334 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



The only question that can arise is with reference to . 

 the origin of the multituberculate molar of the'Proto- 

 theria. 



It is further questioned by Forsyth-Major, whether 

 the tritubercular molar has been derived from the tri- 

 conodont. He believes, on the contrary, that it is de- 

 rived from the multitubercular type by reduction. 

 There are two objections to this view : (i) the cones 

 of the tritubercular tooth or trigon should be subequal, 

 were they derived from a multitubercular source. On 

 the contrary, the two external cones of the upper, and 

 the two inner cones of the lower series are in the 

 earliest (Jurassic), as well as most of the tritubercular 

 types, smaller than the single opposite cusp or proto- 

 cone, precisely as are the anterior and posterior cones 

 of the triconodont molar. (2) No paleontologic series 

 from the multitubercular to the tritubercular types has 

 been traced, while the series from the .triconodont to 

 the tritubercular is well known. Forsyth-Major's evi- 

 dence that such a transition exists in the Glires, is 

 better explained by tracing the moderate complexity 

 he describes to a tritubercular origin. 



It is also alleged by Allen and Scott that the inter- 

 nal cusps of the premolars, when present, originate by 

 the development of internal cingula, and have no prim- 

 itive tritubercular ancestry. The evidence at our dis- 

 posal from paleontological sources is in favor of this 

 view; hence it is reasoned that the history of the mo- 

 lar teeth must have been identical. This however does 

 not follow, especially as the paleontologic evidence 

 points the other way. The history of the two series 

 has been different. In the first place the premolars 

 have been subjected to much less use than the true 

 molars ; hence they retained the primitive reptilian 



