ancestor, who perhaps existed in 

 the Mesozoic age and possessed a greater number of molars. As 

 the construction of our denture was derived from a platyrrhine 

 form by the metamorphosis of a milk molar into a permanent 

 element, and the loss of a third molar, we need not go further 

 back in the line of descent than the last phase of human evolu- 

 tion, i.e. to a predecessor with three premolars and three molars. 

 8uch forms existed among the eocene Primates. The divergence 

 of the two groups of Primates, the Old World and the New 

 World type from a common stem took place during the eocene 

 period. The New World stem maintained the primitive struc- 

 ture of the denture, save the Hapalidae in which the third perma- 

 nent molar was reduced and eliminated. The Old World Pri- 

 mates progressed a step further, by the evolution of the third 

 milk molar into a permanent element of the denture. This 

 demonstrates that in man, as established by Zuckerkandl, the 

 distal end of the dental lamina shows a disposition to produce 

 the germ of a fourth molar, which often leads, particularly in 

 the black races, to the development of a real tooth. This is 

 easily understood. The reduction and final elimination of the 

 tooth took place in a period relatively recent. The fact that in 

 apes and especially in the Orang, the supernumerary molar is 

 met with very often, is perhaps indicated by an elongation of 

 the jaws. This circumstance, of course, favors the further de- 

 velopment of the latent germ of a fourtli mohii'. 



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