KINE TOGENESIS. 331 



rium), and to one (Halicore), is identical with what 

 has taken place in the Proboscidia and Glires, and 

 has resulted in the production of an effective digging- 

 tool. In other genera it may be supposed that their 

 habits of browsing on soft growing materials did not 

 necessitate the use of digging incisors, hence these 

 teeth became atrophied, as in the manatee and Rhy- 



tina. 



c. Development of Molars. 



In fishes and reptiles where teeth occasionally pre- 

 sent very primitive conditions, the theory of the origin 

 of particular types of molar teeth is more simple than 

 in the case of Mammalia. The observations of Hiiter 

 on the action of osteoblasts under stimulus show that 

 under moderate irritation osseous tissue is deposited, 

 while under severe pressure osseous tissue is removed. 

 Koelliker has shown that the action of these bodies is 

 the same in dentine as in true bone. Hence modifica- 

 tions of dental structure must stand in close relation to 

 the uses to which they are put. Thus severe pressure 

 on a simple tooth crown would, if long continued, cause 

 it to expand laterally, or in the direction of least re- 

 sistance, and to grow but little in its vertical axis, i. e., 

 in the direction of greatest resistance. The molar 

 teeth have been subjected to much more severe direct 

 irritation from use than any others in the jaws, and 

 this will account for their increased diameters. 



In the case of the eutherian Mammalia, molar teeth 

 are not traceable back to ancestral types of reptilian mo- 

 lars, but to simple conic (haplodont) reptilian teeth. 

 The process of the evolution of the complex mamma- 

 lian molars from these, forms the subject of a paper in 

 the American Journal of Morphology for 1889, from which 

 I quote extensively in the present work. 



