204 EVOLUTION OF MAMMALIAN MOLAR TEETH 



In support of the simplification hypothesis as applied to the pre- 

 molars, the author endeavours to show (op. cit., 1899): 



(1) The true molars of all these orders can be reduced to this type 

 (pp. 558-565). 



(2) The so-called premolars of Proteodidelphys (pp. 556, 557) are 

 more complex than those of its descendants, Eodidelphys, Microbio- 

 f/irrium, Didclphys. [To the present author the premolars of 

 Proteodidelphys seem less complex and more primitive than those 

 of DidelphysJ] When complex lower premolars do occur in different 

 orders they are to lie interpreted as reversions to the ancestral 

 plexodont type (p. 566, 2). 



(3) The less complex condition of lower premolars generally is 

 secondary (p. 568), due to the want of space for the complete develop- 

 ment of these teeth, which resulted from a fore-and-aft shortening of 

 the whole tooth row : for (a) modern placental cheek teeth represent 

 two series (p. 569), that is, the first series = true molars + milk molars, 

 the second series = replacing teeth or premolars; (b) formerly (p. 569) 

 (in South American forms) the first series was all in use at one time 

 and ctliilnted the same form from one end of the series to the other; 

 (c) on account of the gradual intercalation of the two series the space 

 left free for the incoming premolars was shortened and that for the 

 milk molars was increased so that the premolars progressively simpli- 

 fied and the milk molars became more complicated. Similar reasoning 

 applies to the upper premolars (p. 569, footnote). 



He concludes (p. 571): " The clear result of all these facts is that the 

 famous theory of the gradual complication, of triconodonty and trituberculy, 

 is an untenable hypothesis. Nowhere do we meet with the stages leading 

 from haplodonty to plexodonty ; all those which have been mentioned 

 are, on the contrary, as I believe I have demonstrated, but the result of 

 simplification of molars which were formerly more complicated." 



In answer to these ingenious arguments for the simplification theory 

 may be adduced the facts newly stated in Chapter VII., pp. 100-192. 



2. OBJECTIONS BY FLEISCHMANN AND MAHX ANSWERED BY SCOTT. 



In 1891 A. Fleischmann * and R. Malm 2 rejected both the nomen- 

 clature and the homologies of Trituberculy for reasons which were 

 discussed in great detail and apparently shown to be insufficient, by 

 Prof. W. B. Scott in 1892. 3 



1 " Die Grundform der Backziihne bei Sangethieren," Sitzungsberichte d. kun. pn-uss. 

 Ahul. ,!. IVis*. zu Berlin, 1891, p. 891. 



2 " Ban uncl Entwicklung d. Molaren bei Mus und Arvicola," Jlorphol. Jahrb., Bd. XVI., 

 1>. 6.V2. 



3 " The Evolution of the Premolar Teetli in the Mammals,'' Proc. A< i ad. A'/. Sd. Phi/a., 

 1892, pp. 409 41'J. 



