264 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



" prophetic echo " of future dentition in the descend- 

 ants of the same group. Impartial observation 

 discloses that neither of these hypotheses is com- 

 pletely justified. The milk molars of the Ungu- 

 lates and of the majority of other orders are dis- 

 tinguished from the second set by their longer 

 form, their lower crown, their thinner enamel, 

 and their more complicated structural details. 

 According to Stehlin, the milk molars of the Im- 

 paridigitse are, on certain points, more conservative, 

 on others more progressive than the permanent 

 molars. On the one hand, they preserve the low- 

 crowned or brachyodont type of the early types of 

 this group in opposition to the high or hypseledont 

 type of its more modern representatives, and, from 

 this fact, we may be allowed to see in them a kind 

 of ancestral reminiscence. On the other hand, how- 

 ever, the complication of the folds of the enamel, the 

 development of ridges, of hooks, of supernumerary 

 tubercules, etc., are more connected with a pro- 

 gressive evolution, which is sometimes shown in 

 the definitive dentition of the descendants, and 

 must then, perhaps, be regarded as a prophetic 

 echo. Yet in many cases, according to Stehlin, 

 these complications of structure may more simply 

 be attributed to the slight density of the enamel. 



