ECHOES OF BATS AND MEN 



generated a fraction of a second earlier. But the naviga- 

 tion of water beetles has not been studied since Eggers' 

 day, and it is typical of the opportunities that await 

 patient and ingenious students of biophysics. In the fol- 

 lowing chapters I shall describe in more detail better 

 known examples of animals' and men's learning a great 

 deal by listening for echoes, and it will become apparent 

 that living sense organs and brains detect echoes that 

 seem on first thought far too faint to be of any possible 

 use. The phenomenon is basically one of discrimination, 

 or sifting out faint but important echoes from much 

 stronger waves of the same type which are not relevant 

 for the purposes of the particular animal. Living nervous 

 systems are superior to artificial machines in making a 

 wide variety of fine discriminations, and the next chap- 

 ter describes experiments you can perform yourself to 

 show how the human ear and brain discriminate among 

 various types of sounds including echoes. 



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