40 THE MAMMALIA. 



mammals we find only the actual jaw-bones fur- 

 nished with teeth. 



By this a concentration of the tooth-material 

 has been accomplished, and connected with it we find 

 that concentration of force by which the mammal 

 more readily and surely overpowers its living prey, 

 and prepares it for use in the intestines, by mastica- 

 tion. In the dentition of the mammal we have not 

 a retrogression but an advance in the organisation, 

 and a further diminution in two directions may be 

 expected. What has taken place with numerous 

 fishes has also happened in the case of some 

 of the Mammalia : under certain conditions of 

 nutrition teeth have become useless and have dis- 

 appeared ; and, secondly, the fuller number of 

 teeth of the geologically older species has given 

 place to a dentition less numerous but more 

 specialised in form and action, and therefore more 

 advantageous. As an instance of the one direction, 

 we may take the jaw of a Euminant, which shows 

 a want of the upper incisors ; of the other direction 

 the jaw of the Cat species. 



In order to understand the manifold forms of 

 teeth, we must have some knowledge of the develop- 

 ment of the various substances that form the tooth, 

 the origin of the hard bright enamel (ebur), of the 



