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    "source_title": "Encyclopaedia Britannica (1926)",
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    "chunk_id": "1926:colours of animals:2df757cb55f2",
    "title": "COLOURS OF ANIMALS",
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    "verified_text": "since 1910 important advances have becn made in the study of mimicry and in our knowledge of adaptive colour changes in fishes, but consideration may first be given to the work of thayer on concealing coloration and to mottram’s researches on the general principles underlying the colour patterns of animals. concealing and revealing coloration.—thayer established that what he terms obliterative countershading is of very general occurrence, #.e., that in most animals the colour grades from dark on the back to pale below, tending to counteract the effect of light falling from above and so to conceal the solidity of the animal when seen against a background of the same tint. he also proved that many patterns that appear very conspicuous when an animal is seen out of its natural habitat may help to conceal it in its usual surroundings. markings tend, by their separate and conflicting patterns, to obliterate the visibility of the details of boundary and form, and when an animal is covered by sharply contrasted bold patterns of pale and dark the contour will be broken up against either a pale or a dark background; such patterns, cutting up the aspect of the animal, thayer terms sec- cant. the white rump marks of the prong-buck would be seen against the sky by a crouching carnivore and at night would merge into this background, rendering the animal almost invisible. although thayer’s researches are valuable, he goes too far in claiming that all patterns are for concealment, and some of the pictures which he gives with specially painted backgrounds are not very convincing. mottram has shown that a pattern of dark and pale squares, spots or stripes blends to a uniform colour at a distance which is dependent on the size of these areas. in many mammals the effect of obliterative countershading is pro- duced by many or large dark spots, or broad dark stripes on the back, gradually changing to few or small spots or narrow stripes, below. the zebra has stripes of this type, and mottram considers that at the blending distance this pattern has the advantage over obliterative countershading, as it produces a blurred outline. with the cheetah the spots are closer together on the back. within the blending distance such patterns may imitate the sur- troundings, but mottram thinks this is unusual, and contrasts the striping of the tiger, effective for concealment among reeds, with that of the zebra, which would in his opinion be ineffective in such an environment. mottram finds that most concealing patterns break out along the animal’s margin and tend to obscure its characteristic shape, whereas revealing patterns often follow 685 the margin and tend to accentuate the animal’s shape and to isolate it from its surroundings. a pattern of this kind, com- monly seen in butterflies, is a broad black band following the outer margins of both wings and often enclosing a brilliant yellow or blue central area. experimental evidence and the study of the habits do not bear out the view of thayer and his school that all coloration is for concealment. . without observation of an animal in its natural surroundings it may be difficult to comprehend the meaning of its coloration. a number of unrelated species of sea-perches of the large genus epinephelus are covered with more or less hexagonal reddish spots separated by a pale network. the reason for this was made clear by alcock. he was ina boat with an indian fisherman who tricd to spear a large fish of this type; the wounded fish at once made for an adjacent clump of corals, where it lay concealed, the red spots exactly resembling the coral polyps; although it could easily have escaped, the fish would not leave its shelter and was eventually captured. mimicry.— poulton, who has made special study of mimicry in butterflies, has recently given an account of the most interesting and elaborate example known. an african swallowtail butterfly, papilio dardanus, is represented in madagascar and abyssinia by closely allied forms, in which both sexes are alike and non- mimetic. elsewhere in africa the male is of the same type, but the females have lost the tails to the hind wings and differ from the males also in coloration, mimicking butterflies of a very different group, the danainae. three forms of female of p. durdanus mimic three very distinct danaine patterns; two of these patterns are modified in passing from one area to another, and in the same regions the mimetic females are correspondingly altered. in uganda and west africa a fourth form of female oc- curs, mimicking two species of acracinae. experimental breed- ing has shown that the different female mimetic forms may occur in the same brood. the work of gahan on mimicry in the coleoptera is an impor- tant contribution to our knowledge of this fasctnating subject. in this group mimicry is not so restricted as in butterflies; beetles may mimic ants or wasps, and may be mimicked by flies or moths. gahan points out that there are some who believe that mimicry is wholly the result of chance. nature, they say, who is engaged in turning out thousands upon thousands of different patterns, is bound sometimes to repeat herself. although this may possibly be the true explanation of a few such resemblances, gahan rejects it for the great majority. pascoe had pointed out a close similarity in appearance between two unrelated species of beetle from widely separated parts of the world, but gahan remarks that this is not due to chance, but to the fact that both species are mimics of ants. the beetles of the family lycidae are conspicuously coloured, fly slowly and secrete distasteful liquids. they show little diversity in form, and, in the same region, but little in colour, most of