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the interesting fact that the latter are essentially swifts— profoundly modified, it is true, for an aerial and flowerhaunting existence, but still bearing in many important peculiarities of structure the unmistakable evidences of a common origin.
1 Recent researches into the anatomy of the swifts and humming - birds have brought to light so many and such important differences that the above conclusion, founded on comparatively superficial characters, becomes doubtful. Dr. Shufeldt considers that both groups are so isolated that they each require to be classed as a distinct order of birds. But while the swifts are believed to have undoubted though remote affinities with the swallows, it cannot yet be determined whether they have any real affinity with the humming-birds, which latter appear to have no special and unmistakable relationship with any other order or family of birds. See “Studies of the Macrochires, Morphological, and otherwise, with the view of indicating their relationships," etc., by R. W. Shufeldt, M.D., in the Journal of the Linnean Society, vol. xx. ; Zoology, pp. 299, 394 : 1889.
THE COLOURS OF ANIMALS AND SEXUAL SELECTION 1
General Phenomena of Colour in the Organic World-Theory of Heat and
Light as producing Colour-Changes of Colour in Animals produced by Coloured Light-Classification of Organic Colours-Protective Colours—Warning Colours-Sexual Colours—Normal Colours—The Nature of Colour-How Animal Colours are produced— Colour a normal product of Organisation—Theory of Protective ColoursTheory of Warning Colours-Imitative Warning Colours-The Theory of Mimicry—Theory of Sexual Colours—Colour as a means of Recog. nition-Colour proportionate to Integumentary Development-Selec. tion by Females not a cause of Colour-Probable use of the Horns of Beetles-Cause of the greater Brilliancy of some Female InsectsOrigin of the Ornamental Plumage of Male Birds—Theory of Display of Ornaments by Males-Natural Selection as neutralising Sexual Selection-Greater Brilliancy of some Female Birds—Colour-development as illustrated by Humming-Birds—Theory of Normal ColoursLocal causes of Colour-development—The influence of Locality on Colour in Butterflies and Birds-Sense-perception influenced by Colour of the Integuments-Summary on Colour-development in Animals.
General Phenomena of Colour in the Organic World THERE is probably no one quality of natural objects from which we derive so much pure and intellectual enjoyment as from their colours. The heavenly blue of the firmament, the glowing tints of sunset, the exquisite purity of the snowy mountains, and the endless shades of green presented by the verdure-clad surface of the earth, are a never-failing source of pleasure to all who enjoy the inestimable gift of sight. Yet these constitute, as it were, but the frame and background of
1 A first sketch of this essay appeared in Macmillan's Magazine of September 1877,
a marvellous and ever-changing picture. In contrast with
Full many a flower is born to blush unseen,
And waste its sweetness on the desert air, might seem to throw some doubt on the sufficiency of the explanation, the answer was easy,—that in the progress of discovery man would, sooner or later, find out and enjoy every beauty that the hidden recesses of the earth have in store for him. This theory received great support from the difficulty of conceiving any other use or meaning in the colours with which so many natural objects are adorned. Why should the homely gorse be clothed in golden raiment, and the prickly cactus be adorned with crimson bells? Why should our fields be gay with buttercups, and the heather-clad mountains be clad in purple robes? Why should every land produce its own peculiar floral gems, and the alpine rocks glow with beauty, if not for the contemplation and enjoyment of man? What could be the use to the butterfly of its gailypainted wings, or to the humming-bird of its jewelled breast, except to add the final touches to a world-picture, calculated at once to please and to refine mankind ? And even now, with all our recently acquired knowledge of this subject, who shall say that, these old-world views were not intrinsically and fundamentally sound; and that, although we now know that colour has “uses”. in nature that we little dreamt of, yet the relation of those colours-or rather of the various rays of light—to our senses
and emotions may not be another and perhaps more important use which they subserve in the great system of the universe ?
We now propose to lay before our readers a general account of the more recent discoveries on this interesting subject; and in doing so it will be necessary first to give an outline of the more important facts as to the colours of organised beings; then to point out the cases in which it has been shown that colour is of use; and lastly, to endeavour to throw some light on its nature and on the general laws of its development.
Among naturalists, colour was long thought to be of little import, and to be quite untrustworthy as a specific character. The numerous cases of variability of colour led to this view. The occurrence of white blackbirds, white peacocks, and black leopards, of white blue-bells, and of white, blue, or pink milkworts, led to the belief that colour was essentially unstable, that it could therefore be of little or no importance, and belonged to quite a different class of characters from form or structure. But it now begins to be perceived that these cases, though tolerably numerous, are, after all, exceptional ; and that colour, as a rule, is a constant character. The great majority of the species, both of animals and plants, are each distinguished by peculiar tints which vary very little, while the minutest markings are often constant in thousands or millions of individuals. All our field buttercups are invariably yellow, and our poppies red, while many of our butterflies and birds resemble each other in every spot and streak of colour through thousands of individuals. We also find that colour is constant in whole genera and other groups of species. The Genistas are all yellow, the Erythrinas all red; many genera of Carabidæ are entirely black; whole families of birds
—as the Dendrocolaptidæ—are brown; while among butterfies the numerous species of Lycæna are all more or less blue, those of Pontia white, and those of Callidryas yellow. An extensive survey of the organic world thus leads us to the conclusion that colour is by no means so unimportant or inconstant a character as at first sight it appears to be ; and the more we examine it the more convinced we shall become that it must serve some purpose in nature, and that, besides charming us by its diversity and beauty, it must be well worthy of our attentive study, and have many secrets to unfold to us.
Theory of Heat and Light as producing Colour In commencing our study of the great mass of facts relating to the colours of the organic world, it will be necessary to consider, first, how far the chief theories already proposed will account for them. One of the most obvious and most popular of these theories, and one which is still held, in part at least, by many eminent naturalists, is, that colour is due to some direct action of the heat and light of the sunthus at once accounting for the great number of brilliant birds, insects, and flowers which are found between the tropics.
But before proceeding to discuss this supposed explanation of the colours of living things, we must ask the preliminary question,—whether it is really the fact that colour is more developed in tropical than in temperate climates in proportion to the whole number of species; and even if we find this to be so, we have to inquire whether there are not so many and such striking exceptions to the rule as to indicate some other causes at work than the direct influence of solar light and heat. As this is a most important branch of the inquiry, we must go into it somewhat fully.
It is undoubtedly the case that there are an immensely greater number of richly-coloured birds and insects in tropical than in temperate and cold countries, but it is by no means so certain that the proportion of coloured to obscure species is much or any greater. Naturalists and collectors well know that the majority of tropical birds are dull-coloured; and there are whole families, comprising hundreds of species, not one of which exhibits a particle of bright colour. Such are, for example, the Timaliidæ or babbling thrushes of the eastern, and the Dendrocolaptidæ or tree-creepers of the western hemispheres. Again, many groups of birds which are universally distributed are no more adorned with colour in the tropical than in the temperate zones ; such are the thrushes, wrens, goat-suckers, hawks, grouse, plovers, and snipe ; and if tropical light and heat have any direct colouring effect, it is certainly most extraordinary that in groups so varied in form, structure, and habits as those just mentioned, the tropical should be in no wise distinguished in this respect from the temperate species.