Page:Popular Science Monthly Volume 5.djvu/487

Rh the metallic coloring; while plants and zoophytes are without reflecting shades. The mollusca take a middle path with their hue of mother-of-pearl. What is the reason of these arrangements in the animal kingdom? It is a question which cannot be satisfactorily answered; but some observations have been made which throw light on the subject. One is, that among animals, the part of the body turned toward the earth is always paler than that which is uppermost. The action of light is here apparent. Fishes which live on the side, as the sole and turbot, have the left side, which answers to the back, of a dark tint; while the other side is white. It may be noticed that birds which fly, as it were, bathed in light, do not offer the strong contrast of tone between the upper and lower side. Beetles, wasps, and flies, have the metallic coloring of blue and green, possess rings equally dark all round the body; and the wings of many butterflies are as beautifully feathered below as above.

On the other hand, mollusca which live in an almost closed shell, like the oyster, are nearly colorless; the larvæ of insects found in the ground or in wood have the same whiteness, as well as all intestinal worms shut up in obscurity. Some insects whose life is spent in darkness keep this appearance all their lives; such as the curious little beetles inhabiting the inaccessible crevasses of snowy mountains, in whose depths they are hidden. They seem to fly from light as from death, and are only found at certain seasons, when they crawl on the flooring of the caves like larvæ, without eyes, which would be useless in the retreats where they usually dwell.

This relation between coloring and light is very evident in the beings which inhabit the earth and the air; those are the most brilliant which are exposed to the sun; those of the tropics are brighter than in the regions around the north-pole, and the diurnal species than the nocturnal; but the same law does not apparently belong to the inhabitants of the sea, which are of a richer shade where the light is more tempered. The most dazzling corals are those which hang under the natural cornices of the rocks and on the sides of submarine grottoes; while some kinds of fish, which are found on the shores as well as in depths requiring the drag-net, have a bright-red purple in the latter regions, and an insignificant yellow brown in the former. Those who bring up gold-fish know well that, to have them finely colored, they must place them in a shaded vase, where aquatic plants hide them from the extreme solar heat. Under a hot July sun they lose their beauty.

The causes to which animal coloring is due are very various. Some living substances have it in themselves, owing to molecular arrangement, but usually this is not the case; the liveliest colors are not bound up with the tissues. Sometimes they arise from a phenomenon like that by which the soap-bubble shows its prismatic hues; sometimes there is a special matter called pigment which is united with the