THE NERVOUS SYSTEM. 185 
known, confer by passing their antenne. All the higher 
animals, too, have similar emotions, as joy, fear, love, and 
anger. 
While instinct culminates in Insects, the highest devel- 
opment of intelligence is presented in Man." In Man 
only does instinct cease to be the controlling power. He 
stands alone in having the whole of his organization con- 
formed to the demands of his brain; and his intelligent 
acts are characterized by the capacity for unlimited prog- 
ress. The brutes can be improved by domestication ; 
but, left to themselves, they soon relapse into their origi- 
nal wildness. Civilized Man also goes back to savagery ; 
yet Man (though not all Men) has the ambition 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, final- 
ly, he is a completely self-determining agent, with a prom- 
inent will and conscience—the highest attribute of the ani- 
mal creation. In all this, Man differs profoundly from the 
lower forms of life. 
3. The Voices of Animals. 
Aquatic animals are mute.’ A world of Radiates, 
Mollusks, and Fishes, therefore, would be silent. Insects 
are about the only Invertebrates capable of producing 
sounds. Their organs are usually external, while those 
of higher animals are internal. Insects of rapid flight 
generally make the most noise. In some the noise is pro- 
duced by friction (stridulation); in others, by the passage 
of air through the spiracles (humming). The shrill notes 
of Crickets, and Grasshoppers 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 
