152 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



type. I have pointed out a corresponding modifica- 

 tion in the structure of the crown of the superior true 

 molars, viz. : the transition from a quadritubercular to a 

 tritubercular structure in passing from the lower to the 

 higher races. As this point has some interesting im- 

 plications in the earlier phylogeny of man, and as its 

 value has been disputed, I give it a little attention. 



The facts of the case are as follows : I have dem- 

 onstrated ^ the fact that all forms of dentition exhibited 

 by the eutherian mammals have been derived from a 

 primitive tritubercular type. Professor Osborn says 

 that he expects to be able to do the same for the multi- 

 tuberculate (? Prototherian) dentition. I have also 

 shown that man exhibits a tendency to revert from his 

 primitive quadritubercular molar to this tritubercular 

 type.^ As to the significance of these facts, I have 

 expressed the view that this acquisition of a tritubercu- 

 lar molar is a reversion to the lemurine type. This 

 conclusion is necessary because the lemurs are the last 

 of the families in the line of the ancestry of man which 

 present this dentition. The monkeys and anthropoid 

 apes are all quadritubercular, except a few limited col- 

 lateral branches of the former, which still retain the 

 lemurine type. There are also a few collateral types 

 of lemurs which have acquired one or more quadri- 

 tubercular nlolars, but they are not typical. In many 

 trituberctilar mammals, a precocious form or two can 

 be found, which has acquired the fourth tubercle. But 

 the further back we go in time, the fewer they become, 

 until, in the Puerco fauna, of eighty-two species of 



^Proceeds. Amer, Philos. Soc, Dec, 1883 ; Origin of the Fittest, xiZ'j,-}^ 

 245, 347. 359- 



^American Jotirnal of Morphology, II., r883, p. 7. 



