36 The Lije Story of the Fish 



only very slowly, and never succeeded in making a good 

 match with the background, while others changed quickly 

 into a very close imitation of whatever bottom they were 

 placed on. Here we have a perfect piece of machinery for the 

 working of selective evolution. Those individuals very expert 

 at color adaptations might be expected to live long and suc- 

 cessful lives, and produce a great host of offspring, whereas 

 those that clung obstinately to one color regardless of back- 

 ground might be expected to perish miserably at an early age, 

 leaving no descendants behind them. And so color adaptabil- 

 ity, once it had occurred by mutation, might be expected to 

 be handed on until it included the whole race, while those 

 unhappy individuals in which it failed to reach its full efficacy 

 would be eliminated by nature before they could spread their 

 misfortune. 



In passing, it is of interest to note that the fish definitely 

 improved with practice, that is, the more often they were 

 shifted from one pattern to another, the more quickly they 

 learned to make the necessary change. One individual even 

 became so adept that, when placed in a tank with one part of 

 the bottom black and the other part white, it could be ob- 

 served to change from dark to light as it swam slowly across 

 the dividing line between the two sections. This, remember, 

 is not a fish story, but a scientist's report. 



It was also found that the eyes played an indispensable 

 part in making color adaptations. Fish which had been excel- 

 lent performers lost their ability entirely when they were 

 prevented from seeing. If, in normal condition, they were 

 placed on a black background until they had adapted them- 

 selves to it, and were then blindfolded in one way or another 

 and moved to a white bottom, they remained dark in color, 

 and the reverse was in principle true if they started on white 

 and went to black. 



To the reader this may not seem a particularly startling 



