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ground and invisible; a Membracid (Homoptera) is entirely unlike an ant, but is concealed by an ant-like shield. When we further realize that in this and other examples of Mimicry “ the likeness is almost always detailed and remarkable, however it is attained, while the methods differ absolutely,” we recognize that natural selection is the only possible explanation hitherto suggested. In the cases of Aggressive Mimicry an animal resembles some object which is attractive to its prey. Examples are found in the flower-like species of Mantis, which attract the insects on which they feed. Such cases are generally described as possessing “ alluring colours” and are regarded as examples of Aggressive (Anticryptic) Resemblance, but their logical position is here. Colours displayed in Courtship, Secondary Sexual Characters, Epigamic Colours.—Darwin suggested the explanation of these appearances in his theory of Sexual Selection (The Descent of Man, London, 1874). The rivalry of the males for the possession of the females he believed to be decided by the preference of the latter for those individuals with especially bright colours, highly developed plumes, beautiful song, &c. Wallace does not accept the theory, but believes that natural selection, either directly or indirectly, accounts for all the facts. Probably the majority of naturalists follow Darwin in this respect. The subject is most difficult, and the interpretation of a great proportion of the examples in a high degree uncertain, so that a very brief account is here expedient. That selection of some kind has been operative is indicated by the diversity of the elements into which the effects can be analysed. The most complete set of observations on Epigamic display was made by George W. and Elizabeth G. Peckham upon spiders of the family Attidce (Nat. Hist. Soc. of Wisconsin, vol. i., 1889). These observations afforded the authors “conclusive evidence that the females pay close attention to the love-dances of the males, and also that they have not only the power, but the will, to exercise a choice among the suitors for their favour.” Epigamic characters are often concealed except during courtship; they are found almost exclusively in species which are diurnal or semi-diurnal in their habits, and are excluded from those parts of the body which move too rapidly to be seen. They are very commonly directly associated with the nervous system; and in certain fish, and probably in other animals, an analogous heightening of effect accompanies nervous excitement other than sexual, such as that due to fighting or feeding. Although there is Epigamic display in species with sexes alike, it is usually most marked in those with secondary sexual characters specially developed in the male. These are an exception to the rule in heredity, in that their appearance is normally restricted to a single sex, although in many of the higher animals they have been proved to be latent in the other, and may appear after the essential organs of sex have been removed or become functionless. This is also the case in the Aculeate Hymenoptera when the reproductive organs have been destroyed by the parasite (Stylops). Cunningham has recently 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 :— Eimer. Orthogenesis der Schmetterlinge. Leipzig, 1898. Poulton. The Colours of Animals. London, 1890.—Beddard. Animal Coloration. London, 1892. Haase. Researches on Mimicry (translation). London, 1896.— Wallace. Natural Selection and Tropical Nature. London 1895. Darwinism.

A-NIMi.LS

[chemistry.

London, 1897. Many of the memoirs and volumes quoted in the text also contain further references. (g, b. p.) II. 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 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 role 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 A) may be read directly. Examples of the absorption bands yielded by colouring matters will be found in Ency. Brit. vol. xx. p. 483. Haemoglobin, the red colouring matter of vertebrate blood, C758H1203N]95S3FeO218, and its derivatives hsematin, C32H30N4FeO3, and haematoporphyrin, C16H1SN203, are