CHAPTER XVI 



WHAT THE FISHES SEE 



There is no doubt that fish can see. But how do they 

 see, and by what means are they assured of vision? 

 That is another question. 



So obvious does it appear that fishes see that it may 

 seem ridiculous to assert the existence of a visual power 

 or faculty in them. They have two eyes in their heads, 

 eyes that are sometimes very large indeed, and these 

 look like those of land vertebrates and seem to be their 

 equal in complexity. So, without looking further, we 

 conclude that they are equal in function, or at least, very 

 similar. On the other hand, fishermen know that, in 

 some of their tackle, in several methods of fishing, they 

 must take into account the visual powers of the fish they 

 hope to catch. A real faculty of sight certainly exists. 

 But is it like our own, and, like our own, is it regularly 

 exercised all the time, as a matter of habit ? When the 

 angler casts his fly, he sees it in the air and from above, 

 but his impression of it is quite different from that of 

 the fish, which sees the bait from below and through the 

 water. The lake trout, living in dark depths, can hardly 

 use his eyes as does his sister, the trout of the streams, 

 which dwells in a stream thoroughly penetrated by the 

 light of day. The fish of the ocean bottom, living 

 in regions entirely without light; though they have 

 eyes, are yet in the same negative situation as regards 

 vision. Here is a puzzle, which we may, if we wish, 

 refrain from trying to set ourselves, but which becomes 

 interesting the moment we allow ourselves to think 

 about it. We must not be content with the general 

 opinion which takes things too much on trust and is 

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