Consequently we find that the dorsal and ventral surfaces are almost invariably coloured differently, and the dorsal is the darker in the great majority of instances. Generally the spine is marked by a more or less defined central line, and hence this system of colouration may be termed axial, because it is in the direction of the axes, or applied about the axes. _Fishes._ Where fishes have not been modified out of their original form, as are the soles, plaice, and other flat fish, we find the dorsal region darker than the ventral, and even here the under surfaces are the lightest. Even in cases like the Char, Fig. 1, Plate IX., where vivid colour is applied to the abdomen, the dorsum is the darker. The dorsum is often marked by a more or less well-defined dark band, as in the mackerel and perch, Fig. 2, Plate IX. There are sometimes parallel bands at right angles to the above, as in the perch and mackerel; and this is a common feature, and apparently a very old one, as we find it in the young of fishes whose adults are without these rib-like marks, such as the trout and pike. It is only necessary to inspect any drawings of fishes to see that their colouration is on a definite principle, although rather erratic. Important functional parts, like the gills, fins, and tail, are generally marked in colour more or less distinctly, as may be seen, for instance, in our common fresh-water fishes, like the roach and perch. The line of mucus-secreting glands running along the sides is usually marked by a dark line. These facts point distinctly to structural decoration. [Illustration: Plate IX. CHAR AND PERCH.] There are in some fishes, like the John Dory, curious eye-like dark spots, which we cannot refer to a structural origin, though a better acquaintance with the class might reveal such significance. The Amphibia have not been well studied by us, and we must leave them with the remark that they seem to bear out the view of structural decoration, as is seen in our English newts. Some are, however, modified out of all easy recognition. _Reptiles._ Among the reptiles, the snakes, Fig. 4, may be selected for illustration. Snakes are practically little more than elongated backbones, and are peculiar from the absence of limbs. The colouring matter does not reside so much in the scales as in the skin beneath, so that the sloughs do not illustrate the decoration. Hence, we might expect to find here a direct effect of morphological emphasis. The ornamentation of snakes is very similar throughout the class, both in water and land snakes; as may be seen by Sir W. Fayrer's work on Venomous Snakes. This ornamentation is of a vertebral pattern, placed along the dorsal surface, with cross lines, which may represent ribs. Where the ribs are wanting, as in the neck, the pattern changes, and we get merely longitudinal markings.