Popular Science Monthly/Volume 28/November 1885/Origin of Color in Animals
|ORIGIN OF COLOR IN ANIMALS.|
OF all the characteristics of organized bodies, color is one of the most fugitive. Trifling variations in the individual constitution, apparently slight changes in the biological conditions to which it is subject, are often sufficient to induce considerable modifications in the exterior coloration. Color in animals may, therefore, be regarded as having a variety of origins. Sometimes it is due to the fact that the tissues are formed from colored material; more frequently to their having imbibed a colored fluid. This is generally the case with the formations of the epidermis, the hairs of animals, the feathers of birds, and the scales of reptiles. The translucid nature of the teguments may also be the cause of external coloration, as in men of the white race, whose delicate skin exhibits the vessels of the underlying tissues. Many invertebrates are so transparent that their internal organs may be seen. In the majority of cases, animals owe their external hues to colored granulations or pigments, which, diffused through the tissues, give tints varying with their abundance or distribution. This substance may be black, or brown, or yellow in the vertebrates, while red, yellow, blue, and green predominate among the invertebrates. The phenomena of interference presented by their laminæ, dependent upon the reciprocal action of parallel waves of light of different velocities, and capable in their different combinations of producing all the colors of the rainbow, or the absence of color, furnish that dazzling chromatic gamut which Nature employs to paint the humming-bird and the butterfly, those two jewels of the organic world. Another class of phenomena has been called cerulescence by M. G. Pouchet. It is a property which he regards as analogous to fluorescence, and as due, in the majority of cases, to stick-shaped bodies inclosed in special cells called iridocytes. The blue reflections presented by the scales of most fishes, the blue color of the caruncles of many birds, and the naked parts of some monkeys, the azure tint of the veins of individuals of the white race, the blue of the iris of some persons' eyes, are examples of cerulescence. These phenomena, however, differ but little from those which give to water having drops of milk suspended in it a bluish color by reflection, or from those which make smoke appear blue when seen upon a black ground. It seems to me that it may be going too far to compare such phenomena with fluorescence.
These causes of coloration may be superposed and combined in a thousand ways. When birds are under the influence of physiological excitations like those of rage or love, the flow of blood contributes to enliven the color of the bare parts to the point of greatly modifying it. The bright, metallic tints of the peacock and the humming-bird are due to phenomena of interference and to the presence of a dark pigment combined; the green tint of the lizard to the association of a yellow pigment and blue-reflecting iridocytes. The Annelids and the Nemertes, of the invertebrates, exhibit the combined effects of three causes of coloration: iridization, produced by the thin cuticle; the rich pigmentation of the dermis, and frequently, also, in case the integuments are transparent, the variable coloration of the sanguineous fluid and of the internal organs.
The intensity of coloration is generally proportioned to the vital activity. As life begins to decline, the pigment retires from the formations of the epidermis; and the hairs on regions which have passed maturity often exhibit a lighter coloring than on the neighboring regions. According to Pruner-Bey, the intensity of the color of the negro is an indication of his health; old negroes grow pale as they age. It is well known that pain and depressing moral trials, which are negative facts in life, provoke the retraction of the pigment. On the contrary, everything that tends to accentuate life occasions an enlivening of the intensity of colors, a fact of which Darwin gives many examples in his "Variation of Animals and Plants." Coloration is strongest in adult animals. Breeders prefer animals rich in pigment-matter, because they will best resist disease, and most easily accommodate themselves to special systems of feeding. The ancients regarded animals having white hair on a black skin as the most vigorous. White parts of animals arc often attacked with disease, while the other parts remain healthy; and light-skinned animals are most troubled by flies and parasites. Albinoism, which is simply a total inaptitude for the production of pigment, is a sure sign of degeneracy.
Vigor of the genital organs is one of the most manifest signs of vital activity. The relation between the reproductive function and pigmentation is so striking that Heusinger has expressed it as a law. Troubles brought upon the sexual functions under the influence of any particular causes, as of domestication, often coincide with the most singular modifications of color.
