PROBLEMS OF HUMAN DENTITION 137 



tionary significance of this anomaly. Among my material there 

 are thirty natural preparations, the others being models. The 

 anomaly occurs much more frequently in the mandible than in 

 the upper jaw, the difTerence being so considerable that one 

 may even maintain that in the latter it is a variation of great 

 rarity. This difference between the jaws is not very surprising. 

 As a rule a given variety is more frequent in one than in both 

 jaws. However, there is a system in these differences. As to 

 the absence of the third molar practically the same difference 

 may be noted regarding the second milk molar. As mentioned 

 in the second paragraph, the third molar is absent more frequently 

 in the lower jaw than in the upper. On the other hand a disto- 

 molar — i.e., a so-called fourth molar — appears in the upper jaw 

 more frequently than in the mandible. There is, I believe, a 

 veiy remarkable relation between the frecjuency of these three 

 anomalies in the two jaws. A distomolar in man is, as I have 

 demonstrated in the first section, a reversion, being the reap- 

 pearance of the third molar of the dentition of the platyrrhine 

 monkeys; on the contrary the reduction of the third molar, and 

 also the persistence of the second milk molar, are progressive 

 variations. And so the conclusion is justified that reversion or 

 regressive variation is more frequent in the upper jaw of man, 

 whereas in the lower jaw the anomalies of a progressive nature 

 preponderate. Therefore, in the way of modifying human den- 

 tition, the lower set averages to slightly precede the upper one. 



If circumstances are favorable, the second milk molar remains 

 in the jaw until the general shedding of the teeth begins, 

 in consequence of senility. I recall the case of a colleague in 

 which the second lower milk molars fell out after the 65th year. 

 Furthermore it is very remarkable, that this milk tooth may 

 still stand firmly in the jaw, even after the first and second 

 molars have fallen out. This is demonstrated very clearly by 

 figure 22, showing the left side of a mandible of about 35 years, 

 in which nio still persists, in quite a sound state, while Mi has 

 fallen out and Mo shows a deeply penetrating defect caused by 

 caries. A case represented in figure 22 shows, at first sight, 

 that a persisting uu in man cannot be interpreted as a patho- 



