When we consider such cases as this we are led to the conclusion that the
usual conception of adaptation is not adequate. We require something more
than function or utility to express the difference between the two kinds
of characters to be distinguished. For example, the absence of
pigmentation from the lower sides of Flat-fishes may have no utility
whatever, but we see that it differs from the specific markings of the
upper side in the fact that it shows a relation to or correspondence with
a difference of external conditions--namely, the incidence of light, while
in such a case as the red spots of the Plaice we can discover no such
correspondence.

We know that the American artist and naturalist Thayer has shown that the
lighter colour of the ventral side of birds and other animals aids greatly
in reducing their visibility in their natural surroundings, the diminution
in coloration compensating for the diminution in the amount of light
falling on the lower side, so that the upper and lower sides reflect
approximately the same amount of light, and contrast, which would be
otherwise conspicuous, is avoided. But the white lower sides of
Flat-fishes are either not visible at all, or, if visible, are very
conspicuous, so that the utility of the character is very doubtful.

We may distinguish then between characters which correspond to external
conditions, functions, or habits, and those which do not. The word
'adaptation,' which we have hitherto used, does not express satisfactorily
the peculiarities of all the characters in the former of these two
divisions. If we consider three examples--enlarged hind-legs for jumping
as in kangaroo or frog, absence of colour from the lower sides of
Flat-fishes, and, thirdly, the finlets on the lower side of
_Zeugopterus_--we see that they represent three different kinds of