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Show 188 DARWINISM CJTAI'. the fixed characters of organic beings have been developed under the action of the law of utility, led to the inevitn,blc conclusion that so remarkable and conspicuous a character as colour, which so often constitutes the most obvious distinction of species from species, or group from group, must al~o have arisen from survival of the fittest, and must, therefore, m most cases have some relation to the wellbeing of its possessors. Continuous observation and research, carried on by multitudes of observers during the last thirty years, have shown this to be the case; but the problem is found to be far more complex than was at first supposed. The modes in which colour is of u. e to different classes of organisms is very varied, and have probably not yet been all discovered; while the infinite variety and marvellous beauty of some of its developments are such as to render it hopeless to arrive at a complete and satisfactory explanation of every individual case. So much, however, has been achieved, so many curious facts have been explained, and so much light has been thrown on some of the most obscure phenomena of nature, that the subject deserves a prominent place in any account of the Darwinian theory. The Problem to be Solved. Before dealing with the various modifications of colour in the animal world it is necessary to say a few words on colour in general, on its prevalence in nature, and how it is that the colours of animals and plants require any special explanation. What we term colour is a subjective phenomenon, due to the constitution of our mind and nervous system; while, objectively, it consists of light-vibrations of different wave-lengths emitted by, or reflected from, various objects. Every visible object must be coloured, because to be visible it must send rays of light to our eye. The kind of light it sends is modified by the molecular constitution or the surface texture of the object. Pigments absorb certain rays and reflect the remainder, an<l this reflected portion has to our eyes a definite colour, according to the portion of the rays constituting white light which arc absorbed. Interference colours are produced either by thin films or by very fine strire on the surfaces of bodies, which cause rays of certain wave-lengths to neutralise each other, leaving the remainder to produce th~ ~ffcct$ of colour. Such VIti ORICIN AND USES OF COLOUR IN ANIMALS 189 arc the colours of soap·bubbles, or of steel or glass on which extremely fine lines have been ruled; and these colours often produce the effect of metallic lustre, and are the cause of most of the metallic hues of birds and insects. As colour thus depends on molecular or chemical constitution or on the minute surface texture of bodies, and, as the matter of which organic beings are composed consists of chemical compounds of great complexity and extreme instability, and is also subject to innumerable changes during growth and development, we might naturally expect tho phenomena of colour to be more varied here than in less complex and more stn.ble compound .. Yet even in the inorganic world we find abundant and varied colours ; in the earth and in the water; in metals, gems, and minerals; in the sky and in the ocean ; in sunset clouds and in the many-tinted rainbow. Here we can have no question of use to the coloured object, and almost as little perhaps in the vivid red of blood, in the brilliant colours of red snow and other low algro and fungi, or even in the universal mantle of green which clothes so large a portion of the earth's surface. The presence of some colour, or even of many brilliant colours, in anirnn,ls and plants would require no other expla.nation than does that of the sky or the ocean, of the ruby or the emerald -that is, it would require a purely physical explanation only. It is the wonderful individuality of the colours of animals and plants that attracts our attention-the fact that the colours are localised in definite patterns, sometimes in accordance with structural characters, sometimes altogether indctlendent of them; while often differing in the most striking and fantastic manner in allied species. We n,re thus compelled to look upon colour not merely as a phy~ical bt~t also as a bio~o~ica.l characteristic which has been drfferentmted and specmhsed by natural s~lection, nnd must, therefore, find its explanation in the principle of adaptation or utibty. The Constancy of Animal Colo·ur indicates Utility. That the colours and markings of animals have been acquired under the fundamental law of utility is indicated by a O'eneral fact which has received very little attention. As a rule, colour and markiug are constant in each species of ':ild animal, while, in almost every domesticated animal, there anses |
