Popular Science Monthly/Volume 5/August 1874/Color in Animals
THE variety of coloring in animal life is one of the marvels of Nature, only now beginning to be studied scientifically. It is vain to say that an animal is beautiful, either in symmetry or diversity of color, in order to please the human eye. Fishes in the depth of the Indian seas, where no human eye can see them, possess the most gorgeous tints. One thing is remarkable: birds, fishes, and insects, alone possess 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 organic substance. Such is the brilliant paint, carmine, which is the pigment of the cochineal insect, and the red color of blood, which maybe collected in crystals, separate from the other particles to which it is united.
Even the powder not unknown to ladies of fashion is one of Nature's beautifying means. That which is left on the hands of the ruthless boy, when he has caught a butterfly, is a common instance; but there are birds, such as the large white cockatoo, which leave a white powder on the hands. An African traveller speaks of his astonishment on a rainy day to see his hands reddened by the moist plumage of a bird he had just killed. The most ordinary way, however, in which the pigment is found is when it exists in the depths of the tissues reduced to very fine particles, best seen under the microscope. When scattered, they scarcely influence the shade; but, when close together, they are very perceptible. This explains the color of the negro: under the very delicate layer of skin which is raised by a slight burn there may be seen abundance of brown pigment in the black man. It is quite superficial, for the skin differs only from that of the European in tone; it wants the exquisite transparency of fair races. Among these, the colors which impress the eye do not come from a flat surface, but from the different depths of layers in the flesh. Hence the variety of rose and lily tints according as the blood circulates more or less freely; hence the blue veins, which give a false appearance, because the blood is red; but the skin thus dyes the deep tones which lie beneath it; tattooing with Indian-ink is blue, blue eyes owe their shade to the brown pigment which lines the other side of the iris, and the muscles seen under the skin produce the bluish tone well known to painters.
The chemical nature of pigment is little known; the sun evidently favors its development in red patches. Age takes it away from the hair when it turns white, the coloring-matter giving place to very small air-bubbles. The brilliant white of feathers is due to the air which fills them. Age, and domestic habits exchanged for a wild state, alter the appearance of many birds and animals; in some species the feathers and fur grow white every year before falling off and being renewed; as in the ermine, in spring the fur which is so valued assumes a yellow hue, and, after a few months, becomes white before winter.
It would, however, be an error to suppose that all the exquisite metallic shades which diaper the feathers of birds and the wings of butterflies arise from pigments; it was a dream of the alchemists to try to extract them. Their sole cause is the play of light, fugitive as the sparkles of the diamond. When the beautiful feathers on the breast of a humming-bird are examined under the microscope, it is astonishing to see none of the shades the mystery of which you would penetrate. They are simply made of a dark-brown opaque substance not unlike those of a black duck. There is, however, a remarkable arrangement; the barb of the feather, instead of being a fringed stem, offers a series of small squares of horny substance placed point to point. These plates, of infinitesimal size, are extremely thin, brown, and, to all appearance, exactly alike, whatever may be the reflection they give. The brilliant large feathers of the peacock are the same; the plates are only at a greater distance, and of less brightness. They have been described as so many little mirrors, but that comparison is not correct, for then they would only give back light without coloring it. Neither do they act by decomposing the rays which pass through them, for then they would not lose their iris tints under the microscope. It is to metals alone that the metallic plumage of the humming-birds can be compared; the effects of the plates in a feather are like tempered steel or crystallized bismuth. Certain specimens emit colors very variable under different angles, the same scarlet feather becoming, when turned to ninety degrees, a beautiful emerald green.
The same process which Nature has followed in the humming-bird is also found in the wing of the butterfly. It is covered with microscopic scales, which play the part of the feather, arranged like the tiles of a house, and taking the most elegant forms. They also lose their color under magnifying power, and the quality of reflection shows that the phenomena are the same as in feathers. There is, however, a difference in the extent of the chromatic scale. While the humming-bird partakes in its colors of the whole of the spectrum from the violet to the red, passing through green, those of the butterfly prefer the more refrangible ones from green to violet, passing through blue. The admirable lilac shade of the Morpho menelas and the Morpho cypris is well known, and the wings of these butterflies have been used by the jewelers, carefully laid under a thin plate of mica, and made into ornaments. A bright green is not uncommon, but the metallic red is rare, excepting in a beautiful butterfly of Madagascar, closely allied to one found in India and Ceylon. The latter has wings of a velvet black with brilliant green spots; in the former, these give place to a mark of fiery red.
