26 NOTES ON FISH AND FISHING. 



heat in any animal body ; and the natural heat of fish is 

 very low. Here, then, I will leave this point ; and my 

 readers will again notice that I leave it, as I have left 

 others, without any special expression of my own opinion ; 

 my object being rather to show how many interesting 

 questions there are in connexion with ichthyology, and 

 the almost endless field of study it still offers us. 



Several fish, I mean fish of the same species, exhibit a 

 diversity and change of Colouring, dependent on the 

 different seasons of the year, the depth of water in which 

 they swim, the nature of the ground and vegetation about 

 them, and the food they take ; but how this diversity and 

 change is brought about seems to be still a vexed question. 

 Sea fish are subject to these variations, particularly had- 

 dock, sea-bream, whiting-pollack, soles, conger-eels, and 

 cod ; while the turbot has the power of darkening its 

 spots, or perhaps better say its spots are darkened under 

 the influence of fear. Among our own fresh-water fish 

 trout are the most susceptible of these changes, and they 

 will vary according to the depth of water and different 

 conditions of the different parts of a lake in which they 

 are taken. If one of two trout of the same colouring be 

 placed in a vessel with yellow glaze inside, and the other 

 in a vessel with brown or black glaze, the fish will 

 quickly assume the hue of the respective vessels; but 

 change them from one vessel into the other, and again 

 they quickly adapt themselves in colour to their surround- 

 ings. The same phenomenon may be observed in the case 

 of minnows, which change their hues in a bait-kettle 

 according as its inside is of bright tin, or lined with a dark 

 colour. Thus some fish seem to have " the perception of 

 congruity, and put their coats in agreement with near 



