ECHOES OF BATS AND MEN 



(Listening in the Dark has a more detailed account of 

 these experiments.) 



What emerges from these several examples of orien- 

 tation based on echoes is the simple fact that bats and 

 porpoises are most adept at locating small and distant 

 objects in this way. Furthermore, they do so with a pre- 

 cision and acuity that are understandable only when one 

 remembers that this is how they make their Uving. If a 

 bat fumbles with its echoes, it goes hungry. Hunger is 

 a powerful incentive, tending strongly to improve any 

 mechanism or process subjected to this selective action. 

 This is what biologists call natural selection, the process 

 responsible for the evolution of plants and animals into 

 their many diversified and complex forms. It is a slow 

 process but an extremely effective one, and in the bats 

 and porpoises we see the end result achieved through 

 natural selection, perfecting over miUions of years the 

 animals' faculties for utilizing echoes. Finally, it is im- 

 portant to realize that the use of echoes requires the bats 

 and porpoises to possess more than merely a means for 

 generating sounds that in turn will yield echoes. It is also 

 essential that these animals discriminate certain impor- 

 tant echoes from complex mixtures of other sounds that 

 are often much louder than those conveying the crucial 

 information about food. 



Discrimination of one portion of a complex sound 

 from louder components is not a special skill of bats 

 and porpoises. All animals endowed with a sense of 

 hearing discriminate, and in many respects the human 

 ear and brain are the best of all. When we listen to 

 speech or music, we sort out a few significant portions 

 of a compUcated mixture of shifting wave forms. If we 

 hear people speaking an unknown foreign language, we 

 receive a similar jumble of sound waves, but one to 

 which we have no key. Footsteps or bat chirps and their 



104 



