258 MAMMALIAN DENTITION" 



were coined by Gregory to take the place of the words morpho- 

 logical and adaptive respectively. A paleotelic or morphological 

 feature is primitive and unaffected or but slightly modified by 

 external conditions. Examples are the auditory ossicles, the 

 foramina of the skull, certain features of the base of the skull, 

 the carpal and tarsal bones and the urogenital system in gen- 

 eral. In all of these there is but restricted variation except in 

 certain special types. On the other hand some features such as 

 the facial skeleton, the fingers and toes, the color of the skin 

 and the character of the hair vary through a considerable range 

 in consequence of their exposure to environmental conditions. 

 Such characters are caenotelic and not primitive, but greatly 

 modified. Should, however, a paleotelic feature become ex- 

 posed to environmental influence as in the case of the auditory 

 ossicles in aquatic Mammalia it becomes greatly modified and 

 thus caenotelic in nature. Now upon this basis the crowns of 

 the teeth, being as a rule much more influenced by external 

 conditions than the roots are likely to show greater changes. 

 In other words their paleotelic characters are more likely to 

 become obscured. Infinite are the variations of cuspidation 

 but strikingly few and slight are the modifications of the roots 

 of teeth. 



No biological "Law" is really an explanation. It is in es- 

 sence a description of sequences. We are not explaining why 

 changes occur in structure Avhen we state more or less vaguely 

 that they occur in part at least in response to environmental 

 influence: we are but correlating facts. When we state that 

 certain changes in structure appear apparently as the result 

 of modification in external conditions we are giving an instance 

 of what may be termed the Law of Environmental Influence. 

 Returning then to the subject of teeth it may be stated that the 

 paleotelic characters of the crowns have become largely ob- 

 scured whereas those of the roots show greater tendency to 

 persist. For this reason the tooth crown has received consid- 

 erable attention in this volume and the root but little. 



One feature wherein the mammalian dentition differs from 



