THE TEETH OF MAMMALS. 



of modifications by -which the ultimate divergence of den- 

 tition was effected. But evidence far short of absolute 

 ocular demonstration serves to satisfy us on most points, 

 and there is sufficient evidence available to enable us to 

 say with some confidence that our "typical" or parent 

 mammalian dentition was, so far as the numbers of the 

 several kinds of teeth go, 



i ' c - prm -. m ' -= 44. 



And when there are less than forty-four teeth, as has been 

 already mentioned, we can in most cases say which they 

 are that are absent. 



Thus, taking a certain bear and a baboon (each having 

 two premolars only on each side), we are able to decide, by 

 comparison with allied creatures, that, in the case of the 

 bear, it is the second and third premolars which are wanting, 

 the first and fourth remaining ; while in the baboon it is the 

 first and second which are wanting, the third and fourth 

 being present. By hoinology we mean such correspondence 

 as is above indicated ; a correspondence which might almost 

 be expressed as a relationship by descent. 



Homology, then, is almost equivalent to identity of origin, 

 or, at all events, to similarity of origin ; but it by no means 

 necessarily involves identity or even similarity in the pur- 

 pose to which a thing is ultimately applied a fact which will 

 be further illustrated in speaking of canine teeth. 



The homologies of the teeth may be treated under two 

 heads : the one, the. homologies of the teeth in their relation 

 to other parts of the body, and the other, their more special 

 homologies, or their relation to one another. 



The relation of the teeth to the skin, which we express by 

 calling them " dermal appendages," as well as the epidermic 

 nature of the enamel, and the dermic nature of the dentine, 

 have been sufficiently discussed at former pages, so that 



