THE TROl'T JIMP. 39 



developed ; the optic nerves arc bi^s and arran;;cd in the 

 ^,'ime manner as amon^^ the hi^lier animals ; and the 

 optic centres form by far the largest portion of the 

 brain. On the other hand, the olfactory nerves and 

 centres arc small and shrivelled. The indications of 

 habit are certainly rather inferential, yet they all point 

 in the same direction as these structural facts. 



Mcst common ush certainly find their food main!)- by 

 means of si<,dit. The careful way in which it is necessary 

 to imitate flies in order to deceive the wary trout shows 

 that they can pretty accurately distin^^uish forms and 

 [colours. 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 amoni,'' man)- 

 species similarly prove how sharp are the eyes of their 

 enemies ; for these resemblances can only have been 

 developed 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 colours and arrancjcment of the sand or 

 pebbles on which they lie : and it is often difficult even 

 for a human eye to detect a sole or a flounder in an 

 aquarium, 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 cha- 

 meleon, 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, 

 ire conscious of minute differences in form and colour. 

 lAU the animals which inhabit the sargasso weed are also 



