THE DECIDUOUS DENTITION 255 



dentition to the extent that there is, with the exception of the 

 two animals above mentioned, no functional remnant of the 

 milk teeth unless it be this replaced tooth. The question which 

 immediately presents itself is the true nature of the so-called 

 deciduous tooth replaced by the last premolar. Tims has up- 

 held that it is really the first of the permanent molars which 

 becomes projected above the last premolar in consequence of a 

 progressively relative shortening of the palate in relation to 

 its breadth as the animal grows older. He maintains that a 

 similar explanation holds in the case of the Dog and the Guinea- 

 pig in which animals the last permanent premolar also is very 

 unlike its forerunner somewhat in front of which it develops. 

 Tims claims that his ingeniously presented theory covers the 

 case of certain Inseetivores in which, however, the successional 

 tooth does resemble its forerunner. It is stated that the replac- 

 ing tooth in Marsupials is not embryonically connected with 

 either of the teeth betAveen which it develops and which it suc- 

 ceeds. On this account it cannot be the homologue of either. 

 Even after reading carefully Dr. Tims' statements and con- 

 sidering the evidence he presents I cannot see that his con- 

 ception clears the situation and while his view may be correct 

 it is not entirely satisfying at present. There is nothing un- 

 usual in a successional tooth being of different appearance from 

 its actual predecessor. Examples of this have been cited from 

 many mammalian orders. We have already noted the special 

 adaptations exhibited in the milk molars both of Man and of 

 the Anthropoids. We have observed moreover that the milk 

 incisors and canines are comparatively small teeth, their suc- 

 cessors relatively large. The developing permanent successors 

 of these teeth must then lie rather lateral and posterior to 

 their predecessors in the jaw. It is only when Ave come to the 

 premolar region that we find the actually smaller teeth which 

 take up less room in the jaw succeeding the larger milk 

 molars. A change in relative position between the deciduous 

 teeth and their successors is therefore to be expected in this 

 area. The successional premolar in the Marsupials is not 



