304 Heir Paul Freund on the Development 



Rodents. Consequently, if in a branch of the ^Mammalian 

 stem it is possible to recognize in actual examples the deve- 

 lopment of an upper and lower pair of incisors, accompanied 

 by the loss of the rest of the incisors and the canines, it is also 

 a probable assumption that in the ancestors of the Rodents a 

 similar process occasioned an analogous result. In the group 

 which was undergoing modification the adaptation of the 

 cutting-teeth to their heightened function would have 

 advanced further, since the rooted incisors attained a pris- 

 matic shape and the faculty of permanent regeneration." 



Another view is supported by Cope *, who derives the. 

 Rodents from the Bunotheria, as represented by the suborder 

 Tillodontia, which, while possessing a more complete den- 

 tition, exhibit a decided rodent-like formation of individual 

 incisors. 



With reference to the results of our embryological investi- 

 gations, it seems worth remarking that, according to this 

 derivation, the large gnawing-teeth of the existing Rodentia 

 w'ould correspond to the second incisors of what Cope supposes 

 to have been their ancestors, since in the latter the first 

 incisor is already rudimentary and small, while the second 

 is developed into the gnawing-tooth. 



An altogether separate position is taken up by Baume f in 

 his speculations. 



This position is in the first place based upon the fact that 

 this author is desirous of regarding continuously growing 

 teeth as the prototypes of Mammalian teeth in general. In 

 forming this conception he relies upon the view that the 

 continuously growing rootless tooth is of relatively simpler 

 construction. At the same time he considers that the organ- 

 ization of such teeth is adapted for the production of a mass 

 of tooth-substance. This property is likewise considered by 

 Baume to be of a primitive character, since, as he urges, in 

 the whole animal scries we may recognize the law of a pro- 

 gressive reduction in the formation of tooth-substance. 

 Moreover, according to Baume, with the exception of the 

 Rodentia, animals with continuously growing teeth belong to 

 old groups, the still existing representatives of which are 

 perhaps on the way to extinction. It is evident that by 



* Cope, " The Mechanical Causes of the Development of the Hard 

 Parts of the Mammalia," Journ. of Morphulofry, vol. iii. 1889 (JJoston), 

 I was unfortunately unable to consult the actual original cf)nimunications 

 of this author upon the present subject in the ' American Naturalist ' of 

 188;i-84 and the ' Iteport of the United .States Geological Survey. — Ter- 

 tiary Vertebrata," 1885. 



t K. Baume, ' Odontologieche Forscbungen, Theil i. — Versuch einer 

 Entwicklungegeschichte des Gebisses ' : Leipzig, 1882. 



