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destroyed by the parasite Stylops. J. T. Cunningham has argued (Sexual Dimorphism in the Animal Kingdom, London, 1900) that secondary sexual characters have been produced by direct stimulation due to contests, &c., in the breeding period, and have gradually become hereditary, a hypothesis involving the assumption that acquired characters are transmitted. Wallace suggests that they are in part to be explained as “recognition characters,” in part as an indication of surplus vital activity in the male.

Authorities.—The following works may also be consulted:—T. Eimer, Orthogenesis der Schmetterlinge (Leipzig, 1898); E. B. Poulton, The Colours of Animals (London, 1890); F. E. Beddard, Animal Coloration (London, 1892); E. Haase, Researches on Mimicry (translation, London, 1896); A. R. Wallace, Natural Selection and Tropical Nature (London, 1895); Darwinism (London, 1897); A. H. Thayer and G. H. Thayer, Concealing-Coloration in the Animal Kingdom (New York, 1910).  (E. B. P.) 

2. Chemistry

The coloration of the surface of animals is caused either by pigments, or by a certain structure of the surface by means of which the light falling on it, or reflected through its superficial transparent layers, undergoes diffraction or other optical change. Or it may be the result of a combination of these two causes. It plays an important part in the relationship of the animal to its environment, in concealment, in mimicry, and so on; the presence of a pigment in the integument may also serve a more direct physiological purpose, such as a respiratory function. The coloration of birds’ feathers, of the skin of many fishes, of many insects, is partially at least due to structure and the action of the peculiar pigmented cells known as “chromatophores” (which W. Garstang defines as pigmented cells specialized for the discharge of the chromatic function), and is much better marked when these have for their background a “reflecting layer” such as is provided by guanin, a substance closely related to uric acid. Such a mechanism is seen to greatest advantage in fishes. Among these, guanin may be present in a finely granular form, causing the light falling on it to be scattered, thus producing a white effect; or it may be present in a peculiar crystalline form, the crystals being known as “iridocytes”; or in a layer of closely apposed needles forming a silvery sheet or mirror. In the iris of some fishes the golden red colour is produced by the light reflected from such a layer of guanin needles having to pass through a thin layer of a reddish pigment, known as a “lipochrome.” Again, in some lepidopterous insects a white or a yellow appearance is produced by the deposition of uric acid or a nearly allied substance on the surface of the wings. In many animals, but especially among invertebrates, colouring matters or pigments play an important rôle in surface coloration; in some cases such coloration may be of benefit to the animal, but in others the integument simply serves as an organ for the excretion of waste pigmentary substances. Pigments (1) may be of direct physiological importance; (2) they may be excretory; or (3) they may be introduced into the body of the animal with the food.

Of the many pigments which have been described up to the present time, very few have been subjected to elementary chemical analysis, owing to the great difficulties attending their isolation. An extremely small amount of pigment will give rise to a great amount of coloration, and the pigments are generally accompanied by impurities of various kinds which cling to them with great tenacity, so that when one has been thoroughly cleansed very little of it remains for ultimate analysis. Most of these substances have been detected by means of the spectroscope, their absorption bands serving for their recognition, but mere identity of spectrum does not necessarily mean chemical identity, and a few chemical tests have also to be applied before a conclusion can be drawn. The absorption bands are referred to certain definite parts of the spectrum, such as the Fraunhofer lines, or they may be given in wave-lengths. For this purpose the readings of the spectroscope are reduced to wave-lengths by means of interpolation curves; or if Zeiss’s microspectroscope be used, the position of bands in wave-lengths (denoted by the Greek letter λ) may be read directly.

Haemoglobin, the red colouring matter of vertebrate blood, C758H1203N195S3FeO218, and its derivatives haematin, C32H30N4FeO3, and haematoporphyrin, C16H18N2O3, are colouring matters about which we possess definite chemical knowledge, as they have been isolated, purified and analysed. Most of the bile pigments of mammals have likewise been isolated and studied chemically, and all of these are fully described in the text-books of physiology and physiological chemistry. Haemoglobin, though physiologically of great importance in the respiratory process of vertebrate animals, is yet seldom used for surface pigmentation, except in the face of white races of man or in other parts in monkeys, &c. In some worms the transparent skin allows the haemoglobin of the blood to be seen through the integument, and in certain fishes also the haemoglobin is visible through the integument. It is a curious and noteworthy fact that in some invertebrate animals in which no haemoglobin occurs, we meet with its derivatives. Thus haematin is found in the so-called bile of slugs, snails, the limpet and the crayfish. In sea-anemones there is a pigment which yields some of the decomposition-products of haemoglobin, and associated with this is a green pigment apparently identical with biliverdin (C16H18N2O4), a green bile pigment. Again, haematoporphyrin is found in the integuments of star-fishes and slugs, and occurs in the “dorsal streak” of the earth-worm Lumbricus terrestris, and perhaps in other species. Haematoporphyrin and biliverdin also occur in the egg-shells of certain birds, but in this case they are derived from haemoglobin. Haemoglobin is said to be found as low down in the animal kingdom as the Echinoderms, e.g. in Ophiactis virens and Thyonella gemmata. It also occurs in the blood of Planorbis corneus and in the pharyngeal muscles of other mollusca.

A great number of other pigments have been described; for example, in the muscles and tissues of animals, both vertebrate and invertebrate, are the histohaematins, of which a special muscle pigment, myohaematin, is one. In vertebrates the latter is generally accompanied by haemoglobin, but in invertebrates—with the exception of the pharyngeal muscles of the mollusca—it occurs alone. Although closely related to haemoglobin or its derivative haemochromogen, the histohaematins are yet totally distinct, and they are found in animals where not a trace of haemoglobin can be detected. Another interesting pigment is turacin, which contains about 7% of nitrogen, found by Professor A. H. Church in the feathers of the Cape lory and other plantain-eaters, from which it can be extracted by water containing a trace of ammonia. It has been isolated, purified and analysed by Professor Church. From it may be obtained turacoporphyrin, which is identical with haematoporphyrin, and gives the band in the ultra-violet which J. L. Soret and subsequently A. Gamgee have found to be characteristic of haemoglobin and its compounds. Turacin itself gives a peculiar two-banded spectrum, and contains about 7% of copper in its molecule. Another copper-containing pigment is haemocyanin, which in the oxidized state gives a blue colour to the blood of various Mollusca and Arthropoda. Like haemoglobin, it acts as an oxygen-carrier in respiration, but it takes no part in surface coloration.

A class of pigments widely distributed among plants and animals are the lipochromes. As their name denotes, they are allied to fat and generally accompany it, being soluble in fat solvents. They play an important part in surface coloration, and may be greenish, yellow or red in colour. They contain no nitrogen. As an example of a lipochrome which has been isolated, crystallized and purified, we may mention carotin, which has recently been found in green leaves. Chlorophyll, which is so often associated with a lipochrome, has been found in some Infusoria, and in Hydra and Spongilla, &c. In some cases it is probably formed by the animal; in other cases it may be due to symbiotic algae, while in the gastric gland of many Mollusca, Crustacea and Echinodermata it is derived from food-chlorophyll. Here it is known as entero-chlorophyll. The black pigments which occur among both vertebrate and invertebrate animals often have only one attribute in common, viz. blackness, for among the discordant results of analysis one thing is certain, viz. that the melanins from vertebrate animals