442 MAMMALIAN ANATOMY 



tively few through the study of variations. For this reason I venture 

 to depart from my rule of avoiding in this volume all theoretical 

 questions, and call attention to some of the possible explanations of 

 the variations which I have observed during the examination of the 

 cheek teeth of many hundred skulls of the domestic cat. 



We must first consider some of the theories of the evolution of 

 teeth, especially those based principally upon the study of the teeth of 

 fossil mammals. These theories have been formulated by Ryder, 1 

 Cope, 2 Allen, 3 Osborn, 4 and Scott, 5 whose publications the student 

 should consult for a full discussion of the subject. The following 

 summary is taken from Professor Cope's last book : 



" The distinction of teeth into incisors, canines, and molars appears 

 independently at various points in the line of Vertebrata. Incisors 

 and molars are distinguished in sparoid fishes and in placodont and 

 diadectid reptiles. Canine-like teeth, or pseudo-canines, appear in 

 clepsydropid and crocodilian reptiles and in saurodont fishes. Canine- 

 like incisors appear in the Clepsydropidse. The variety of character 

 in these structures presented by the Mammalia to be considered is 

 great, and the principles deduced from observation of them are appli- 

 cable to the Vertebrata in general. 



"As mechanical causes of the origin of dental modifications, I have 

 enumerated the following : 



" 1. Increase of size of a tooth, or a part of a tooth, is due to 

 increased use, within a certain maximum of capacity for increased 

 nutrition. 



" 2. The change of direction and use of a tooth take place away 

 from the direction of greatest and in the direction of least resistance. 



" 3. It follows, from their greater flexibility, that crests of crowns 

 of teeth yield to strains more readily than do the cusps. 



" 4. The increase in the length of crests and cusps in all direc- 

 tions, and therefore the plications of the same, is directly as the irrita- 

 tion from use to which their apices and edges are subjected, to the 



1 Proc. Phila. Acad. Nat. Sci., 1878, p. 45. 



2 The Origin of the Fittest, New York, 1887 ; The Mechanical Causes of the 

 Development of the Hard Parts of the Mammalia, Jour. Morph., vol. iii., 1889, 

 pp. 137-290 ; Primary Factors of Organic Evolution, Chicago, 1896. 



3 Dental Cosmos, 1888. 



4 American Naturalist, Dec., 1888, p. 1067. 



5 Proc. Phila. Acad. Nat. Sci., 1892, p. 405. 



