colored brown or gray, and still more those which were actually black or light pink, would be at once spotted, snapped up, and devoured. Hence in every generation the ever-surviving sole or turbot was the one whose spots happened most closely to harmonize with the general coloration of the surrounding bottom. As these survivors would alone intermarry and bring up future families of like-minded habits, it would naturally result that the coloration would become fixed and settled as a hereditary type in each particular species. Meanwhile, the eyes of the enemies of flat-fish, ever on the lookout for a nice juicy plaice or flounder, would become educated by experience, and would grow sharper and ever sharper in detecting the flimsy pretenses of insufficiently imitative or irregularly colored individuals. Natural selection means in this case selection by the hungry jaws of starving dog-fish. When once the intelligent dog-fish has learned to appreciate the fact that all is not sand that looks sandy, you may be sure he exercises a most vigilant superintendence over every bank he happens to come upon. None but the most absolutely indistinguishable soles are at all likely to escape his interested scrutiny.
The mere nature of the bottom upon which they lie has thus helped to become a differentiating agency for the various species and varieties of flat-fish. Soles, which easily enough avoid detection on the sandy flats, would soon be spotted and exterminated among the pebbly ridges beloved of plaice, or the shingly ledge especially affected by the rough-knobbed turbot. Flounders, whose coloring exactly adapts them to the soft ooze and shallow mud-banks at the mouths of rivers, would prove quite out of place on the deep pools of the channel, covered with pale-yellow sand, where the pretty lemon sole is most at home. In the case of the true sole, too, the long, graceful, sinuous fringe of fins is so arranged that it can fit accurately to the surface on which the fish is lying, and so add in a great measure to the appearance of continuity with the neighboring sands. A sole, settling down on a ribbed patch of sand, can thus accommodate its shape to the underlying undulations, so that it is almost impossible to distinguish its outline, even when you know exactly where to look for it. Soles are very clever at choosing such deceptive hiding-places, and very seldom openly expose themselves on a flat horizontal surface. Moreover, whenever they settle, they take care partially to bury themselves in the sand, with a curious sidelong flapping motion, and so still more effectually screen themselves from intending observers.
I may note in passing that such correspondence in color with the general hue of the surrounding medium is especially common wherever a single tone predominates largely in the wider aspect of nature. Arctic animals, as everybody knows, are always white. Ptarmigan and northern hares put on a snowy coat among the snows of winter. The uncommercial stoat needlessly transforms himself on the approach of cold weather into the expensive and much-persecuted ermine.
