THE MAMMALS 235 



appear to be correlated with the habits of the animal. 

 Among the species of subterranean habits the sense of hear- 

 ing is largely deficient ; but, on the other hand, it is ex- 

 ceedingly keen in the ruminants, and enables them to detect 

 their enemies at surprisingly great distances. In these 

 creatures the outer ears are of large size and great mobility, 

 and, placed as they are on the top of the head, serve to con- 

 centrate the sound-waves on the delicate apparatus within. 

 In the mammals the sense of smell reaches its highest de- 

 velopment, especially among the carnivores which scent their 

 prey. On the other hand, it is said to be absent in the 

 whales and very deficient in the seals. The sense of taste, 

 closely related to that of smell, is located in taste-buds on 

 the tongue, and is also more acute than in any other class of 

 animals. The sense of touch, located over the surface of 

 the body, is especially delicate on the tips of the fingers, 

 the tongue, and lips, which often bear long tactile hairs, 

 called whiskers or vibrisscB. 



221. Mental qualities. Correlated with the high degree 

 of perfection of the brain and sense-organs the mammals 

 show a higher degree of development of the intellectual 

 faculties than any other class of animals. In many cases 

 their acts are instinctive, and not the result of previous 

 training and experience. Just as the duck hatched in an 

 incubator instinctively takes to the water and pecks at its 

 food, or as the bee builds its symmetrical comb, many of 

 the mammals perform their duties day by day. On the 

 other hand many other mammals are also undoubtedly in- 

 telligent. They possess the faculty of memory ; they form 

 ideas and draw conclusions ; they exhibit anger, hatred, and 

 self-sacrificing devotion for their companions and offspring 

 that is different from that in man only in degree and not 

 in kind. In fact, intelligence differs from instinct primar- 

 ily in its power of choice among lines of action. 



222. Classification. Of the eleven orders into which the 

 mammals have been divided eight are represented in this 