the asiatic species being red and the south american black and yellow. thus they are easily recognised, and it is established by observa- tion and experiment that they are singularly free from the at- tacks of insectivorous birds and mammals. wherever the lycidae occur, they form mimetic associations and are mimicked by day-flying beetles of other families, and sometimes also by day- flying moths and other insects. some critics have suggested that mimetic resemblances are due to the direct action of the environ- ment, or to similarity in habits and mode of life, or even to re- lationship, the species concerned being supposed to have retained their ancestral coloration, while they have diverged in structural characters. to this gahan replies that the lycidae and their longicorn mimics belong to widely separated groups; they differ from one another as larvae as much as they resemble one another when they reach the perfect state, and they lead a different kind of life. gahan contrasts with the uniformity of the lycidae the diversity of the cerambycidae, which are not protected by a distasteful secretion. many of these look quite unlike their nearest allies, but show a close resemblance in form, size and colouration to un- related insects, either noxious beetles of other families or stinging 686 insects such as wasps. in the coloured plate! (see note below) are shown a number of cerambycidae and the insects that they resemble, the mimics (cerambycidae) being placed on the left (figs. 1 to 7), their models, wasps (figs. 1a to 4a, 7a) and noxious beetles (figs. 5a, 6a) on the right. attention may particularly be directed to the fact that the beetles have one pair of wing-covers, or elytra, and one pair of wings, whereas the wasps have two pairs of wings, and that the one pair of wings of the mimics gen- erally resemble the two pairs of their wasp models in form, size and colour. as a rule the elytra are small and inconspicuous in the mimicking beetles, but there are some exceptions: for ex- ample, in acyphoderes odyneroides (fig. 7) the wing-cases have a glistening appearance, imitating transparent wings. in an australian species, tragocerus formosus (fig. 4), the elytra are quite long and are coloured with bands of black and red to look like the banded abdomen of the wasp; moreover, the elytra are strongly emarginate on the outer edges near the _ base, giving free play to the wings when the elytra are kept close to- gether. the same wasp serves as a model for another beetle (esthesis ferrugineus) belonging to quite a different group from tragocerus, and in this the resemblance is brought about in another way, the elytra being short and the abdomen banded. gahan points out that mimicry is unknown in beetles that fly by night, which as a rule have a colouration that harmonises with the object on which they rest during the day. he concludes that one of the chief operating causes in bringing about mimicry _ must be some agency that is at work only in the hours of day- light, and he thinks that the facts of mimicry strongly support the theory of natural selection, which is the only one that offers an adequate explanation of them. critics of the natural selection theory have naturally turned their attention to mimicry, especially in relation to butterflies, the main group that has been studied in this connection. some critics have asserted that butterflies are not attacked by birds, the enemies believed to be responsible for the deceptive resem- blance of mimics to models. attention having been thus directed to the subject many naturalists have made a special study of it. and have proved this assertion to be unfounded. under the heading of mimicry may also be considered the work of stuart baker (1923) and jourdain (1925) on the eggs of cuckoos. these birds form a large family with a nearly cosmo- politan distribution; most of them have the habit of laying their eggs in the nests of other birds, which hatch and rear the young cuckoos. the eggs of those species of cuckoo which make their own nests and rear their own young are white or pale blue in colour, but the eggs of the so-called parasitic cuckoos exhibit a great diversity, generally resembling those of the birds in whose nests they are laid. some cuckoos—for example our british species—are polymorphic as regards the colour of the eggs; individuals of such forms not infrequently lay in nests of birds whose eggs they do not match; some of these birds appear to take no notice, whereas others may eject the egg or desert the nest. baker gives some interesting examples of dimorphism, e.g., cuculus pliocephalus, which in japan lays chocolate eggs only, in the himalayas white ones, but in assam both kinds, generally in appropriate nests; and hierococcyx sparveroides, which lays pale blue eggs in the nests of laughing-thrushes and olive brown ones in those of the giant spider-hunter. baker, who has formed and studied a collection of over 2,500 eggs of cuckoos from europe, asia and australia, says that several species lay only one type of egg, and these always choose the same foster-parents. thus the two commonest indian species, clamator jacobinus and hierococcyx varius, lay their deep blue eggs in nests of birds of 1explanation of plate-——mimetic beetles and their models. mimics (cerambycidae) models habitat fig. 1. nothopeus auricomus. 1a. salius ceylonicus (wasp) . ceylon fig. 2. nothopeus hemipterus 2a. macromerus splendida (wasp) burma fig. 3. nothopeus fasciatipen- nis . . ; . . 3a. mygnimia princeps (wasp) borneo fig. 4. tragocerus formosus . 4a. abispa ephippium (wasp) . oueensland fig. 5. ites plagiatus ; . 8a. mesomphalia latevittata (beetle, fam. cassididae) . 