DISTINCTIVE CHARACTERISTICS. 39 



as regards form as well as the mode of life. The 

 study of the teeth has acquired an entirely new 

 interest since Gegenbauer has proved that the teeth 

 of sharks and rays are perfectly identical with the 

 scale and plate formations of their outer skin ; and 

 thus in the case of these fishes we can at once 

 see the transition of the outer body-skin into the 

 mucous membrane of the mouth- cavity, also the 

 direct transition of the hard formation of the skin 

 into movable teeth. Oscar Hertwig has given us 

 a supplement to and a further application of these 

 fundamental inquiries. Accordingly, teeth have 

 originated from skin developments having been 

 used for the purpose of seizing and crushing food. 

 In the higher vertebrates we are no longer reminded 

 of this first origin of the teeth. We there find 

 the adaptation to the new activity completed, 

 the organ has long since come to stand in a closer 

 relation to the skeleton as a whole. In the case of 

 every mammal the tooth, in its development, can be 

 traced to have proceeded from the membranous 

 covering of the mouth. And while the capacious 

 mouth and gullet of fishes has been able in almost 

 every case to cover itself with teeth, the Amphibians 

 and Eeptiles show a reduction in the number of 

 teeth and of the bony supports, and finally in the 



