24 CLASSIFICATION AND ADAPTATION 



utility, but relation to surroundings or to function 

 or to habit. In this case colour is related to in- 

 cidence of light, absence of colour to absence of light. 

 Position of eyes is also related to light; they are 

 situated where they can see, absent from the side 

 which is shut off from light. The marginal fins are 

 extended where their movements best support and 

 move the body. 



It is to be noted also that these adaptations of 

 different organs of the body, eyes, fins, colour, are 

 entirely independent of each other physiologically. 

 It may appear on first consideration that eyes and 

 colour, being both on the upper side, may have 

 been somehow connected in the constitution of the 

 body, whereas the only connexion is external in 

 their common relation to light. This independence 

 is well shown in the modification of the dorsal 

 fin: if this were physiologically affected by the 

 change in the eyes, which is brought about by 

 the twisting of the interorbital region of the skull, 

 the anterior end of the fin would be between the 

 two eyes, since the morphological median line of 

 the body is in that position. In fact, on the con- 

 trary, the attachment of the dorsal fin is continued 

 forward where it is required for its mechanical 

 function, regardless entirely of the morphology of 

 the head. 



This is even more clearly evident in the structure 

 of the jaws and teeth. These are entirely un- 

 affected by the torsion of the interorbital part of 

 the skull. In cases where the mouth is large and 

 teeth are required on both sides, the prey being 

 active fish of other species, as in Turbot, Brill, and 

 Halibut, the jaws and teeth are equally developed 



