BY WAY OF INTRODUCTION 15 



The same occurs with bird life on the 

 surface. 



The concealment of a bird on the surface is 

 probably both protective and aggressive. 



The young water-hen only becomes obvious 

 when it swims into the pike's " window," but 

 before the fish is ready to attack the bird has 

 again disappeared into the area of total reflec- 

 tion. Again, if a sprat is swimming, say, a foot 

 under the water, the arc of his " window " is 

 only a foot in front of him, and the surface- 

 feeding gull can be almost over him before the 

 fish detects his enemy. 



It might be thought that the white streak 

 where the body of the bird cuts the water 

 would reveal the gull, but this is not so, for 

 the slightest ripple on the surface causes 

 flickers of white light among which the streak 

 is lost. 



Although reflection is undoubtedly the main 

 factor in the concealment of subaquatic life, 

 mimicry, pure and simple, also plays an im- 

 portant part. 



Not only do many fishes, crustaceans, 

 anemones, &c, mimic vegetation perfectly, but 

 various marine organisms are pigmented so as 



