RESPONSIVE LIFE OF ORGANISMS 447 



mechanical effect when they fall upon a sensitive surface. 

 And doubtless tones are perceived as such only by special- 

 ized organs of hearing. Sound receptors are variously 

 located about the body, and are wonderfully different in 

 structure in different groups of animals. The antennal hairs 

 of a male midge that vibrate to sound waves of a length 

 corresponding to the pitch of the humming of the female, 

 are freely exposed. But sound receptors are more often 

 sequestered in some part of the body behind a mechanism 

 (such as the tympanum and small bones of the middle ear in 

 ourselves) capable of taking up vibrations and passing them 

 on to appropriate nerve endings. Since sound waves tend 

 to fill all the space they traverse, there is not the same need 

 as in the case of eyes, that ears should be located in a well 

 exposed part of the body. Hence, we find the sound recep- 

 tors of certain grasshoppers located in the fore legs, and in 

 others, upon the base of the abdomen. These are so 

 different, however, from the ears of vertebrates, that we can 

 form hardly any conception of how they work, nor any at all 

 of the sort of sensations to which they may give rise. The 

 ears of vertebrates, situated on the head, are placed in the 

 direct path of passing sound waves, and trumpet-like exter- 

 nal ears are added for concentrating these waves upon the 

 ear drum. 



Ears possess the great advantage of giving notice of the 

 presence of friends or enemies in the darkness as well as in 

 the light, and when hidden as well as when in the open. 

 Well developed, they confer great powers of discrimination 

 upon their possessor. For social organisms ears have their 

 value wonderfully enhanced by the development of sound- 

 producing organs. Together, these constitute the funda- 

 mental equipment for social intercourse, and for exchange of 

 ideas. Eye and ear are in ourselves the receptors to which 

 we have learned as social organisms to make appeal. 



