302 Herr Paul Freuiul on the Development 



reduction in the number of tlic incisors (in the mandible there 

 is always only one on eacii side, in the upper jaw there are 

 two in the Leporidie only, in all other cases there is likewise 

 one) and the disappearance of the canines, and, at any rate, 

 of a portion also of the premolars, are related to the modifica- 

 tion of the persisting incisors to form chisel-like gnawing- 

 teeth, by means of which the annuals are able to consume 

 even very solid vegetable food-substances, such as the bark of 

 trees and grains, the latter of which possess an especially 

 high nutritive value. It lias already been demonstrated by 

 numerous authors that the peculiarity of the continuous 

 growth of the gnawing-teeth also belongs to the complete 

 efficiency of the entire arrangement. The effectiveness of 

 this form of dentition in tiie struggle for existence is at once 

 evident from the great wealth of species possessed by the 

 order Rodentia, the universal geographical distribution of its 

 representatives, as well as the enormous number of indi- 

 viduals com])osing the various species. The latter pheno- 

 menon naturally results from the proverbial fertility of the 

 Kodents; but this, as we know, is directly proportional to 

 the abundance of food and the ease with which it is obtained. 

 It is characteristic that the advantages in the struggle for 

 existence, which are ultimately occasioned by the form of the 

 dentition, are so great that Nature has almost entirely dis- 

 [)( used ■with the additional gift of other means of defence. 

 Tlie Rodents are among the most defenceless of Mammals. 



(3ur views as to the reduction of the dentition in particular 

 depend upon the conceptions -which we have formed as to the 

 phylogenetic derivation of the Rodents. A whole series of 

 such conce])tions, however, have already been foruiulated by 

 various authors. The fullest and most recent treatment of 

 this subject, which is based upon an exhaustive comparison 

 of the dentition, of various parts of the skeleton, the uro- 

 genital system, and the mammary organs, is furnished by 

 A. Fleischmann *. 



Fleischmann is inclined to derive the Rodents from mar- 

 supial-like ancestors, with a complete insectivorous dentition. 

 In his seventh chapter, which he terms a " Pliylogenetic 

 Sketch," he sums uj) his results as follows : — " In tlic organ- 

 ization of the Rodents two different stages are accordingly 

 recognizable, which are full of meaning for the history of 

 Mammals. On the one side many peculiarities are empha- 

 sized which are of functional importance in a lower stage, 



• ' Enibryologische UilterHUclmngen von A. Fleisclnuann, I'rivat- 

 docent in Erlangen,' ii. Heft. : A. " Die Staniniesf^esciiichte der Nage- 

 thiere." 



