146 COLOURS
OF
1. Bionomics. THE scope of this article includes the uses of colour in the struggle for existence among animals and in their sexual relationships, but not the physiological uses of coloured pigments or the discussion of pigments of uncertain meaning. Use of Colour for Concealment.—Cryptic Colouring is by far the commonest use of colour in the struggle for existence. It is employed for the purpose of attack (Aggressive Resemblance or Anticryptic Colouring) as well as of defence (Protective Resemblance or Procryptic Colouring). The fact that the same method, concealment, may be used both for attack and defence has been well explained by Belt (The Naturalist in Nicaragua, London, 1888), who suggests as an illustration the rapidity of movement which is also made use of by both pursuer and pursued, which is similarly raised to a maximum in both by the gradual dying out of the slowest through a series of generations. Cryptic colouring is commonly associated with other aids in the struggle for life. Thus well-concealed mammals and birds, when discovered, will generally endeavour to escape by speed, and will often attempt to defend themselves actively. On the other hand, small animals which have no means of active defence, such as large numbers of insects, frequently depend upon concealment alone. Protective Resemblance is far commoner among animals than Aggressive Resemblance, in correspondence with the fact that predaceous forms are as a rule much larger and much less numerous than their prey. In the case of insectivorous Vertebrata and their prey such differences exist in an exaggerated form. Cryptic colouring, whether used for defence or attack, may be either General or Special. In General Resemblance the animal, in consequence of its colouring, produces the same effect as its environment, but the conditions do not require any special adaptation of shape and outline. General Resemblance is especially common among the animals inhabiting some uniformly coloured expanse of the earth’s surface, such as an ocean or a desert. In the former, animals of all shapes are frequently protected by their transparent blue colour; on the latter, equally diverse forms are defended by their sandy appearance. The effect of a uniform appearance may be produced by a combination of tints in startling contrast. Thus the black and white stripes of the zebra blend together at a little distance, and “ their proportion is such as exactly to match the pale tint which arid ground possesses when seen by moonlight ” (Galton, South Africa, London, 1889). Special Resemblance is far commoner than General, and is the form which is usually met with on the diversified surface of the earth, on the shores, and in shallow water, as well as on the floating masses of Algae on the surface of the ocean, such as the Sargasso Sea. In these environments the cryptic colouring of animals is usually aided by special modifications of shape, and by the instinct which leads them to assume particular attitudes. Complete stillness and the assumption of a certain attitude play an essential part in General Resemblance on land; but in Special Resemblance the attitude is often highly specialized, and perhaps more important than any other element in the complex method by which concealment is effected. In Special Resemblance the combination of colouring, shape, and attitude is such as to produce a more or less exact resemblance to some one of the objects in the environment, such as a leaf or twig, a patch of lichen, or flake of bark. In all cases the resemblance is to some object which is of no interest to the
ANIMALS.
enemy or prey respectively. The animal is not hidden from view by becoming indistinguishable from its background, as in the cases of General Resemblance, but it is mistaken for some well-known object. In seeking the interpretation of these most interesting and elaborate adaptations, attempts have been made along two lines. First, it is sought to explain the effect as a result of the direct influence of the environment upon the individual (Buffon), or by the inherited effects of effort and the use and disuse of parts (Lamarck). Second, natural selection is believed to have produced the result, and afterwards maintained it by the survival of the best concealed in each generation. The former suggestions break down when the complex nature of numerous Special Resemblances is appreciated. Thus the arrangement of colours of many kinds into an appropriate pattern requires the co-operation of a suitable shape and the rigidly exact adoption of a certain elaborate attitude. The latter is instinctive, and thus depends on the central nervous system. The cryptic effect is due to the exact co-operation of all these factors; and in the present state of science the only possible hope of an interpretation lies in the theory of natural selection, which can accumulate any and every variation which tends towards survival. A few of the chief types of methods by which concealment is effected may be briefly described. The colours of large numbers of Vertebrate animals are darkest on the back, and become gradually lighter on the sides, passing into white on the belly. Abbot H. Thayer (The Auk, vol. xiii., 1896) has suggested that this gradation obliterates the appearance of solidity, which is due to shadow. The colour-harmony, which is also essential to concealment, is produced because the back is of the same tint as the environment (e.g., earth) bathed in the cold blue-white of the sky, while the belly, being cold bluewhite bathed in shadow and yellow earth reflexions, produces the same effect. Thayer has made models (in the Natural History Museums at London, Oxford, and Cambridge) which support his interpretation in a very convincing manner. This method of neutralizing shadow for the purpose of concealment by increased lightness of tint was first suggested by E. B. Boulton in the case of a larva (Trans. Ent. Soc. Lond., 1887, p. 294) and a pupa (Trans. Ent. Soc. Lond., 1888, pp. 596, 597), but he did not appreciate the great importance of the principle. In an analogous method an animal in front of a background of dark shadow may have part of its body obliterated by the existence of a dark tint, the remainder resembling, e.g., a part of a leaf (Muller, Zool. Jahr. J. W. Spengel, Jena, 1886). This method of rendering invisible any part which would interfere with the resemblance is well known in Mimicry. A common aid to concealment is the adoption by different individuals of two or more different appearances, each of which resembles some special object to which an enemy is indifferent. Thus the leaf-like butterflies (Kallima) present various types of colour and pattern on the under side of the wings, each of which closely resembles some well-known appearance presented by a dead leaf; and the common British Yellow Underwing Moth (Tryphcena pronuba) is similarly polymorphic on the upper side of its upper wings, which are exposed as it suddenly drops among dead leaves. Caterpillars and pupae are also commonly Dimorphic, green and brown. Such differences as these extend the area which an enemy is compelled to search in order to make a living. In many cases the cryptic colouring changes appropriately during the course of an individual life, either seasonally, as in the ptarmigan or Alpine hare, or, according as the
