230 ANIMAL ACTIVITIES. 
in many cases even changing with the seasons to afford 
a better protection. The arctic fox and the northern 
hare become near as white as snow in winter. It has 
been found that the fact that most animals are lighter 
in color beneath than above is really a device for con- 
cealing them. 
Horns. These defensive weapons are formed from 
the epidermis in much the same way as hair. In 
FIG. 193.—1, horns of sheep; 2, horns of cow; 3, horns of deer. 
domestic cattle the horns have an interior core of bone 
over which the skin of horn fits. The horns of deer 
are solid, but in the velvet are covered with epidermis. 
They fall off each autumn and grow again in the spring. 
Structure of Teeth. The teeth of mammals are 
composed chiefly of a bony substance called dentine, 
