22 CLASSIFICATION AND ADAPTATION 



different explanations, to two different processes 

 going on throughout the organic world and affecting 

 every individual and every group in classification. 



The Flat-fishes, now regarded not as merely a 

 family but a sub-order of Teleosteans, afford 

 a good example of the contrast between adaptive 

 and non-adaptive diagnostic characters. For the 

 whole group the adaptive characters are diagnostic, 

 distinguishing it from other sub-orders. It is 

 conceivable that different phyletic groups of fishes, 

 that is fishes of different descent, might have been 

 modified in the same way, as, for instance, grass- 

 hoppers and fleas have been adapted for leaping 

 without being closely related to each other. It is 

 generally held, however, that the Flat-fishes are 

 of common descent. In this group the adaptive 

 characters are diagnostic ; that is to say, they dis- 

 tinguish the group from other sub-orders, though 

 there are other non-adaptive characters which indi- 

 cate the relationship to other groups and which are 

 not adapted to the horizontal position of the original 

 median plane of symmetry. The principal adaptive 

 characters are : both eyes and the pigmentation 

 on the side which is uppermost in the natural 

 position, lower side without eyes and colourless ; 

 dorsal and ventral fins continuous and extending 

 nearly the whole length of the dorsal and ventral 

 edges ; dorsal fin extending forwards on the head, 

 not along the morphological median line, which is 

 between the eyes, but between the more dorsal eye 

 and the lower side of the body, in the same horizontal 

 plane as the posterior part of the same fin. The 

 4 adaptive ' quality in these characters, as in other 

 cases, does not necessarily consist in their utility 



