239 ANIMAL LIFE 
the case of man the sense of smell is not nearly so well 
developed as among many of the other vertebrates. This 
inferiority is largely due to degeneration through lessened 
need; for in Indians and primitive races the sense of 
smell is keener and better developed than in civilized 
races. Where man has to make his living by hunting, and 
has to avoid his enemies of jungle and plain, his special 
senses are better developed than where the necessity of 
protection and advantage by means of such keenness of 
scent and hearing is done away with by the arts of civi- 
lization. 
122. The sense of hearing.—Hearing is the perception 
of certain vibrations of bodies. These vibrations give rise 
to waves—sound waves as they are called—which proceed 
from the vibrating body in all directions, and which, com- 
ing to an animal, stimulate the special auditory or hearing 
organs, that transmit this stimulation along the auditory 
nerve to the brain, where it is translated as sound. These 
sound waves come to animals usually through the air, or, 
in the case of aquatic animals, through water, or through 
both air and water. 
The organs of hearing are of very complex structure 
in the case of man and the higher vertebrates. Our ears, 
which are adapted for perceiving or being stimulated by 
vibrations ranging from 16 to 40,000 a second—that is, for 
hearing all those sounds produced by vibrations of a rapid- 
ity not less than 16 to a second nor greater than 40,000 to 
a second—are of such complexity of structure that many 
pages would be required for their description. But among 
the lower or less highly organized animals the ears, or au- 
ditory organs, are much simpler. 
In most animals the auditory organs show the common 
characteristic of being wholly composed of, or having as 
an essential part, a small sac filled with liquid in which 
one or more tiny spherical hard bodies called otoléths arc 
held. This auditory sac is formed of or lined internally by 
