278 MAMMALIAN DENTITION 



occlusal surface. At present one cannot conceive of the in- 

 heritance of an acquired* character: that an individual should 

 transmit to its progeny dental features which it exhibits as the 

 result of wear is utterly at variance with all known principles 

 of heredity. The tooth, it must be remembered, is pre-formed 

 in the jaw and its enamel-covered crown cannot grow or 

 change actively in response to conditions of occlusion. Again 

 the tooth is not firmly fixed in the bone: it is capable of con- 

 siderable change in position. That teeth do move is obvious 

 from the wearing which occurs upon the proximal and distal 

 aspects of the molars. The exceedingly loose manner in which 

 the incisors of the Cow are imbedded in the mandible has pre- 

 viously been noted. The alteration in level of a tooth in con- 

 sequence of damage to or loss of its opponents is well known. 

 The raising and loosening of a molar, the occlusal face of 

 which is worn down or carious is especially manifest immediately 

 after it is crowned by the dentist. At this time the tooth can 

 be "wobbled" between the fingers but in the course of a few 

 weeks during which the patient will notice the resilience of 

 the tooth in its bed of softer tissue before the bony socket 

 again grasps its roots closely, becomes firmly imbedded once 

 again. 



The change in position of teeth in the jaws in some animals 

 is very well marked. We have already stated that replace- 

 ment of teeth in all Mammals except the Proboscidea is verti- 

 cal ; in these animals alone it is oblique. We must now modify 

 this statement somewhat. As a molar of the Elephant erupts, 

 it moves forward in the jaw and hence changes its position 

 with respect to the bone. It is not immovably fixed in the jaw 

 as if it were imbedded in a matrix of Portland cement. When 

 we come to examine more minutely the jaws of Mammals in 

 which replacement is vertical we find many instances of the 

 same forward movement of teeth even resulting in the shed- 



*The term is a poor one. Archdall Reid pointed out some years ago that all 

 characters can be said to be the result of interaction between internal potentiality and 

 external stimulus. Nevertheless the precise sense in which the term is used in the 

 present instance is sufficiently explained by the latter part of the sentence. 



