Coloration (biology)

“ No one has yet shown any principles which will better explain the great color scheme of the animal world, and until such explanation is forthcoming I believe it will not be wise for us to discard the idea of protective coloration. ”

“ Thus, for example, the action of light on the surface of the wings when in the resting posture may well inhibit the production of vivid pigment owing to some inherent physiological idiosyncrasy. But any individuals which lack this inhibiting factor—as some species which, though resting, are brightly coloured, appear to do—will be eliminated, if they live in an environment harbouring eliminating factors, which the exceptions to the rule we must suppose do not. But on this interpretation the fundamental factor in the determination of the coloration is the action of light. ”

“ To those who believe that every spot or stripe or patch of colour on every animal is a beautiful illustration of the truth of the theory of protective coloration, this may seem a very fanciful idea. Yet I feel convinced that the influence of environment has played a greater part than is generally believed in the evolution of colour in living organisms. The weak point in the theory of protective coloration when applied to large mammals is the fact that all carnivorous animals are nocturnal and seek their prey habitually by night and by scent, and only occasionally by daylight and by sight. ”

“ Warning Coloration and Mimicry
The colors of a great many animals, including a considerable percentage of American butterflies and their larvae, have been commonly explained by the theory of warning colors. According to this theory animals which [Pg 34] were for any reason not edible by birds and mammals have developed various striking combinations of color such as black and yellow, red and black, or black and white, in order to advertise to their foes their inedible qualities. ”

“ In a very suggestive work [12] , Alfred Tylor has shown in how far the different markings and the diversified coloration of animals follow the chief lines of structure; and A. R. Wallace has readily admitted that, while the fundamental or ground colors of animals are due to natural selection, the markings are probably due to internal physiological causes. [13] Coloration responds to function; and there is a law in the distribution of colors and the development of the markings, while there ought to be none under the hypothesis of selected accidental variations. ”

“ 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. ”

“ The examples now given lead us to the conclusion that colours acquired for the purpose of serving as a danger-signal to enemies are very widespread in nature, and, with the corresponding colours of the species which mimic them, furnish us with a rational explanation of a considerable portion of the coloration of animals which is outside the limits of those colours that have been acquired for either protection or recognition. ”

“ We have further the dusky hue of creatures that haunt the night, such as mice, moles, and bats; and the gorgeous tints of fishes that swim among the coral reefs. These local color adaptations of animals are of great use to them, either enabling them to escape the notice of their enemies or to come upon their prey with the least risk of being detected. Certain groups of animals have a local color-adaptation, and may be noticed under the heading of Protective Coloration ”

“ Changes of coloration sometimes take place, which either have no discernible relation to age, condition, or surroundings, or are brought about by domestication; and in individuals of the same species there is often a wide range of colour-variation, which is sometimes, but not always, associated with particular localities. ”

“ We may, therefore, discuss the subjects of colors of flowers as a means of attracting bees, warning coloration, protection by mimicry, and certain phases of protective coloration in connection with sexual selection, since it is clear that the same objection, namely, the need of intelligence, may be urged against the assumption of warning coloration. And warning coloration is effective against some animals with distance receptors, but not necessarily against animals without such sense organs. ”

“ The property of changing color depends on the presence of certain dark cells in the tissue of the skin, called chromatophores or color-bearers, which, contracting, under reflex influences of the nervous system, permit the full display of the ground-color of the animal, or, expanding to a certain extent, overlie it. ”

“ We must, therefore, assume that assimilative colouration, as here understood, signifies a close similarity of colouring between an organism and its environment, due to the direct action of the latter upon the organism, either through nerve stimuli or through the direct absorption of the environment into its system, such colouring being essentially non-significant, any utility it may possess being entirely fortuitous. ”

“ A like deposit of pigment, in greater amounts, in the skins of races who live in or near the tropics, gives rise to the characteristic coloring of the black, brown, and yellow races. The pigment, or coloring matter, is of exactly the same kind in all, from the negro to the white. The brown race having a little less of it than the negro, the yellow race a little less yet, and the white least of all, though there is some of it in even the whitest of skins. ”

“ But what makes the pigment-cells change? The fact that a blind flat-fish does not change its colour gives us the first part of the answer. The colour and the pattern of the surroundings must affect the eye. ”

“ Numerous facts and experiments show that while pigment may be, and is, developed in total darkness, the amount of color in an individual animal depends, other things equal, directly on the amount of light to which it is habitually exposed. ”

“ Now in domesticated animals there is great variation in colouring, but not in the majority of wild species. What the causes are that operate in the painting of the skin of an animal no one can say, any more than one can say how particular spots are arranged on the petal of a flower or the wing of a butterfly. ”

“ They are at once less definite and less generally evident, but more delightful—more frequent in nature, though rarer in common art, than the like relations of the secondaries and primaries. There is very little pure colour in the world: now and then a gleam dazzles us, like a burst of sunshine through grey mists; but as a rule, nature prefers broken colours to absolute hues. Most pure in spring, most full in summer, most mellow in autumn, [312] most sober in winter, her tints and shades of colour are always more or less interlaced, from white and the primaries to the semi-neutral and black. ”

