THE TKOU'I JUMP. 33 



means of sight. The careful way in which it is neces- 

 sary to imitate flies in order to deceive the wary trout 

 shows that they can pretty accurately distinguish forms 

 and colors. The rapidity and certainty with which other 

 fish will rise to an artificial minnow on a trolling-line 

 sufficiently proves the rapidity of their perceptions. The 

 imitative devices or mimicry which exist among many 

 species similarly prove how sharp are the eyes of their 

 enemies ; for these resemblances can only have been de- 

 veloped in order to deceive the senses of other fishes, 

 and would not, of course, go beyond the point at which 

 they proved useful to the species. All flatfish closely 

 imitate the colors and arrangement of the sand or peb- 

 bles on which they lie ; and it is often difficult even for 

 a human eye to detect a sole or a flounder in an aqua- 

 rium, although one may be perfectly sure that it is to be 

 found at the bottom of a particular tank. Some of 

 them have special pigment cells, like those of the chame- 

 leon, which they squeeze out in varying proportions till 

 they exactly resemble their surroundings ; and as this 

 action ceases when the fish is blind, it shows that the 

 protected fish themselves, as well as their enemies, are 

 conscious of minute differences in form and color. All 

 the animals which inhabit the sargasso weed are also col- 

 ored exactly like it, and so closely imitate it in many 

 ways that I have often narrowly examined a piece of the 

 weed freshly brought up in a bucket, and yet failed to 

 detect any sign of life till 1 lifted the spray from the 

 water and so compelled the hide-aways to reveal them- 

 selves. There is one pipe-fish, indeed, from the Austra- 

 lian coasts, which so exactly mimics the fucus in which 

 it lurks that nobody would believe it is a fish rather than 

 a branch of the weed round which it curls, until he has 

 dissected it. The necessity for such close resemblances 



