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refer our readers to Mr. Darwin's Descent of Jan, chapters x. to xviii., and to chapters üi., iv. and vii. of my Contributions to the Theory of Natural Selection.
Typical Colours.—The fourth group--of Typicallycoloured animals-includes all species which are brilliantly or conspicuously coloured in both sexes, and for whose particular colours we can assign no function or use. It comprises an immense number of showy birds, such as Kingfishers, Barbets, Toucans, Lories, Tits, and Starlings; among insects most of the largest and handsomest butterflies, innumerable bright-coloured beetles, locusts, dragon-flies, and hymenoptera; a fuw mammalia, as the zebras ; a great number of marine fishes; tlousinds of striped and spotted caterpillars ; and abundance of mollusea, star-fish, and other marine animals. Among these we have incluiled some which, like the gaudy caterpillars, have warning colours ; but as that theory does not explain the particular colours or the varied patterns with which they are adorned, it is best to include them also in this class. It is a suggestive fact, that all the brightly-coloured birils mentioned above build in holes or form covered nests, so that the females do not need that protection during the breeding season which I believe to be one of the chief causes of the dull colour of female birids when their partners are gaily coloured. This subject is fuily argued in my Contributions, &c., chapter vii.
As the colours of plants and flowers are very different from those of animals both in their distribution and functions, it will be well now to consider how the general facts of colour liere sketched out can be
explained. We lave first to inquire what is colour, and how it is produced ; what is known of the causes of change of colour; and what theory best accords with the whole assemblage of facts.
The Nature of Colour.—The sensation of colour is caused by vibrations or unulations of the ethereal medium of different lengths and velocities. The whole body of vibrations caused by the sun is termed radiation, or, more commonly, rays; and consists of sets of waves which vary considerably in their dimensions and rate of recurrence, but of which the middle portion only is capable of exciting in us scusations of light and colour. Beginning with the largest waves, which recur at the longest intervals, we have first those which produce heat-sensations only; as they get smaller and recur quicker, we perceive a dull red colour ; and as the waves increase in rapidity and diminish in size, we get successively sensations of orange, yellow, green, blue, indigo, and violet, all fading imperceptibly into each other. Then come more invisible rays, of shorter wave-length and quicker recurrence, which produce, solely or chiefly, chemical effects. The red rays, which first become visible, have been ascertained to recur at the rate of 458 millions of millions of times in a second, the length of each wave being 37ooth of an inch ; while the violet rays, which last remain visible, recur 727 millions of millions of times per second, and have a wave-length of atboth of an inch. Although the waves recur at different rates, they are all propagated through the ether with the same velocity (192,000 miles per second); just as different musical sounds, which are produced by waves of air of different lengths and rates of recurrence, travel at the same speed, so that a tune played several bundred yards off reaches the ear in correct time. There are, therefore, an almost infinite number of different colour-producing undulations, and these may be combined in an almost infinite variety of ways, so as to excite in us the sensation of all the varied colours and tints we are capable of perceiving. When all the different kinds of rays reach us in the proportion in which they exist in the light of the sun, they produce the sensation of white. If the rays which excite the sensation of any one colour are prevented from reaching us, the remaining rays in combination produce a sensation of colour often very far removed from white. Thus green ray's being abstracted leave purple light; blue, orange-red light; violet, yellowish-green light, and so on. These pairs are termed complementary colours. And if portions of differently coloured lights are abstracted in various degrees, we have produced all those infinite gradations of colours, and all those varied tints and hues which are of such use to us in distinguishing external objects, and which form one of the great charms of our existence. Primary colours would therefore be as numerous as the different wave-lengths of the visible radiations, if we could appreciate all their differences; while secondary or compound colours, caused by the simultaneous action of any combination of rays of different wave-lengths, must be still more numerous.
In order to account for the fact that all colours appear to us to be produced by combinations of three primary colours---red, green, and violet-it is believed that we have three sets of nerve fibres in the retina, each of which is capable of being excited by all rays,
but that one set is excited most by the larger or rul Waves, another by the medium or green waves, and the third set chiefly by the violet or smallest waves of light; and when all three sets are excited together in proper proportions we see white. This view is supported by the phenomena of colour-blindness, which are explicable on the theory that one of these sets of nerve-fibres (usually that adapted to perceive rel) has lost its sensibility, causing all colours to appear as if the red rays were abstracted from them.
It is a property of these various radiations, that they are unequally refracted or bent in passing obliquely through transparent bodies, the longer waves being least refracted, the shorter most. IIence it becomes possible to analyse white or any other light into its component rays. A small ray of sunlight, for example, which would proluce a round white spot on a wall, if passed through il prism is lengthened out into a band of coloured light, exactly corresponding to the colours of the rainbow. duy one colour can thus be isolated and separately examined; and by means of reflecting mirrors the separate colours can be again compounded in various ways, and the resulting colours observed. This band of coloured light is called a spectrum, and the instrument by which the spectra of various kinds of light are examined is called a spectroscope. This branch of the subject has, however, no direct bearing on the mode in which the colours of living things are prodluced, and it has only been alluded to in order to complete our sketch of the nature of colour.
The colours which we perceive in material substances are produced either by the absorption or by the inter
ference of some of the rays which form white light. Pigmental or absorption-colours are the most frequent, comprising all the opaque tints of flowers and insects, and all the colours of dyes and pigments. They are caused by rays of certain wave-lengths being absorbed, while the remaining rays are reflected and give rise to the sensation of colour. When all the colour-producing rays are reflected in due proportion, the colour of the object is white; when all are absorbed the colour is black. If blue rays only are absorbeel the resulting colour is orange-red ; and generally, whatever colour an object appears to us, it is because the complementary colours are absorbed by it. The reason why rays of ouly certain refrangibilities are reflected, and the rest of the incident light absorbed by each substance, is supposed to depend upon the molecular structure of the body. Chemical action almost always implies change of molecular structure, hence chemical action is the most potent cause of change of colour. Sometimes simple solution in Water effects il marvellous change, as in the case of the well-knowu aniline dyes; the magenta and violet dyes exhibiting, when in the solid form, various shades of golden or bronzy metallic green.
leat alone often produces change of colour without effecting any chemical change. Mr. Ackroyd has recently investigated this subject," and has shown that a large number of bodies are changed by heat, returning to their normal colour when cooled, and that this change is almost always in the direction of the less refrangible rays or longer wave-lengths; and he connects the change with the molecular expansion caused by heat. I "Metachromatism, or Colour-Change," Chemical Nero, August, 1876.