The coloring-matter is also intimately connected with the nervous system. Thus, it is at the extremity of a nerve, the optic nerve, that is localized, in all species of animals, the maximum of aptitude for the production of pigment. In the lowest types of the series, when the eye begins to become differentiated, and while it can hardly yet be considered an organ of vision, a pigment-spot may be observed to make its appearance. At the same time other parts of the optical apparatus that have a much greater functional importance, the refracting media, for example, may not yet be existing even in a rudimentary state. These considerations lead me to believe that the optical pigment-spot owes its existence not solely to the advantages which the individual may derive from it, but chiefly to the proximity of a nerve, the elements of which are disturbed by a continuous vibrating movement, or by light. This kind of election of pigment exists, moreover, not only in reference to the organ of sight, but frequently also in other special sensitive terminations—at the ends of the auditory nerves with some invertebrates, at the end of the proboscis in the Nemertes. In the chameleon, the turbot, the cuttle-fish, and some other animals, the connection of the pigmentary system with the nerves is so close that a simple nervous excitation is enough to modify the distribution of the colored granulations in the integuments.
On the other hand, certain constitutional defects induce a diminution or absence of coloring-matter, of which I can give no better illustration than to cite Darwin's curious observation that white cats generally have blue eyes and are deaf.
What we have said tends to prove that the positive facts of life, or the complete development of the organs of the individual—health, strength, fullness of functions, display of activity and accentuation of animal vigor in the nervous system and the organs of relation—correspond closely with an abundant production of coloring-matter; while the negative facts of life—age, constitutional weakness, disease, apathy, and degeneration generally—lead to a more or less complete disappearance of the same substance. Nevertheless, we notice in some cases the contrary fact, or a deposition of coloring-matter, or an increase of its production in connection with some pathological condition of the organism. But these cases, which seem opposed to our theory, are generally susceptible of special explanations, and their contradiction of the other facts is only apparent.
In consideration of the influence of external agencies on coloration, we distinguish between two classes: those forces which can be resolved into a rapid vibration—light, heat, and electricity—the action of which is very marked; and other more complex agencies, among which we include food, captivity, moisture, and the colorizing and decolorizing action of some secretions. Light is the principal excitant capable of provoking the development of coloring-matter. Very significant on this point is M. Paul Bert's account of his experiments with the larvæ of the axolotl: "Pale on issuing from the egg, they become colored by the deposition of pigment under the influence of light. In the dark, or in red light, the pigment is not developed." From this we learn that the less refrangible rays have no influence on the production of pigment; it is therefore by the rapidity, and not by the amplitude of its vibrations, that light acts upon the formation of coloring-matter. An analogous example is furnished by the Proteus, which, having been drawn out from its dark hole, becomes gradually colored by light. We may compare with these observations that the negro baby is, when first born, of only slightly different color from the white; and the fact that certain parts of his body may already show the negro tinge does not contradict our theory of the dependence of the color on the action of light, but is only the mark of a hereditary tendency to become black. I do not intend to assert that light is the sole cause of pigment-coloration, for that would be contrary to the facts; but it is generally the exciting and sometimes the necessary means for the development of the coloring-matter. It plays a part like that of the spark in combustion, which has no effect upon an incombustible body, in the same way that light produces no colorizing effect upon an albino. There is, then, an aptitude to become colored, which varies according to races, and may not always exist. The question, however, of the ultimate cause of coloration is not solved, but only pushed back; for we are ignorant of the cause of this aptitude, and are obliged, to explain it, to have recourse to the laws of heredity and natural selection.
The rich coloration of deep-sea animals apparently contradicts the facts we have cited, but does not really do so. For it is principally the red, or less refrangible, neutral rays, the passage of which is interrupted by the water, while the blue, violet, and ultra-violet rays, which are the active ones in coloration, pass through it to a considerable depth. Furthermore, we know that the molecules composing the tissues of these animals are subject to vibratory movements analogous to those of light, which are represented to us by phosphorescence; and we may conceive those vibrations to be intense enough to produce a coloration like that which is the effect of sunlight.
