168 



EVOLUTION AND GENETICS 



trunk is correspondingly increased. By this adaptation the animal 

 is also freed from the necessity of supporting the weight of its head 

 at the end of a long neck, although it has an ample reach and great 



power in the combination of short, 

 powerful neck, short skull, and long, 

 flexible proboscis. 



Teeth. The molar teeth of ele- 

 phants are made up of relatively 

 thin plates of dentine surrounded 

 by enamel and connected by cement. 

 They lie in such a position that 

 their ends form ridges across the 

 grinding surface (Fig. 93). Since 

 the teeth grow obliquely toward the 

 plane of contact with those of the 

 opposed jaw, successive parts come 

 into use as they grow, with the result 

 that a limited number of teeth or 

 parts are in use at any time. The 

 maximum number of teeth is twenty- 

 eight, but these come into use and 

 are worn out and shed in such a way 

 that not more than two molars in each half-jaw are functional at 

 the same time, making eight molars in all. In addition to these 

 the tusks or upper incisors are present from their first appearance; 

 they are preceded by a pair of small 

 milk tusks which are shed early in 

 life. Lull records a pair of tusks of 

 an African elephant which were 10 

 feet ^ inch and 10 feet 3}/^ inches 

 long respectively, and weighed 224 

 and 239 pounds, an almost incredi- 

 ble weight for an animal to carry in Fig. 94.— Skull of MoeriiheHum 

 two teeth alone. The tusks are com- ^■'/"«'^'*"' ^"e tenth natural size, 

 posed of dentine, excepting a small 

 enamel tip, and grow throughout life. 



Specialization in three directions is shown by this dentition: (1) 

 departure from the primitive numl)er of sixty teeth by reduction 

 to a total of twenty-eight, (2) modification of the primitive mam- 

 malian molar to form a finely ridgc^l grinding tooth, and (3) de- 



FiG. 93.— Molar tooth of Indian 

 elephant. A, crown; B, lon- 

 gitudinal section. Enamel is 

 black, dentine shaded with 

 oblique lines and cement stip- 

 pled. (From Lull.) 



From the Eocene, Africa. 

 (From Lull, after Andrews.) 



