The Living Age 1569, 4.7.1874
From Chambers' Journal.
The variety of colouring 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 colour, in order to please the human eye. Fishes in the depths 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 colouring; whilst plants and zoophytes are without reflectig shades. The mollusca take a middle path with their hue of mother-of-pearl. What is the reason of these arrangements in the animal kindgom? Its 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 oft he body turned towards 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; whilst 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 colouring 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 alsmot closed shell, like the oyster, are nearly colourless; 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 curous 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 floowing of the cases like larvæ, without etes, which would be useless in the retreats where they usually dwell.
This relation between colouring 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 coloured, 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 colouring 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 colours are not bound up with the tissues. Sometimes they arise from a phenomenon like that by which the soap-bubble shews 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 colour of blood, which may be 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 too see his hands reddened by the moist plumage of a bird he has 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 a 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 colour 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 wuite superficial, for the skin differs only from that of the European in tone; it wants the exquisite transparency of fair races. Among these, the colour 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 apprearance, because the blood is red; but the skin thus dyes the feep 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 luttle known; the sun evidently favours its development in red patches. Age takes it away from the hair when it turns white, the colouring-matter giving place to very small air-bubbles. The brilliant white of feathers is due to the air which fills tem. 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 butterflues 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 colouring it. Neither do they act by decomposing the rays which pass through them, for then tehy 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 colours 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 to the most elegant forms. They also lose their colour under magnifying power, and the quality of redlection shews that the phenomena are the same as in feathers. There is, however, a difference in the extent of the chromatic scale. Whilst the humming-bird partakes in its colours off the whole of the spectrum from the violet to the red, passing through green, those of the butterflies 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 jewellers, 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 scales, but to extremely thin layers lying below the scales under the skin and round the bloodvessels, 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 defence which colour supplies to animals: hares, rabbits, stags, and goats possess the most favourable 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 colour for the riflemen wasdiscussed, 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 colour of the doe. Among hunting quadrupeds, such as the tigerm the leopard, the jaguar, the panther, there is a shade of skin which man has always been anxious to use. The old Egyptian tombs have painting of the negroes of Sudan, 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 colours, 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 colour - 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 colour when in the sun, or when it was teased; whilst the idea that it took its colour 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 colours 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 colour. Beneath the African sun, its livery is inscessantly 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, &c. They offered a contrast of colour, 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 colour. 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 colouring-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; whilst, if the nerve which animates the head is thus treated, the turbot growing pler on hte 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 colour 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 colour. It was watched, and died after a few days, evidently of some injury which it had received. The subject offers a field of imemnse inquiry: the chemical and physical stury of pigments, the conditions which regulate 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, in England, and to M. Ponchet, in France, the subject is indebted for much research, which will no doubt be continued an occasion offers.