THE NERVOUS SYSTEM 



399 



While instinct culminates in insects, the highest devel- 

 opment of intelligence is presented in man. 167 In man 

 only does instinct cease to be the controlling power. 

 He stands alone in having the whole of his organization 

 conformed to the demands of his brain; and his intelli- 

 gent acts are characterized by the capacity for unlimited 

 progress. The brutes can be improved by domestica- 

 tion ; but, left to themselves, they soon relapse into their 

 original wildness. Civilized man also goes back to 

 savagery ; yet man (though not all men) has the ambi- 

 tion to exalt his mental and moral nature. He has a 

 soul, or conscious relation to the infinite, which leads 

 him to aspire after a lofty ideal. Only he can form 

 abstract ideas. And, finally, he is a completely self- 

 determining agent, with a prominent will and conscience 

 the highest attribute of the animal creation. In all 

 this, man differs profoundly from the lower forms of 

 life. 



3. The Voices of Animals 



Most aquatic animals are mute. Some crabs make 

 noises by rubbing their fore legs against their carapace ; 

 and many fishes produce noises in various ways, mostly 

 by means of the swim bladder. Insects are the inverte- 

 brates which make the most noise. Their organs are 

 usually external, while those of vertebrates are internal. 

 Insects of rapid flight generally make the most noise. 

 In some the noise is produced by friction (stridulation) ; 

 in others, by the passage of air through the spiracles 

 (humming). The shrill notes of crickets and grasshop- 

 pers are produced by rubbing the wings against each 

 other, or against the thighs ; but the cicada, or harvest 

 fly, has a special apparatus a tense membrane on the 

 abdomen, acted upon by muscles. The buzzing of flies 

 and humming of bees are caused, in part, by the vibra- 



