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 characters, all related to habits or external conditions. We may say that the third kind are correlated with some other character that has a relation to function or external conditions, as the extension of the fins on the under side of _Zeugopterus_ is correlated with the enlargement of the fins, whose function is to cause the adhesion of the fish to a vertical surface. With regard to the specific characters of the species of _Zeugopterus_ nothing is known of peculiarities in mode of life which would give an importance in the struggle for existence to the concrescence of the pelvic fins with the ventral in _punctatus_, to the absence of this character and the elongation of the first dorsal ray in _unimaculatus_, or to the absence of both characters in _norvegicus_. No use is known for any of the other specific characters, which tend in each case to form a series. Thus in size _norvegicus_ is the smallest, _unimaculatus_ larger, and _punctatus_ largest, the last reaching a of 8-1/2 inches. The subcaudal fin-flaps are developed in _norvegicus_, most in _punctatus_; each has four rays in _norvegicus_ and _unimaculatus_, six in _punctatus_. The shortening and spinulation of the scales are greatest in _punctatus_, least in _norvegicus_. In _punctatus_ there are teeth on the vomer, in _unimaculatus_ none, in _norvegicus_ they are very small. If we consider fishes in general, we see that there is no evidence of any relation between many of the most important taxonomic characters and function or external conditions. In the seas Elasmobranchs and Teleosteans exist in swarming numbers side by side, but it is impossible to say that one type is more adapted to marine life than the other. There is good reason to believe that bony fishes were evolved from Elasmobranchs in fresh water which was shallow and foul, so that lungs were evolved for breathing air, and that marine bony fishes are descended from fishes with lungs; but no reason has been given for the evolution of bone in place of cartilage or for the various kinds of scales. Professor Houssaye, on the other hand, believes that the number and position of fins is adapted to the shape and velocity of movement of each kind of fish.