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726 refuge. Many grasshoppers and moths look red and blue when flying, but on settling, these bright colours are at once hidden.

Swynnerton has suggested the substitution of " distinctive coloration " for" warning coloration "; the assumption is that the preyer will remember the distinctive colours and patterns, and associate them with their varying palatability. In support of this there is much evidence to show that many animals have a good memory in this respect.

Mimkry. In view of the fact that insects can be graded in respect of their palatability, the distinction between Mullerian and Batesian mimicry appears difficurt to maintain. A set of animals presenting common warning coloration (Mullerian mimicry) are never equally unpalatable, and therefore it may be said that the relatively palatable of this set are of the nature of Batesian mimics. Although in extreme cases a distinction may be drawn, nevertheless intermediate cases occur which it is impossible to classify in this way. In the study of mimicry many notable advances have been made, chiefly among butter- flies. Several insects which were thought to be different species, or varieties, have lately been shown by breeding-experiments to be polymorphic forms: the same species mimicking sometimes one species and sometimes another, both forms being bred from the same mother. Also it has been shown that, in situations where models are scarce, the mimicking species presents tran- sitions between its various polymorphic forniSj and this fact is considered to indicate that natural selection is required to maintain a mimicry. At one time it was thought that butter- flies had few enemies, and that therefore their remarkable mimicry could bear no relation to natural selection; however, evidence that they are eaten by birds to a considerable extent has been brought forward by several observers.

Sexual Coloration. Secondary sexual coloration still gives rise to much speculation. The Darwinian view that it represents selection by the female is still held by some observers. Others consider that it serves the purpose of stimulating the sexual instinct of the female; or that it is related to the different habits of the male and female, as, for instance, the incubation of eggs; or that it represents a difference in value to the species between male and female, making the conspicuous but less valuable male more likely to be destroyed by enemies than the inconspicuous and valuable female. These various theories are mentioned to show that no general law to explain these colorations has been accepted. Much valuable field work has been done in which many new facts as regards sexual displays have been collected.

Chemistry of A nimal Colours. It has been shown that, in the case of the lobster and salmon, the colours which the males assume at the breeding-season are due to the laying down of a coloured waste product in the scales and shell. This waste product is finally got rid of when the scales become worn and the shell cast. In the case of the female the waste product is dis- charged in quite a different manner; it is deposited in the eggs and disposed of when they are laid. In certain parts of England and Germany, chiefly in the neighbourhood of large towns, many species of moths have developed melanotic forms, or these dark forms have greatly increased in number. Much work has been done in an endeavour to discover the cause of this change, as it was at one time thought to be a case of the acquirement of a dark coloration for concealment against sooty surroundings, and thus to be an example of the rapid action of natural selection. Melanism, however, occurs in other districts, distant from large towns, more particularly near the sea. Recently evidence has been brought forward that this change is due to the particular feeding of the caterpillar; that, in fact, a deposit occurs on the leaves near large towns and near the sea, which causes this change.

Physics of Animal Coloration. Several eminent physicists have taken an interest in, and attempted to explain on a physical basis, the brilliant metallic and iridescent colours of many insects and birds. Although a physical explanation of the coloration of most objects is available the brilliant colours of these animals remain a mystery. A recent summary by the late Lord Rayleigh in the Philosophical Transactions may be quoted: " These colours are probably structural rather than pigment, but still much remains to be effected towards a complete demonstration of the origin of these effects. Even if we admit an interference character questions arise as to the particular manner and there are perhaps possibilities not hitherto contemplated."

It has been suggested that fluorescence plays a part, and to test this insects have recently been examined in a beam of ultra- violet light. It was found that the brilliantly coloured species are not fluorescent. A few Lepidoptera were found to be fluorescent and this character has been found to be of some service in classi- fication, as the property appears to be limited to closely allied species when it occurs in a group.

AUTHORITIES : Carpenter, Naturalist on L. Victoria; Dixey, Presi- dential Address Ent. Soc. (1911); Howard, British Warblers; Longstaff, Butterfly Hunting in Many Lands (1912); Marshall, Trans. Ent. Soc. (1909); Mottram, Proc. Zool. Soc. (1915, 1916, 1917) and Controlled Natural Selection (1914); Poulton, Essays on Evolu- tion (1908) ; Punnett, Mimicry of Butterflies; Pycraft, The Courtship of Animals (1913); Swynnerton, Journ. Linn. Soc. (1919); Thayer, Concealing Colouration in the Animal Kingdom (1909). See also the article CAMOUFLAGE. (]. C. Mo.)

COLOUR-VISION AND COLOUR-BLINDNESS (see 28.139). Much new work has been done in research on these questions, which have become increasingly important in practical life; and a restatement is needed (1921) of the accepted views.

The Physical Basis of Normal Colour-Vision. White light can by means of a prism be split up into its constituent parts to form a spectrum which shows a number of colours. The spectrum or rainbow consists of a series of waves of light of different refrangibility extending from the red, which are the largest waves, to violet, which are the smallest. These waves are similar to those of the sea, only infinitely smaller. Similar waves, differing in size and not giving rise to a sensation of light, are found above the violet and below the red. In the visible spectrum we have a physical series arranged in consecutive order, each member of the series differing in wave-length.

Special Physiological Facts in the Appearance of the Spectrum. If a number of persons be asked to state how many definite colours they see in the spectrum, very different answers will be obtained. The large majority will say that they see six colours, red, orange, yellow, green, blue and violet. A few will state that there are seven colours, indigo being added as a colour, being seen in the region of blue-violet. Newton appears to have seen the spectrum in this way.

The spectrum may be examined in another way, certain portions of it being isolated between two shutters. An extraordinary fact then becomes apparent, viz. that large divisions of the spectrum appear monochromatic, as if they had been painted with one brush of colour, though physically every part of the division differs. Most normal-sighted persons divide the spectrum into about 18 mono- chromatic divisions: those with super-normal colour-perception, into about 25, and those with diminished colour-perception a less num- ber. For instance, those who see three colours in the spectrum generally divide it into ten monochromatic divisions. These divi- sions when examined by a normal-sighted person appear quite wrong and to contain several colours instead of one. It is obvious that a man who sees only ten colours instead of 18 will confuse col- ours which appear different to the normal-sighted.

The Anatomical Basis of Vision and Colour-Vision. It is upon the outer layer of the retina, the membrane lining the back of the eye, that the images of external objects are formed. The outer layer of the retina is the layer farthest away from the front of the eye, so that light has to pass through all the other layers before it reaches the sensitive portion. This sensitive layer consists of two elements, which are called respectively, on account of their shape, the rods and cones. A little dip in the centre of the retina, the fovea, is the region of most distinct vision. In the fovea only cones are present. External to the fovea the rods are arranged in rings round the cones, and the proportion of rods to cones increases as portions of the retina farther from the fovea are taken, except at 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 photochemically sensitive to light. This visual purple i-. not found in the cones, but only in the rods. It was for this reason that 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 the visual purple is not present in the cones of the fovea, it is found between them, four special canals aiding the flow from the periphery to the centre of the fovea. When there is no visual purple in the fovea it is blind. 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