328 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



That the increased size of canine teeth is due to 

 strains is strongly indicated by the huge development 

 of these teeth in the walrus. This animal uses its ca- 

 nines for the breaking of ice, and for lifting itself from 

 the water on to the edge of strong ice. The fact that 

 canines and not incisors have been thus developed is 

 a necessary result of the fact that the walrus is a de- 

 scendant of a line of animals which had already re- 

 duced incisors and larger canines. 



a 



Fig. 96. Esthonyx burmeisterii Cope, dentition: a, profile; b, superior; 

 c, inferior dentition, grinding faces. Reduced. 



b. Development of the Incisors. 



The history of the incisor teeth of the Mammalia 

 exhibits three processes, viz. : hypertrophy (e. g. 

 Glires), specialization (e. g. Galeopithecus, Lemur- 

 idae), and atrophy (e. g. Booidea, Phacochrerus, Glos- 

 sophaga, etc.). 



