Fig. 1.—Egg of Sole, and Stages in its Growth.
The difference in the colouration of these two sides is a matter to which we must now refer. As everybody knows, the upper side is dark-coloured, while the under side is white. Why is this? Why are not the colours reversed, or why are not both sides coloured? These questions open up a most fascinating study—the use and meaning of the colours of animals. And you will find, when you come to look into the matter, that there is a very close relation between the colour of an animal and the nature of its surroundings. In the case of the sole, the brown upper surface, from its resemblance to the mud and sand at the bottom of the sea, serves to conceal it from the sharp eyes of prowling fishes on the look-out for a meal. A broad expanse of white would at once betray it to the enemy. No colour is developed on the under surface, for it would be a waste of energy to produce colour for a surface that was kept constantly concealed from view.
Fig. 2.—Full-grown Sole.
Although, in our picture, all these fish can be seen quite plainly, in real life they are quite hard to find. The young, being well-nigh transparent as glass, are almost invisible as they float in the water; while later, when these wanderings cease, and they settle down to a quiet life, the dark colour forms an equally invisible covering.
Prairie Dogs.