There is the same difference between the metallic hues of creatures endowed with flight and the iris shades of fishes, that there is between crystallized bismuth and the soft reflections of the changing opal. To have an idea of the richness of the fish, it is only necessary to see a net landed filled with shad or other bright fish. It is one immense opal, with the same transparency of shade seen through the scales, which afford the only means of imitating pearls. It is due, however, not to the seales, but to extremely thin layers lying below the scales under the skin and round the blood-vessels, which look like so many threads of silver running through the flesh. Réaumur first noticed and described them; sometimes their form is as regular as that of a crystal, and of infinitesimal size and thickness. The art of the makers of false pearls is to collect these plates in a mass from the fish, and make a paste of them with the addition of glue, which is pompously named "Eastern essence." This is put inside glass beads, and gives them the native whiteness of pearls.
Many observations have been made lately by our naturalists as to the defense which color supplies to animals: hares, rabbits, stags, and goats, possess the most favorable shade for concealing them in the depths of the forest or in the fields. It is well known that when the Volunteer corps were enrolled, and the most suitable color for the riflemen was discussed, it was supposed to be green. Soldiers dressed in different shades were placed in woods and plains, to try which offered the best concealment. Contrary to expectation, that which escaped the eyes of the enemy was not green, but the fawn color of the doe. Among hunting quadrupeds, such as the tiger, the leopard, the jaguar, the panther, there is a shade of skin which man has always been anxious to appropriate for his own use. The old Egyptian tombs have paintings of the negroes of Soudan, their loins girt with the fine yellow skins for which there is still a great sale. All the birds which prey upon the smaller tribes, and fishes like the shark, are clothed in dead colors, so as to be the least seen by their victims.
There is an animal which, for two thousand years, has excited the curiosity and superstition of man by its change of color—that is, the chameleon. No reasonable observation was ever made upon it, until Perrault instituted some experiments in the seventeenth century. He observed that the animal became pale at night, and took a deeper color when in the sun, or when it was teased; while the idea that it took its color from surrounding objects was simply fabulous. He wrapped it in different kinds of cloth, and once only did it become paler when in white. Its colors were very limited, varying from gray to green and greenish brown.
Little more than this is known in the present day; under our skies it soon loses its intensity of color. Beneath the African sun, its livery is incessantly changing; sometimes a row of large patches appears on the sides, or the skin is spotted like a trout, the spots turning to the size of a pin's head. At other times, the figures are light on a brown ground, which a moment before were brown on a light ground, and these last during the day. A naturalist speaks of two chameleons which were tied together on a boat in the Nile, with sufficient length of string to run about, and so always submissive to the same influences of light, etc. They offered a contrast of color, though to a certain degree alike; but, when they slept under the straw chair which they chose for their domicile, they were exactly of the same shade during the hours of rest—a fine sea-green that never changed. The skin rested, as did the brain, so that it seemed probable that central activity, thought, will, or whatever name is given, has some effect in the change of color. The probability is that, as they become pale, the pigment does not leave the skin, but that it is collected in spheres too small to affect our retina, which will be impressed by the same quantity of pigment when more extended.
It is undoubtedly the nerves which connect the brain with organs where the pigment is retained. By cutting a nerve, the coloring-matter is paralyzed in that portion of the skin through which the nerve passes, just as a muscle is isolated by the section of its nerve. If this operation be performed on a turbot when in a dark state, and thrown into a sandy bottom, the whole body grows paler, excepting the part which cannot receive cerebral influence. The nerves have, in general, a very simple and regular distribution; if two or three of these are cut in the body of the fish, a black transversal band following the course of the nerve will be seen; while, if the nerve which animates the head is thus treated, the turbot, growing paler on the sand, keeps a kind of black mask, which has a very curious effect.
These marks will remain for many weeks, and what may be called paralysis of color has been remarked in consequence of illness or accident. Such was seen in the head of a large turbot, the body being of a different color. It was watched, and died after a few days, evidently of some injury which it had received. The subject offers a field of immense inquiry; the chemical and physical study of pigments, the conditions which regulated their appearance, their intensity, and variations under certain influences; the want of them in albinos, and the exaggerated development in other forms of disease. To Mr. Darwin, and to M. Ponchet, in France, the subject is indebted for much research, which will no doubt be continued as occasion offers.—Chambers's Journal,