of being excessively long and pointed; their plumage is arranged differently; and their feet are long and strong, instead of being excessively short and weak. There remain only the superficial characters of small size and brilliant metallic colours to assimilate them with the humming-birds, and one structural feature-a tubular and somewhat extensile tongue. This, however, is a strictly adaptive character, the sun-birds feeding on small insects and the nectar of flowers, just as do the humming-birds; and it is a remarkable instance of a highly peculiar modification of an organ occurring independently in two widely-separated groups. In the sun-birds the hyoid or tongue-muscles do not extend so completely over the head as they do in the humming-birds, so that the tongue is less extensible; but it is constructed in exactly the same way by the inrolling of the two lamina of which it is composed. These The tubular tongue of the sun-birds is a special adaptive modification acquired within the family itself, and not inherited from a remote ancestral form. This is shown by the amount of variation this organ exhibits in different members of the family. It is most highly developed in the Arachnotheræ, or spider-hunters of Asia, which are sun-birds without any metallic or other brilliant colouring. have the longest bills and tongues, and the most developed hyoid muscles; they hunt much about the blossoms of palmtrees, and may frequently be seen probing the flowers while fluttering clumsily in the air, just as if they had seen and attempted to imitate the aerial gambols of the American humming-birds. The true metallic sun-birds generally cling about the flowers with their strong feet; and they feed chiefly on minute hard insects, as do many humming-birds. There is, however, one species (Chalcoparia phoenicotis), always classed as a sun-bird, which differs entirely from the rest of the species in having the tongue flat, horny, and forked at the tip; and its food seems to differ correspondingly, for small caterpillars were found in its stomach. More remotely allied, but yet belonging to the same family, are the little flower-peckers of the genus Diceum, which have a short bill and a tongue twice split at the end; and these feed on small fruits, and perhaps on buds and on the pollen of flowers. The little white-eyes (Zosterops), which are probably allied to the last, eat soft fruits and minute insects. Conclusion Here, then, we have an extensive group of birds, considerably varied in external form, yet undoubtedly closely allied to each other, one division of which is specially adapted to feed on the juices secreted by flowers and the minute insects that harbour in them; and these alone have a lengthened bill and double tubular tongue, just as in the humming-birds. We can hardly have a more striking example of the necessity of discriminating between adaptive and purely structural characters. The same adaptive character may coexist in two groups which have a similar mode of life, without indicating any affinity between them, because it may have been acquired by each independently to enable it to fill a similar place in nature. In such cases it is found to be an almost isolated character, apparently connecting two groups which otherwise differ radically. Non-adaptive or purely structural characters, on the other hand, are such as have probably been transmitted from a remote ancestor, and thus indicate fundamental peculiarities of growth and development. The changes of structure rendered necessary by modifications of the habits or instincts of the different species have been made to a great extent independently of such characters; and as several of these may always be found in the same animal their value becomes cumulative. We thus arrive at the seeming paradox that the less of direct use is apparent in any peculiarity of structure, the greater is its value in indicating true, though perhaps remote, affinities; while any peculiarity of an organ which seems essential to its possessor's wellbeing is often of very little value in indicating its affinity for other creatures. This somewhat technical discussion will, it is hoped, enable the general reader to understand some of the more important principles of the modern or natural classification of animals as distinguished from the artificial system which long prevailed. It will also afford him an easily remembered example of those principles, in the radical distinctness of two families of birds often confounded together, the sun-birds of the Eastern Hemisphere and the humming - birds of America; and in the interesting fact that the latter are essentially swiftsprofoundly 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 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. Z V 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 Colours— Theory of Warning Colours-Imitative Warning Colours-The Theory of Mimicry-Theory of Sexual Colours-Colour as a means of Recognition-Colour proportionate to Integumentary Development-Selection 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 Colours— Local 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 these broad and soothing tints, we have presented to us in the vegetable and animal worlds an infinite variety of objects adorned with the most beautiful and most varied hues. Flowers, insects, and birds are the organisms most generally ornamented in this way; and their symmetry of form, their variety of structure, and the lavish abundance with which they clothe and enliven the earth, cause them to be objects of universal admiration. The relation of this wealth of colour to our mental and moral nature is indisputable. The child and the savage alike admire the gay tints of flower, bird, and insect; while to many of us their contemplation brings a solace and enjoyment which is both intellectually and morally beneficial. It can then hardly excite surprise that this relation was long thought to afford a sufficient explanation of the phenomena of colour in nature; and although the fact that Full many a flower is born to blush unseen, 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 |