210 ANIMAL BIOLOGY 



tinguish friends from foes within their own species; so 

 the world of smells is one of great importance to them. 

 The keen interest which the dog takes in the various odors 

 encountered along his path is doubtless a trait inherited 

 from his wild ancestors for whom the detection of odors 

 often meant the prevention of starvation or escape from 

 being eaten by some larger animal. It is perhaps because 

 of the keen sense of smell in mammals that there have 

 been developed scent glands in many species which doubt- 

 less enable individuals to find one another with greater 

 readiness. Musk which is the product of the scent 

 glands of the musk deer is used as a perfume and also 

 in medicine. In the skunk the scent glands which are 

 situated near the base of the tail are developed to a very 

 unusual degree and constitute an efficient means of 

 defense. The animal can discharge the fluid secreted by 

 the glands to a distance of several feet; consequently there 

 are few creatures that care to molest him, and he can 

 go about with the impudent boldness which is one of the 

 prominent traits of his character. 



Mammals are found in most parts of the earth except 

 upon oceanic islands where there are none except bats 

 which may have flown there, or more rarely very small 

 species, such as mice, which may have been transported on 

 floating trees or other drift. Mammals have very limited 

 powers of migration, so that those inhabiting cold coun- 

 tries cannot escape the winter like the birds. Some species 

 have developed a remarkable aptitude for finding food 

 even when the ground is thickly covered with snow; some, 

 such as many kinds of squirrels, store up food during summer 

 which is used during the winter; while others, such as the 

 ground hog, undergo what is called hibernation, remaining 

 in a dormant condition in which they subsist mainly on 

 their own fat. 



