32 The Life Story of the Fish 



black pigment which forms in that portion of the skin. It is 

 a temptation to say that this shading is a beautiful example of 

 protective coloration, the dark back being almost invisible to 

 an enemy like a fish-hunting bird looking down from above 

 into the dark water, while to an enemy looking up from 

 below the light belly would be inconspicuous against the 

 light coming from above. However, there is a school of 

 thought which claims that protective coloration is a myth, 

 and that what seem to be examples of it can all be explained 

 on purely mechanical grounds. They believe that the fish's 

 back is dark solely because the light shining on it from above 

 turns it dark, and the under side is pale only because it re- 

 ceives no light. One worker, to prove this, carried out an 

 experiment on the flounder. The flounder habitually rests on 

 bottom, and its under side is completely without coloring 

 matter, whereas the upper side is well supplied with it. It 

 being impossible to persuade the flounder to reverse its posi- 

 tion, this man resorted to trickery. He put it in a glass tank 

 through which by means of mirrors a constant stream of light 

 was brought to play on it from beneath. The flounder did 

 eventually succeed in producing colored pigment on its 

 theretofore immaculate under side, but the fact that it took 

 over a year to do it leaves one uncertain whether this scien- 

 tist proved his theory, or merely showed what changes can 

 be made in nature's arrangements when they are forcibly 

 submitted to unnatural conditions. In any case, that light 

 plays at least some part in the distribution of pigment is 

 shown by a species of African catfish which, swimming habitu- 

 ally wrong side up, has reversed the conventional color 

 scheme: its belly is dark, its back almost white. 



To return to our guanin crystals, the interesting point 

 about them is that they are by-products. They were not put 

 there to give the fish iridescence. They are there because, at 

 the time she designed the fish. Nature had not yet got around 



