56 



EVOLUTION OF MAMMALIAN MOLAR TEKTH 



have four cusps. 1 Half a century ago this would have been con- 

 sidered as something ultimate, simply as an adaptation to human 

 diet ; but now that we have come to understand the doctrine of 

 evolution, we ask ourselves, What is the meaning of these cusps ? 

 what is their history ? what is their origin ? Now, these four cusps 

 which are present on the four corners of the teeth might be explained 

 by evolution in three ways. We might imagine that the crown of 

 the tooth was originally a low rounded summit, and that on the 

 summit these four cusps appeared at each angle ; no one has 

 advocated this. Or we might imagine that they represent the coming 

 together of a number of tips of pointed teeth, such as we see in the 

 jaw of this member of the dolphin family ; this is the theory which 

 has been recently advanced in Germany, and it has been called the 

 " cusp concrescence " theory. Or, again, we might imagine that these 



me. 



me. 



me. 



Kii;. 40. Evolution of the Human Upper Molars, '.i. Aiiiiiito,iiii,-/i/nix. a lower Eocene monkey. 

 10. An upper Eocene monkey. 11 and 12. Human: 11, Esquimaux; 12, Negro. See addition of 

 "talon," /i if, ti> ''trigon" composed of pit, jn-, i,n. 



cusps have originated by a gradual addition to the sides of a primitive 

 single cone ; this we call the " cusp differentiation " theory, or the theory 

 of cusp addition, in distinction from concrescence. The differentiation 

 theory is supported by Cope, by myself, and others in this country. 



Now, suppose an evolutionist were to trace back the history of the 

 monkeys and of other animals among their fossil ancestors, he would 

 find that the further back his researches extended the more simple 

 the types of the teeth would be ; he would find that the teeth 

 of the oldest types of ancestral mammals have a simple conical 

 form, the form that is preserved in the teeth of the whales and the 

 dolphins of the present day (Fig. 42* A, p. 64), or in the Edentates 

 as represented in the group to which the sloth and the armadillo of 

 South America belong.* 



We have the same type of conical tooth preserved in the human 

 canines, and if we turn from the teeth of man, in which the canine 

 has almost entirely lost its original laniariform or flesh-tearing shape, 

 to that of the lower monkeys, we see that the canine is really a 



1 E. D. Cope, "On the Tritubercular Molar in Human Dentition," Jour, of Mor- 

 phology, July, 1888, p. 7. 



*[Iii both groups the simple conical form is now believed to be secondary (pp. 79, 151, 

 191). En.] 



