THE TEETH OF MAMMALS. 267 



rudimentary condition, instead of in its ordinary size and 

 functional activity. 



This is as true of teeth as of any other organs ; indeed 

 the study of odontology reveals many admirable examples 

 of the law. 



Thus the tusks of the boar or of the Sus babirussa, large 

 and peculiar though they be, are not new developments, but 

 are merely the canine teeth which in these species attain to 

 unusual dimensions. In the same way the enormous 

 straight tusk of the Narwal (see Fig. 133) is nothing more 

 than an incisor tooth of one side, the fellow to which has 

 been checked in its development ; but this is not missing, for 

 it remains throughout the life of the animal buried within 

 its socket. In the female Narwal both of the teeth, being 

 rudimentary, are permanently enclosed within the sockets, 

 and are of course not of the smallest service to the animal, 

 directly or indirectly ; furthermore, as has been shown by 

 Professor Turner, in young specimens, a second pair of 

 rudimentary aborted incisors are to be found, which in the 

 adults have disappeared. 



The modern school of biologists, rejecting this "arche- 

 type " theory as a far-fetched and unsatisfactory hypothesis, 

 refer these resemblances detected between dentitions upon the 

 whole dissimilar to one another to a more intelligible cause, 

 namely, inheritance. Assuming, as the balance of evidence 

 compels us to assume, that the many divergent forms which 

 we observe have been derived by progressive modifications 

 and differentiations from fewer ancestral forms, we shall have 

 no difficulty in seeing how, by such processes as we full well 

 know to occur, namely, the dwindling of disused organs and 

 the exaggerated development of those used in an unusual 

 degree, great differences may ultimately result. 



To illustrate what is meant by this so-called " adaptive 

 modification," this suppression of things that are not 

 needed, and increased development of those most used, we 