“ The colour of the human skin has always held an important place among physical criteria of race. Physiology explains colour as a consequence of climate and even diet. The pigment or colour- ing matter under the epidermis, or rather under the second or Malpighian skin, is not peculiar to the Negroid and other coloured races, but is common to all human beings. ”

“ We took it for granted above that birds and mammals could discriminate between the red or yellow berry and the green foliage in whose midst it grows. Indeed, were other proof wanting, we should be justified in concluding that animals generally are possessed of a sense for the discrimination of color, from the mere fact that all those fruits and flowers which depend for their dispersion or fertilization upon animal agency are brightly tinted, while all those which depend upon the wind, or other insentient energies, are green or dull-brown in hue. ”

“ Under natural selection, as we have seen, the development of colour is fostered under certain conditions. The colour is either protective, rendering the organism inconspicuous amid its normal surroundings, or it is of warning value, advertising the organism as inedible or dangerous, or, in the form of recognition-marks, it is of service in enabling the members of a species to recognize each other. ”

“ If this idea be correct, it follows that a uniformly coloured flower or animal must be of extreme rarity, since it necessitates not merely the entire suppression of the tendency to emphasize important regions in colour, but also the adjustment of all the varying parts of the organism to one uniform molecular condition, which enables it to absorb all but a certain closely related series of light waves no matter how varied the functions of the parts. ”

“ In fair individuals with blue eyes and a pink and white skin, the pigment seemed to be entirely wanting. In individuals with a brown skin, black hair, and a very dark iris, “not only the brain enveloped by its membranes assumes a deeper shade, but a layer of black matter, in every way comparable to that of the Negro, covers the protuberance, the pineal gland, and some other points of the nervous centres.” ”

“ All these rapid changes of colour are due to changes in shape or position of superficial pigment cells controlled by the nervous system. That the latter is itself stimulated by light through the medium of the eye and optic nerve has been proved in many cases. Animals with a short life-history passed in a single environment, which, however, may be very different in the case of different individuals, may have a different form of variable cryptic colouring, namely, the power of adapting their colour once for all (many pupae) , or once or twice (many larvae) . ”

“ The blue colours in birds' feathers and insects' wings are produced without blue pigment by special effects of reflection, and where green is the colour it is often due to the addition of a small quantity of yellow pigment to what would otherwise look blue: though some caterpillars and grasshoppers have a real green pigment in their skin. ”

“ If we suppose a few of the pigment cells here and there to become coloured, we should have irregular brilliant dottings, just as we actually see in many butterflies, along the costa. The grouping together of these colour dots would give rise to a spot, from which point all is clear.
That some such grouping or gathering together, allied to segregation, does take place, a study of spots, and especially of eye-spots, renders probable. What the nature of the process is we do not know, nor is it easy to imagine. ”

“ This view is in accordance with the well-known fact, of colours which rarely or never appear in the species in a state of nature, continually occurring among domesticated animals and cultivated plants; showing us that the capacity to develop colour is ever present, so that almost any required tint can be produced which may, under changed conditions, be useful, in however small a degree. ”

“ Nature, however, does not proceed quite in this way. The yellow pigment varies greatly in hue; it is muddy yellow, brown, or earthy color, and it never spreads itself uniformly over the surface, but occurs in patches grouped about the pupil and spreads in dull rays or lines and spots, so that the eye which science says “ought to be called green” is usually a very dull blue-grey or brownish-blue, or clay color, and in some rare instances shows a changeable greenish hue. ”

“ It therefore seems possible that assimilative colouration may have been a first and a very general consequent in animal development; and that the subsequent protective resemblance acquired by numerous living creatures through the process of natural selection, when life had advanced to the competitive stage, is far too frequently used as an explanation for whole series of uniform phenomena in colouration, which have probably survived unaltered from remote antiquity. ”

“ 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. ”

“ Here, then, we have to deal with the simplest forms of life, and if colouration depends upon structure, these structureless transparent creatures should lack all colour-pattern, and such is really the case. Possessing no organs, they have no colouration, and are generally either colourless or a faint uniform brown colour, and through their colourless bodies the food particles show, often giving a fictitious appearance of colouring. ”

“ Nature provides pigments of color to guard the skin against the inclemencies of sun and weather. Every modern man knows and 70has experienced the result of strong sun and wind in his own skin. “Tan” it is called, and sometimes, within a few weeks the color of a white man’s skin is transformed into a yellow or a dark brown. If the exposure continues, the color remains.
In the opinion of noted scientists, it is certain that the difference between the races as to color is merely skin deep. ”

“ It gives the color to the person by means of pigment cells which lie in its midst.
The black man is dark because of the abundance of pigment cells in his scarf skin. The albino is light because of their absence. The colors of hair and feathers are due to these cells in their receptacles, but white and iridescent feathers are doubtless so partly because of their absence and partly because of hollow spaces which catch and reflect or refract the light. ”