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As examples may be mentioned mercuric oxide, which is orange yellow, but which changes to orange, red, and brown when heated; chromic-oxide, which is green, and changes to yellow; cinnabar, which is scarlet, and changes to puce; and metaborate of copper, which is blue, and changes to green and greenish yellow.
Ilow Animal Colours are Produced. The colouring matters of animals are very varied. Copper has been found in the red pigment of the wing of the turaco, and Mr. Surby has detected no less than seven distinct colouring matters in birds' eggs, several of which are chemically related to those of blood and bile. The same colours are often produced by quite different substances in different groups, as shown by the red of the wing on the burnetmoth changing to yellow with muriatic acid, while the red of the red-admiral-butterfly undergoes no such change.
These pigmental colours have a different character in animals according to their position in the integument. Following Dr. IIagen's classification, epidermal colours are those which exist in the external chitinised skin of insects, in the hairs of mammals, and, partially, in the feathers of birds. They are often very deep and rich, and do not fade after death. The hypodermal colours are those which are situated in the inferior soft layer of the skin. These are often of lighter and more vivid tints, and usually fade after death. Many of the reds and yellows of butterflies and birds belong to this class, as well as the intensely vivid hues of the naked skin about the heads of many birds. These colours sometimes exude through the pores, forming an evanescent bloom on the surface.
Interference colours are less frequent in the organic world. They are caused in two ways : either by reflection from the two surfaces of transparent films, as seen in the soap-bubble and in thin films of oil on water; or by fine strive which produce colours either by reflected or transmitted light, as seen in mother-of-pearl and in finely-ruled metallic surfaces. In both cases colour is proclucel by light of one wave-length being neutralised, owing to one set of such waves being caused to be half a wave length behind the other set, as may be found explained in any treatise on physical optics. The result is, that the complementary colour of that neutralised is seen; and, as the thickness of the film or the fineness of the striæ undergo slight changes, almost any colour can be produced. This is believed to be the origin of many of the glossy or metallic tints of insects, as well as those of the feathers of some birds. The iridescent colours of the wings of Iragon-flies are caused by the superposition of two or more transparent lamellae; while the shining blue of the Purple-Emperor and other butterflies, and the intensely metallic colours of humming-birds, are probably due to fine striæ.
Colour a Normal Product of Organization.—This outline sketch of the nature of colour in the animal world, however imperfect, will at least serve to show us how numerous and varied are the causes which perpetually tend to the production of colour in animal tissues. If we consider, that in order to produce white, all the rays which fall upon an object must be reflected in the same proportions is they exist in solar light-whereas, if rays of any one or more kinds are absorbed or neutralised, the resultant reflected light will be coloured ; and that this colour may be infinitely varied according to the proportions in which different rays are reflected or absorbedwe should expect that white would be, as it really is, comparatively rare and exceptional in nature. The same observation will apply to black, which arises from the absorption of all the different rays. Many of the complex substances which exist in animals and plants are subject to changes of colour under the influence of light, heat, or chemical change, and we kuow that chemical changes are continually occurring during the plıysiological processes of development and growth. We also find that every external character is subject to minute changes, which are generally perceptible to us in closely allied species; and we can therefore have no doubt that the extension and thickness of the transparent lamelle, and the fineness of the striæ or rugosities of the integuments, must be undergoing constant minute changes ; and these changes will very frequently produce changes of colour. These considerations render it probable that colour is a normal and even necessary result of the complex structure of animals and plants; and that those parts of an organism which are undergoing continual development and adaptation to new conditions, and are also continually subject to the action of light and heat, will be the parts in which changes of colour will most frequently appear. Now there is little doubt that the external changes of animals and plants in adaptation to the environment are much more numerous than the internal changes; as seen in the varied character of the integuments and appendages of animals—hair, horns, scales, feathers, &c. &c.--and in plants, the leaves, bark, flowers, and fruit, with their various modifications -as compared with the great uniformity in the texture and composition of their internal tissues; and this accords with the uniformity of the tints of blood, muscle, nerve, and bone throughout extensive groups, as compared with the great diversity of colour of their external organs. It seems a fair conclusion that colour per se may be considered to be normal, and to need no special accounting for ; while the absence of colour (that is, either white or black), or the prevalence of certain colours to the constant exclusion of others, must be traced, like other modifications in the cconomy of living things, to the needs of the species. Or, looking at it in another aspect, we may say, that amid the constant Variations of animals and plants colour is ever tending to vary and to appear where it is absent; and that natural selection is constantly eliminating such tints as are injurious to the species, or preserving and intensifying such as are useful.
This view is in accordance with the well-known fact, of colours which rarely or never appear in the species in a state of nature, continually occurring among domesticated animals and cultivated plants ; showing us that the capacity to develop colour is ever present, so that almost any required tint can be produced which may, under changel conditions, be useful, in however small a degree.
Let us now see how these principles will enable us to understand and explain the varied phenomena of colour in nature, taking them in the oriler of our functional classification of colours.
Theory of Protective Colour's.- We have seen that obscure or protective tints in their infinitely varied degrees are present in every part of the animal kingdom, whole families or genera being often thus coloured.
Now the various brown, earthy, ashy, and other neutral tints are those which would be most readily produced, because they are due to an irregular mixture of many kinds of rays ; while pure tints require either rays of one kind only, or definite mixtures in proper proportions of two or more kinds of rays. This is well exemplified by the comparative difficulty of producing definite pure tints by the mixture of two or more pigments; while a haphazard mixture of a number of these will be almost sure to produce browns, olives, or other neutral or dingy colours. An indefinite or irregular absorption of some rays and reflection of others would, therefore, produce obsure tints; while pure and viviil colours would require a perfectly definite absorption of one portion of the coloured rays, leaving the remainder to produce the true complementary colour. This being the case we may expect these brown tints to occur when the need of protection is very slight or even when it does not exist at all; always supposing that bright colours are not in any way useful to the species. But whenever a pure colour is protective,—as green in tropical forests or white among arctic snows, there is no difficulty in producing it, by natural selection acting on the innumerable slight variations of tint which are ever occurring. Such variations may, as we have seen, be produced in a great variety of ways; either by chemical changes in the secretions, or by molecular changes in surface structure; and may be brought about by change of food, by the photographic action of light, or by the normal process of generative variation. Protective colours therefore, bowever curious and complex they may be in certain cases, offer no real difficulties.