266 A MANUAL OF DENTAL ANATOMY. 



in many particulars ; or, again, the gap between the Monkeys and 

 the Insectivora is bridged over by the Lemurs, which are inter- 

 mediate forms. 



We do not yet know enough of extinct Mammalia to feel quite 

 sure of the true line of affinities between all the orders, but the 

 foregoing table serves to give a more true idea of our present know- 

 ledge than any arrangement in linear series can convey. There is 

 no animal to which we can point and say that we know its whole 

 line of descent ; but the ancestry of some of the Ungulata has been 

 greatly elucidated of late years, and the chain of progressive modi- 

 fication by which so highly specialised a form as the Horse has been 

 arrived at, starting from a very much more generalised form, is now 

 pretty complete. 



In a treatise dealing only with the teeth, in which the orders 

 must necessarily be taken in succession, it will be convenient to 

 deviate somewhat from the natural order for the sake of taking 

 first those animals whose dentitions are of the simplest character. 

 Thus it is convenient to describe in succession the Edentata and 

 the Cetacea, which have little or nothing to do with one another, 

 because they alike have teeth of simpler form than the rest of the 

 Mammalia. But, as far as possible, the arrangement indicated in 

 the table, which the student will do well to impress upon his mind, 



will be followed in these pages. 







INTRODUCTORY REMARKS. 



Not many years ago it was customary to explain the 

 various facts which were revealed by the study of com- 

 parative anatomy upon the supposition that there was some 

 sort of type or standard organization, and that all others 

 were arrived at by modifications and departures from this 

 type, these modifications being introduced with a direct 

 purpose in view, in order to fit the creature to a special 

 habit of life. 



Among the matters which this " type " theory sought to 

 account for was this : when an animal possesses some pecu- 

 liar organ, it is found on close examination that it, however 

 specialised, is after all only something which allied animals 

 also possess, only it has been exaggerated or developed in an 

 unusual manner and degree ; or, on the other hand, that 

 when an organ is wanting, the suppressed organ is not 

 absolutely abolished, but is to be found stunted and in a 



