order to meet the requirements of the change of structure.

The present writer, on the other hand, believes that not only are adaptive
characters distinct from non-adaptive specific characters, and from
non-adaptive diagnostic characters in general, but that their origin and
evolution are entirely distinct and different. There are two separate
problems, the origin of adaptations and the origin of species, and the
investigation of these two problems leads not to one explanation common to
both, but to two entirely 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, 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