228 ANIMAL FORMS 
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 vibrisse. 
215. 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. 
216. Classification.—Of the eleven orders into which the 
mammals have been divided eight are represented in this 
