112 THE POPULAR SCIENCE MONTHLY. 



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 insuffi- 

 ciently 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, cov- 

 ered 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 appear- 

 ance 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. More- 

 over, 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. 



