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, grasshoppers 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 distinguish the group from other sub-orders, though there are other non-adaptive characters which indicate 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 'adaptive' quality in these characters, as in other cases, does not necessarily consist in their utility to the animal, but in the definite relation between them and the external conditions. When the relation 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 resemblance 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