EVOLUTION OF MOLARS 123 



have descended from ancestors with brachyodont multituberculate 

 molars, in which the tubercles both in upper and lower molars 

 were triserially arranged. In the most primitive Nesomyinae and 

 Murinae, the transverse complication, occasioned by the triserial 

 arrangement of the tubercles, is more completely preserved than 

 in the other subfamilies ; in the most primitive Microtinae, on the 

 other hand, the no less archaic longitudinal complexity of the 

 teeth is preserved to an unusually large extent, although in all 

 other respects the teeth of some of these primitive Microtinae have 

 reached the highest possible level of specialization. This association 

 of archaic and progressive features in one and the same form, 

 often in one and the same organ, is a familiar one to the student 

 of mammals ; over and over again we find that what is funda- 

 mentally a lowly type has preserved itself and prospered in 

 competition with more highly organized forms, by adapting all 

 its plastic characters to the needs of some special environment. 



But if the views expressed above be sound, then those brilliant 

 generalizations constituting the famous Tritubercular Theory, 

 which has stimulated so much productive palaeontological 

 research, do not apply to the Simplicidentate Rodents. Equally 

 inapplicable is the still more brilliant though less known theory 

 of Winge. In the teeth of these mammals, whatever may be the 

 case elsewhere, we have nowhere to deal with increasing complica- 

 tion of the molar crown ; on the contrary in every family molar 

 evolution resolves itself into a tale of reduction. There is through- 

 out this great group a general tendency for the molars to become 

 hypsodont, and increasing hypsodonty implies, as a rule, pro- 

 gressive simplification. 



Forsyth Major taught me many years ago two things : — (1) 

 that a triangular tooth is not necessarily tritubercular; (2) that 

 we are not entitled to neglect any element of a molar, no matter 

 how small and inconspicuous it may be, if we can identify it when 

 it occurs. He was the first ^ to show that the primitive rodent 

 molar must have been a very complex thing, and the first to 

 suggest a Multituberculate origin for the whole group. The views 

 expressed above are merely extensions of his theory; but they 

 have been arrived at quite independently, as the result of many 

 years' work, upon the basis of material far richer than that at the 

 disposal of my late friend. 



In conclusion it may be stated, in order to put my meaning 

 beyond doubt, that like Forsyth Major I regard the Allotheria of 

 Marsh (= the Multituberculata of Cope) as the probable source 

 of the Simplicidentate Rodents, as well as of all the other 

 Placentals. Although as regards general characters members 



^ The chief papers in which Forsj-th Blajor expressed and developed his 

 views are: 1873, " Nageriiberreste aus Bohnerzen Siiddeutschlands und 

 der Schweiz," PaU-eontographica, 22; 1893, " On some Miocene Squirrels," 

 P.Z.S., 1893, p. 179; 1897, -'On the Malagasy Rodent Genus Brathy- 

 vromyst" P.Z.S., 1897, p. 695; 1899, " On Fossil and Recent Lagomorpha," 

 Trans. Linn. Soc. London, 2nd Ser. Zool., 7, p. 433. 



