CHAPTER XXI 



THE EVOLUTION OF TYPES 



Biological interpretations and laws — External and internal 

 factors inducing variation — Examples of pronounced diver- 

 gence within a single family — Theories of modifications in 

 dentition — Migration of teeth in the jaw — Variations and 

 mutations once more — Creative Evolution. 



Having now completed our brief survey of mammalian teeth, 

 we turn our attention to certain features of the dentition as a 

 whole which, though deeply interesting, are until now imper- 

 fectly understood. It has already been emphatically declared 

 that a truly satisfying explanation of phenomena is not given 

 by a statement of sequences nor does one answer the question 

 how by making an adequate reply to the question why. In all 

 so-called biological interpretations and in all biological laws one 

 must be on one \s guard against confusing these two simple inter- 

 rogatives how and why. AVe have seen that differences in life 

 habits, especially differences in diet, are associated with changes 

 in tooth form and we infer that the former at least in part 

 called forth the latter. But if this be a true statement of fact 

 it does not throw any light on how the changes are brought 

 about. One may wonder if the alterations in tooth form are 

 compelled by some dominating force of which Ave are ignorant 

 or if perhaps they are mere chance variations pounced upon 

 and utilized by natural selection. 



Every dental student knows that individual variations occur 

 in all the teeth yet the complete dentition of any individual, 

 broadly speaking, is true to type. We have observed that the 

 dentition may differ considerably from species to species. 



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