As a rule, the parts of animals most exposed to rays of light are, other things being equal, richest in coloring-matter. The backs of wild animals are usually and with few exceptions (as among nocturnal and burrowing animals) more strongly colored than their bellies. Another class of exceptions may be seen among fishes of certain families which lie on their sides instead of on their bellies, and expose, not their backs, but one of their sides to the light. In these fishes the upper side is colored, while the under side, next to the ground and the darkness, is not. Articulates also have their upper sides most strongly colored, although what in them answers most nearly to the dorsal column is next to the ground. The parts of the shells of mollusks which are in contact with the ground are uncolored, while the parts exposed to the light shine with varied tints; and this, whatever may be the peculiar positions assumed by particular shells.
For individuals of the same race, the abundance of the coloring matter is generally proportioned to the intensity of the light to which they are exposed. This fact is generally understood, though exact observations bearing upon it are not as numerous as it is desirable they should be. It is well known that the skin is tanned by light, that people from the north are browned by living in the south, and that ruddiness and freckles appear under the action of the sunlight. Some peoples of the white race, like the Hindoos and the Moors, that live in southern climates, are frequently darker-skinned than the negroes themselves. Still, we can not affirm that light is the only cause of these changes.
Mr. Gould has observed that birds are more strongly colored when they live in countries having a clear sky than on islands or the seashore. Berchstein says that the colors of the plumage of cage-birds are affected by the shade in which they are kept. Mr. Allen has shown that the color of several species in the United States changes as we go from north to south.
On account of their close relations with one another, it is hard to distinguish the effects of heat on color from those of light. External temperature can not have much effect upon the skin of warm-blooded animals whose bodies are kept by the internal heat at a uniform degree; but with the fur it is different, and it is possible that cold may induce an abstraction of coloring-matter from the hairs, and that the white color of animals of the polar zone may be partly owing to this fact. According to Pallas, the horse and the cow in Siberia become paler during the winter. The ermine seldom becomes as white during winter in England as in Norway. Its summer color persists till late in the season, when the extreme cold comes on, and then changes in a few days. The Mr. Nicholas Wagner, using an exceedingly sensitive galvanometer, has discovered fixed currents in the wings of butterflies; and, with the aid of electric currents, has succeeded in producing changes in the color and disposition of their pigments. What part electricity may play in this matter is still, however, unsettled.fox, which in the polar regions becomes white in winter from brownish-gray, changes but little when taken to Europe. The Alpine hare does not put on its white dress at a fixed period, but at a time that depends on the greater or less earliness of the beginning of winter.
In regard to the effects of feeding, Darwin cites cases of complete changes in the color of birds brought about by modifications of their alimentation. Bullfinches, fed with hemp-seed, turned black. The common green paroquet, fed with the fat of certain fishes, became striped with red and yellow.
Volumes have been written on the influence of natural selection upon color, and have elucidated the subject so fully that we need not dwell on it at length. The principal aspect in which the influence asserts itself is that in which the prevailing color among animals gives them a kind of resemblance to the ground on which or the medium in which they live, or to the objects by which they are surrounded, so that they are more readily hidden from their enemies. In other cases they are made conspicuous in color or to resemble disagreeable objects, so that their enemies, mistaking them for something else, shall avoid them. Such cases belong to the classes of phenomena which Mr. Wallace has grouped under the designation of protective mimicry. In other cases, certain colors may be associated with peculiarities that render the animal more capable of resisting peculiar conditions to which it may be exposed; when natural selection, aided by selection by the breeder, may contribute to preserve this color to the exclusion of others. Thus, according to Darwin, in Virginia, black hogs alone can endure a course of feeding consisting largely of the roots of Lachnantes tinctoria; so a race of black hogs became established in that country.
Much more might be said on this subject. We might consider the phenomena of sexual selection to which male birds largely owe their bright plumage; the heredity of colors, correlative variations, and the complex and obscure action of domestication; the action of moisture and of some secreted principles; and the distribution of colors as related to geographical regions. What I have said has been really only introductory to the subject, and for the purpose of reminding investigators what a full field of work they might find in exhaustively following it up.—Translated for the Popular Science Monthly from the Revue Scientifique.