DISTINCTIVE CHARACTERISTICS. 43 



are accompanied by the other back cheek-teeth, the 

 molars. But apart from the fact that in several 

 groups of mammals the whales and armadilloes 

 milk teeth do not occur at all (Owen's Monophy- 

 oclonta), there are among those which do show a 

 change of teeth (Diphyodonta), so many deviations 

 and exceptions to the rule that, as Baume has 

 proved, the prevailing idea of change of teeth as 

 a succession of two distinctly different sets, can 

 scarcely hold good. The origin of the so-called 

 milk teeth can be traced back to the fact that with 

 the shortening of the facial portion of the skull, 

 the place for the incoming of the tooth-germ 

 naturally became smaller as well, and the germs, 

 in place of lying side by side, came to lie one above 

 the other. Hence those placed uppermost had to 

 be used first, before the lower ones had developed 

 and could uproot and finally expel their prede- 

 cessors by pressure. The weakening of an indi- 

 vidual milk tooth or of the whole set of milk teeth 

 will, accordingly, in general be a question of time, 

 and depend upon the delay in the development of 

 their successors. The milk teeth are at a disad- 

 vantage, owing to the inevitably hostile position 

 which their successors must sooner or later assume 

 towards them ; and they have to face a certain 



