THE TEETH 441 



drawn out into columns, while the neck is deep in the socket, and the 

 fangs small, the tooth belongs to the hypsodont 1 type. 



Every group of mammals is characterized by a special kind of 

 dentition ; hence the teeth are used as a means of classification, 

 especially since from their hard and resistant qualities they are those 

 parts of extinct animals which are most likely to be preserved in fossil 

 form. It is by these fossil forms that the naturalist is able to trace 

 the descent of existing animals, and their present relations to one 

 another ; some mammals which appear to be widely separated in any 

 system of classification based only on the living forms are linked 

 together by a long chain of fossil species so gradually that at no one 

 point can they be said to be distinct. The study of the evolution of 

 the different groups of existing animals is therefore largely dependent 

 upon a study of the evolution of the different forms of teeth. We 

 may pursue this study by at least three different methods : 



(1) By comparing the fossil with the existing forms and tracing 

 the development, through long periods of time, of cusp after cusp, 

 noting their gradual change, or differentiation, in various directions, 

 from the simple to the complex type. 



(2) By investigating the complete embryological development of 

 the teeth of one species at a time, in order to trace the order of 

 appearance and the relative importance of the different cusps, and 

 thereby learn to what more simple types the finished form is related, 

 or through what grades it has passed to reach its present apparently 

 permanent structure. 



(3) By examining the adult dentition of very many individuals 

 of one species and observing all the variations of each tooth in order 

 to learn their meanings, namely, whether they represent reversions 

 to a more simple stage of development through which the animal has 

 passed, or are the result of mechanical causes still operating to produce 

 change in tooth structure. 



All three methods of study should be employed in the solution of 

 the problem ; the results gained by one alone of them must necessarily 

 be incomplete ; the facts discovered by all of them must, of course, 

 within certain limits agree. 



Many investigations have been made in the evolution of tooth 

 forms through palseontological and embryological study, but compara- 



1 From (Gr.) hypsos, height, and odoits. 



