OBJECTIONS AND DIFFICULTIES AND OTHKl! THEORIES 207 



yield the tri tubercular sectorial tooth common among the Marsupials 

 and Placentals : while the loss of the inner cusps would result in the 

 formation of a triconodont molar. The conclusion reached is, therefore, 

 that the primitive mammalian molar bore a crown with several cusps." 



This seems to be a clear statement of the polybuny theory, although 

 the reader must consult the authors themselves for the detailed argu- 

 ments by which it is supported. According to the author last quoted. 

 not onlv the tritubercular but the triconodont molar is of multitubercular 



origin. 



4. SUMMARY OF OBJECTIONS TO THE POLYBUNY THEORY. 



Our own contrary view may be advanced with equal show of 

 evidence that even the most prilnitive Multituberculates known had 

 already passed tli rough previous fewer-cusped or even tritubercular 

 stages (see p. 80). A third and perhaps preferable alternative is 

 that the simple " multituberculate " lower molar of the Triassic Micro- 

 lestes was derived from the triconodont type by the transverse broadening 

 of the base of the tooth and the upgrowth of the internal basal border. 

 Other possible modes of derivation are discussed in pages 103-105. 



The general answer to this line of reasoning is that which applies 

 equally to many other generalizations which are founded chiefly upon 

 anatomical and zoological comparison, namely : that there is a danger 

 of inverting a series, of placing the most specialized teeth at the bottom 

 of a theoretical scale of evolution, while the most primitive are placed 

 at the top. This criticism certainly can be demonstrated as correct so 

 far as the argument applies to Arctocyon (see p. 133) and blunts (see 

 p. 142), as we feel reasonably certain that the presumed ancestors or 

 oldest representatives of each of these types have elevated and distinct 

 cusps, and in each case the low-crowned, tuberculate condition is 

 secondary. Among the Rodents we have, it is true, not yet traced 

 the phyletic succession as fully as in other series, but we have strong 

 grounds for considering trituberculate upper molars such as those of 

 Plcsiarctomys as the most primitive (see pp. 145-151). 



The chief difficulties in the ' polybuny theory ' may be summarized 

 as follows : ( 1 ) The examples cited as primitive are tubercular low- 

 cusped crushing teeth ; all zoological and palaeontological evidence goes to 

 show that such crowns are secondary as compared with more pointedly 

 cusped, piercing-cutting-crushing teeth such as those seen in Polyproto- 

 dont Marsupials (p. 109), Insectivores (p. 117), Creodonts (p. 132). (2) 

 Palaeontology offers strong negative evidence against it, for Microles1<-* 

 very probably 1 leads up through Plagiaulax and Ctenacodon into the 



1 Osborn, H. F., "Structure and Classification of the Mesozoic Mammals," Jew. Acad. 

 Nat. So'., Fhila., 1888, pp. 214-216. 



