1 1 2 The Life Story of the Fish 



ting suicide by warning him when the water which he is 

 entering is colder or warmer than the water he has left is 

 a duty of the lateral line. 



And only recently, one of the long-standing misconceptions 

 about the lateral line has been cleared up. Scientists of old 

 looked at the lateral line, scratched their heads, and tried to 

 imagine what uses it might be put to. Someone suggested 

 that the fish ought to have some way of sensing currents, 

 and that seemed like a happy thought. Contentedly the scien- 

 tists wrote down in their books, "Lateral line: current de- 

 tector," and put the matter out of their minds. This was not 

 even theorizing j it was plain guesswork. 



The fact is that the fish detects currents in the same way 

 you do. If you are swimming in the middle of the Gulf 

 Stream out of sight of land, the whole body of water sur- 

 rounding you, on account of the uniformity of its motion 

 and the fact that you can see nothing else, appears to be 

 standing still, and you do not realize that you are being car- 

 ried along at anywhere from two to four miles an hour. If, 

 now, the water were suddenly to grow so shoal that you 

 could touch bottom, or if the shore-line were suddenly 

 brought out to where you could see it, you would know that 

 the water was not stationary, for the speed at which you 

 dragged over the bottom or at which things on shore moved 

 by you would be quite at variance with the exertions you 

 were putting forth in swimming. In the same way, the fish 

 recognizes that it is in a current by the fact that it has to 

 swim in order to keep up with things on shore, or to keep 

 from being dragged over things on the bottom. Differential 

 currents, coming into still water or striking at an angle to or 

 a different speed from the main current, may conceivably 

 have an effect on the lateral line, but the principal "current 

 detectors" are the eyes and the skin. 



