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
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 utility, but relation to surroundings or to
function or to habit. In this case colour is related to incidence of