9 ecuador fig. 6. ctenodes decemmaculata 6a. cephalodonta spinipes brazil (beetle, fam. hispidae) fig. 7. acyphoderes odyne- aed rcides ‘i , ‘ . « 474, polybia liliacea (wasp) brazil colour vision and colour blindness the common indian genera argyra and turdioides, the eggs of which are extremely similar; and a large indian cuckoo, eudyna- mis scolopaceus, has its eggs pale blue, blotched with red, coloured like those of the corvidae which it uses to rear its young. colour changes in fishes —a great advance has been made in our knowledge of colour changes in fishes, especially through the experiments of mast (1916). the tropical sea-perches from bermuda in the new york aquarium are a source of constant interest to visitors on account of their changes of colour and markings, one species suddenly changing from crimson to dull green, another from yellow to red or dark brown, whilst a third may turn on or off cross-bars, spots or other markings. these changes appear to have little meaning under such conditions, although townsend has noted that alarm generally produces a pale colouration and excitement and intensification of pattern; but under natural conditions the different colour-phases may harmonise with different environments. the flat-fishes have long been known to simulate closely the ground on which they lie. sumner, experimenting with a mediterranean: flat-fish (platophrys podas), obtained some remarkable results by placing them on patterns such as black and white squares, to which they were not accustomed in nature; on such backgrounds the fish responded more slowly than when placed on sand, gravel or mud, but with practice acquired the power of changing more rapidly than at first. the capacity of this species to adapt itself is limited to the black, brown, grey and white of its customary habitat; also it has definite spots and markings, which vary in their relative intensity and may disappear, but when present always have the same form or position. thus pale and dark areas are reproduced by areas of about the same size, but not of the same form; squares and circles produce the same effect. mast has carried the matter much further;for he has found that flat-fishes of the genus puralichthys on white, grey, black, brown, blue, green, yellow, orange or pink assume a colour very near that of the background, but that reds are less accurately simulated. yellows and browns take much less time to copy than reds, greens or blues; for the latter some days may elapse before the full effect is produced. these effects are produced by the concentration or distribution of granules of pigment in the pigment-cells of the skin, under the control of stimuli received through the eyes. the flat-fish has to see the ground before it can resemble it, but there is no visual comparison of skin with ground. one important result of mast’s experiments is that he con- cludes that in respect to motion and colour the vision of these fishes is much the same as human vision, although it is less acute in distinguishing differences of size and form. it is a fair assump- tion that, if the colour vision of these fishes is like our own, this is true of vertebrates in general; thus an argument that has sometimes been urged in relation to the adaptive coloration of insects, that we have no evidence that the exact resemblance to the surroundings or to other insects is necessary to deceive birds, loses much of its validity. bibliography.—a. w. alcock, a mnaturafist in indian seas (1902); g. h. thayer, concealing coloration in the animal kingdom, new ed. (1919); e. b. poulton in phases of modern science (1925). see also c. h. townsend, report new york zool. soc. (1908); f. b. sumner, jour. exp. zool. (1911); c. j. gahan, proc. south london ent. soc. (1913); j. c. mottram, proc. zool. soc. (1915, 1916, 1917); s. o. mast, bull. u.s. fisheries bureau (1916); e. c. s. baker, proc. zool. soc. (1923); f. c. r. jourdain, proc. betis colour vision and colour blindness.—the images of external objects are formed upon the outer layer of the retina, the layer furthest away from the front of the eye, so that the light has to pass through all the other layers before it reaches the sensi- tive portion. this sensitive layer consists of two elements, which are called respectively, on account of their shape, the rods and the cones. a slight depression in the centre of the retina, the fovea, is the region of most distinct vision. in the fovea, only cones are present. in the region external to the fovea each cone is surrounded by a group of rods, and the number of rods to cones increases as portions of the retina further from the fovea are taken, except at oo guitiiten, a ee we % 4a mimetic beetles compared with their models palatable beetles of the tamily cerambycidae (1-7), compared with wasps (14-44 and 72), and unpalatable beetles (52 and 62), showing the striking similarity in appearance. columbia university the extreme periphery, where, again, only cones are found. in the outer segment of each rod there is a rose-coloured substance, the visual purple, which is photo-chemically sensitive to light. this visual purple is not found in the cones, but only in the rods. for this reason it was not considered to be essential to vision, because it was absent from the cones, and only cones are to be found in the fovea, the region of most distinct vision. though visual purple is not present in the cones of the fovea, it is found between them, four special canals aiding the flow from the periph- ery to the centre of the fovea. when there is no visual purple in the fovea it is blind, as shown by the varying sensitiveness of the fovea, and the disappearance of objects in the central area of vision when there is no stimulation of the periphery. the rods and cones project into a thin layer of fluid, which is kept in its place by a membrane. the visual purple is diffused into this liquid and on being de- composed by light stimulates the cones, thereby setting up a nerve impulse, which causes the sensation of vision. the move- ment of after-images shows that the stimulus in vision is fluid and situated outside the cones. the rods are not percipient elements but regulate the formation and distribution of the visual purple. the decomposition of the visual purple by light stimulates the ends of the cones, and a visual impulse is set up which is con- veved through the optic nerve