194 THE SENSES OF ANIMALS 



is distorted by nearby objects when they are near the bottom 

 or the bank. The distortions are reflected to the fish, and 

 appear to be received by the sense organs of the lateral line, 

 so that the fish is made aware of its surroundings although 

 it cannot see them. If suitable electrodes are put into the 

 water where these fish are swimming, and are connected 

 with an amplifier, each electric pulse can be translated into 

 a sound so that the pulses are heard as a series of clicks. 

 Once this discovery had been made the evolution of the more 

 powerful electric organs of the torpedo and electric eel be- 

 came comprehensible ; moreover it was found that although 

 these fishes can give a strong shock occasionally they are in 

 addition continually giving mild ones wherever they move 

 about, in order to probe their surroundings. The electric 

 fishes have indeed anticipated man, for by using electro- 

 magnetic waves instead of sound for echo-location they have 

 evolved a form of true radar. 



The sense of sight is so dominant with us that it is difficult 

 for us to imagine how the world must appear to all the 

 animals that get their main information about it not by sight 

 but by echo-location. Is there any reason why it should 

 appear materially different? On the whole there seems to be 

 none, for it is not the means by which signals are received 

 from the outside world that matters, but the mental image 

 that is built up in the brain as the result of decoding the 

 signals. And it is obvious from the behaviour of these animals 

 that their mental image built up from the receipt of echoes 

 is about as complete and detailed as the image we build up 

 as a result of light falling on the receptor cells at the back of 

 our eyes. 



