92 The Life Story of the Fish 



their indifference to a common field of vision, the flounder's 

 eyes move independently. They swivel around without any 

 regard for each other, whereas most fishes' eyes move in 

 unison, as ours do, swinging to the right or the left at the 

 same time. This means that, in a fish like the trout, the left 

 eye moves toward the front and center at the same time that 

 the right eye moves toward the rear and out. If it is directly 

 in front of him, in the common field, that the trout sees best, 

 as we proved to ourselves a little further back, why are not 

 his eyes arranged so that both move toward the front and 

 center at the same time? Just another one of those questions 

 which no one can answer. It is no wonder that people end in 

 a daze when they try to puzzle out the fish's optical ap- 

 paratus, and find themselves setting forth as their conclusion 

 the very fact which started them on their investigation, 

 namely: that the fish's eyes are different from ours, and very 

 hard to understand. 



We have, however, gained one thing. We are now in 

 a position to appreciate the point of view of those who claim 

 that fish do not see very well, and to appraise the validity of 

 their arguments. Such people think that the unvarying 

 amount of light admitted by the unvarying iris must be 

 unfavorable to distinctness of vision. They maintain that 

 the globular lens, with its back-and- forth system of adjust- 

 ment, cannot be capable of bringing light to a proper focus 

 and of producing a clear image on the retina. And they ques- 

 tion whether fish can judge distance with anything like our 

 speed and accuracy. For it is now believed by leading authori- 

 ties that our partial decussation is a device for putting our 

 oculomotor apparatus in control of the pair of images and 

 making it responsible for the co-ordination of eye move- 

 ments which is essential for precise and rapid estimation of 

 distances. The fish's eyes, as we have seen, do not work in 

 that way. 