fibres to the brain. the char- acter of the impulse differs according to the wave-length of the light causing it. therefore, in the impulse itself we have the physiological basis of the sensation of light, and in the quality of the impulse the physiological basis of the sensation of colour. the impulse, being conveyed along the optic nerve to the brain, stimulates the visual centre causing a sensation of light, and then, passing on to the colour-perceiving centre, causes a sensa- tion of colour. but though impulses vary in character according to the wave-length of the light causing them, the colour-per- ceiving centre is not able to discriminate between adjacent impulses, the nerve cells not being sufficiently developed for the purpose. even with the normal-sighted, there is room for much further development in the discrimination of colour, but when the de- velopment is not up to the normal standard, or there is a defect in any portion of the apparatus diminishing the power of dis- crimination, colour blindness is the result. on numerous points in the above theory there is no evidence. we do not know whether one nerve cell is able to appreciate all colours according to its stimulation by impulses of varying character, or whether distinct nerve cells are required for dif- ferent colours. we do not know whether nerve impulses differ in character, but we do know that light and colour perception are quite distinct and, therefore, we require a different set of nerve cells for the perception of light and colour. the colour-per- ceiving cells may be only indirectly connected in the visual path. though the position of the visual centre in the brain has been accurately determined, the position of the colour vision centre is still unknown. it may, indeed, be in some portion of the visual tract between the eye and the visual centre. for instance, one portion of the end of the cone when stimulated may possess fibrils which are either connected with, or pass through, a portion of brain giving rise to the sensation of red, whilst stimulation of the other end of the cone stimulates fibrils which, with their neural connections, give rise to the sensation of violet. the in- termediate portions give rise to the sensations of green, yellow and blue. there is a considerable range both at the red and the violet ends of the spectrum which does not vary in hue, but only in luminosity, and it is therefore extremely probable that rays from these regions affect only one portion of the cone respectively, but in different degrees. colour blindness —now, whether the colour vision centre be above or below the visual centre, the results will be the same. the nerve fibrils from one end of the cone may be connected with one side of the colour vision centre so that stimulation of one side of the centre causes the sensation of red and the other side the sensation of violet, and stimulation of the intervening por- 687 tions the sensations of yellow, green and blue. it should be noted, however, that this does not support the trichromatic theory, because there is the strongest evidence showing that colour sensation can be abolished without interfering with light sensation and that the colour perceiving centre—whether it be above or below the visual centre—is more easily affected than the latter. a man may entirely lose his sensation of colour without interference with his perception of light and form, as in a case of a signalman on the railway who became totally colour blind after tetanus. again, the continued use of the eyes for colours of high lumin- osity may cause colour blindness. after an illness a person may become temporarily colour blind. a man, after an attack of influenza, was so colour blind as to designate the whole of the green region of the spectrum as white. tested with the lantern he called green, white and red; red, white; and white, red. a few months afterwards his colour sense was quite normal. there is much less likelihood of an accident occurring through acquired colour-blindness than congenital, because in acquired colour- blindness the man knows that his colour vision has altered. cases of colour blindness may be divided into three classes, which are quite separate and distinct from each other though one or more may be present in the sameperson. in the first class there is light as well as colour loss. in the second class the per- ception of light is the same as in the normal-sighted, but there is a defect in the perception of colour. in the first class certain rays are either not perceived at all, or very imperfectly. both these classes are represented by analogous conditions in the per- ception of sounds. the first class of the colour blind is repre- sented by those who are unable to hear very high or very low notes. the second class is represented by those who possess what is commonly called a defective musical ear. colour blind indi- viduals belonging to this class can be arranged in a series. at one end of this series are the supernormal-sighted and at the other end the totally colour blind. in the third class of the colour blind there is defective perception of colour through the fovea or central region of the retina not being normal. tests.—the test which should be used for the marine and railway services is a lantern in which the requisite conditions are represented. a lantern of this kind is used in great britain by the admiralty and the board of trade. a man who cannot dis- tinguish the red, green and white lights in these lanterns will not be able to do so in actual practice, and this fact is easily proved by testing with signal lights. the wool test is a failure. it is now obsolete, as it allows over 30% of dangerously colour blind persons to pass, and it will be noticed in certain reports that of those who were rejected by the wool test andi who appealed, over 50% were found to be normal- sighted and had been wrongly rejected. .see f. w. edridge- green, the physiology of vision (1920). (f. w. e.-g.)",
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