CLASSIFICATION AND ADAPTATION 23 



to the animal, but in the definite relation between 

 them and the external conditions. When the re- 

 lation is one of function, the organ may be said to be 

 useful : for example, the position of the two eyes 

 is adaptive because they are on the upper side where 

 alone light can reach them, the other side resting 

 on the ground ; and the adaptation is one of function, 

 and therefore useful, because if the eyes were in 

 their normal position one of them would be useless, 

 being generally in contact with the ground or buried 

 in it. Similarly with the extension of the dorsal 

 and ventral fins, the undulations of which serve to 

 move the fish gently along in a plane parallel to the 

 ground. If the dorsal fin was not extended forward, 

 the head would not be so well supported. But when 

 we consider the pigmentation of the upper side 

 and the normally white lower side, although the 

 adaptation is equally obvious, the utility is by no 

 means certain. To any naturalist who has observed 

 these fishes in the living state the protective re- 

 semblance of the pigmentation of the upper side is 

 very evident, especially because, as in many other 

 fishes and amphibians, the intensity of the colour 

 varies in harmony with the colour of the ground on 

 which the fish rests. But the utility of the white 

 lower side is not so easy to prove. Would the fish 

 be any worse off if the lower side were coloured like 

 the upper ? Probably it would not, although it 

 has been maintained that the white lower side serves 

 to render the fish less visible when seen against the 

 sky by an enemy below it. Ambicolorate specimens 

 occur, and there is no evidence that their lives are less 

 secure than those of normal specimens. The essential 

 and universal quality of adaptation, then, is not 



