330 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



No. 2. In Calamodon the first incisor has become ru- 

 dimentary from the same cause, and in Anchippodus 

 it has disappeared altogether, leaving a truly rodent 

 incisor dentition, consisting of the second incisors 

 only, in the lower jaw. Continued use as chisels has 

 developed these teeth to the great proportions seen 

 in such Glires as Castoroides, etc. (Fig. 107). 



The use which the Proboscidia and Sirenia (Hali- 

 core) give their incisors, is, from a mechanical point 

 of view, like that which the Carnivora give their ca- 

 nines ; that is, it consists of impacts in the long axis, 

 and strains transverse to the long axis of the tooth. 

 The elephants use their tusks for prying up the vege- 

 tables on which they feed, or for pushing aside the 

 vegetation through which they wish to pass. The an- 

 cestors of the Proboscidia are not certainly known, 

 but they possessed incisors of enlarged proportions, 

 such as we find in the Toxodontia and other late rep- 

 resentatives of some of the primitive Ungulata. Use 

 of such teeth in the manner referred to, without oppo- 

 sition from the inferior incisors, will account for the 

 tremendous proportions which they ultimately reached 

 in some of the species of Elephas. 



The use made by the narwhal of its single huge 

 superior incisor, that of an ice-breaker, indicates the 

 origin of its large dimensions. So with the straight 

 incisors of the hippopotamus; use as diggers has 

 straightened them to a horizontal from their primitive 

 vertical direction, a change which is also partially ac- 

 complished in the true pigs (Sus). 



In the Sirenian genus Halicore the upper incisors 

 have been used in excavating vegetable growths from 

 the banks and bottom of shallow seas. The transition 

 from three incisors (Prorastomus) to two (Dioplothe- 



