278 A MANUAL OF DENTAL ANATOMY. 



THE HOMOLOGIES OF THE TEETH. 



A superficial survey of the teeth of those mammals which 

 possess two sets of teeth (diphyodonts) will indicate that, 

 notwithstanding the apparent anomalies brought about by 

 adaptive modifications, a close correspondence between the 

 several teeth of different animals exists. That is to say, we 

 can generally identify incisors, premolars, and molars ; nay, 

 more, when an animal has less than the full typical number 

 of a particular class of teeth, we can ordinarily say with 

 certainty which of them it is that are absent. 



As it is impossible, or at least inconvenient, to avoid the 

 use of the term "typical" dentition, it will be well to explain 

 at the outset what is, and what is not, meant by it. 



That the great majority of biologists reject utterly the 

 " archetype " theory, by which all those resemblances which 

 really exist were referred to the influence of a sort of gene- 

 ralised " pattern " animal, according to the model of which 

 all other animals were fashioned, has already been mentioned : 

 this, then, is what is not meant by a "typical" dentition. 

 What is meant, is a form, so simplified, so little modified in 

 in any special direction, that we can conceive it to be near 

 to a common parent form whence, by progressive modification 

 in successive generations, other forms have been derived. 

 We cannot point to any mammalian dentition at present 

 known to us, and say this may have been the parent; 

 this is a typical form of mammalian dentition ; but we 

 do know many fossil forms which approximate to it far 

 more closely than do any at present in existence, and as 

 transitional forms of animals, and animals of highly gene- 

 ralised characters, are every day coming to light, we do not 

 doubt that such forms once did actually exist, and may 

 one of these days be found. Absolute proof would be ob- 

 tainable only if we could refer to its place every mammal 

 that had ever existed, and show every step in the series 



