Page:Popular Science Monthly Volume 29.djvu/121

Rh, that our primitive ancestor was as clear as glass, and had his eye inside his brain, as is still the case with the ascidian larva. As soon as his descendants began to grow opaque, the eye was forced to push itself outward, so as to reach the surface of the body; and thus at last, we may imagine, it came to occupy its present prominent position on the full front of all vertebrate animals.

To return to our sole, however, whom I have left too long waiting in the sand to undergo his next transformation: as soon as he has selected a side on which to lie, he begins to grow dark, and a pigmentary matter forms itself on the upper surface exposed to the light. This is a very common effect of exposure, sufficiently familiar to ladies and others, and therefore hardly calling for deliberate explanation. But the particular form which the coloring takes in the true sole and in various other kinds of flat-fish is very characteristic, and its origin is one of the most interesting illustrations of, natural selection to be found within the whole range of animated nature. In every case it exactly resembles the coloration of the ground on which the particular species habitually reposes. For example, the edible sole lies always on sandy banks, and the spots upon its surface are so precisely similar to the sand around it that in an aquarium, even when you actually know from the label that there is a sole to be found in a particular tank, you can hardly ever manage to spot him as long as he lies perfectly quiet on the uniform bottom. Turbot, on the other hand, which prefers a more irregular pebbly bed, is darker brown in color, and has the body covered on its upper side with little bony tubercles, which closely simulate the uneven surface of the banks on which it basks. The plaice, again, a lover of open, stony spots, where small shingle of various sorts is collected together in variegated masses, has its top side beautifully dappled with orange-red spots, which assimilate it in hue to the party-colored ledges whereon it rests. In this last case the brighter dabs of color undoubtedly represent the bits of carnelian and other brilliant pebbles, whose tints of course are far more distinct when seen in water by refracted light than when looked at dry in the white and common daylight. We all know how much prettier pebbles always seem when picked up wet on the sea-shore than under any other circumstances.

Some few flat-fish even possess the chameleon power of altering their color, in accordance with the nature of the bottom on which they are lying. The change is managed by pressing outward or inward certain layers of pigment-cells, whose combination produces the desired hues.

The origin of this protective coloration must once more be set down to that deus ex machinâ of modern biology, natural selection. In the beginning, those flat-fish which happened to be more or less spotted and speckled would be most likely to escape the notice of their ever-watchful and rapacious foes; while those which were uniformly