ECHO-LOCATION I93 



issues in search of its food, which so far as is known, consists 

 entirely of oily nuts or fruits . . . some of them sought, it 

 would seem, at a very great distance, for M. Funk . . . states 

 that in the stomach of one he obtained at Caripe he found 

 the seed of a tree which he believed did not grow nearer 

 than 80 leagues." It has been shown that these birds find 

 their way about in the darkness of the crowded caves by 

 means of sound echo-location. (See Plates 17 and 18.) 



Finally there remains to be mentioned another method of 

 orientation analogous in some ways to sonar, but which 

 makes use of pulses of electrical energy to create an electric 

 field that informs its possessor about its surroundings. This 

 is found in some of the fishes, and its existence was discovered 

 only a few years ago. It has long been known that some 

 fishes, such as the torpedo and the electric eel, possess electric 

 organs that can give a shock strong enough to stun their 

 prey; it has equally been known that others, such as the 

 skates and rays and some of the fresh- water fishes of Africa, 

 have much smaller and less powerful electric organs, the 

 function of which was unknown. The electric organs in some 

 fishes are formed from highly modified muscular tissue, and 

 in others from similarly modified glandular tissue. When- 

 ever a muscle fibre in any animal contracts an electrical 

 change is produced in it; it is therefore conceivable that in 

 the course of evolution some muscle fibres may have lost 

 their contractile function and become specialized for the 

 production of electricity. A similar argument can be applied 

 to glandular tissue. But the evolution of electric organs has 

 been a great puzzle to biologists, for it seemed impossible 

 that the powerful organs of the torpedo and electric eel could 

 have evolved by a single mutation, and yet no reason could 

 be seen for their evolution by smaller stages because no 

 function could be found for the comparatively feeble or 

 ''rudimentary" electric organs of other fishes. 



The riddle was solved when Lissmann discovered that the 

 low-powered electric or^ns of certain fishes which live in 

 the turbid muddy waters of some of the rivers in Africa have 

 a very important function. These fishes generate pulses of 

 electrical energy which produce a symmetrical electric field 

 around them when they are floating in mid-water, but which 



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