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that the tongue is less extensible; but it is constructed in exactly the same way by the inrolling of the two laminæ of which it is composed.

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 spiderhunters, of Asia, which are sun-birds without any metallic or other brilliant colouring. These 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 aërial gambols of the American humming-birds. The true metallic sunbirds 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 phenicotis) 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.

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 well-being 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 swifts—profoundly modified, it is true, for an aërial and flower-haunting existence, but still bearing in many important peculiarities of structure the unmistakable evidences of a common origin.

V.

THE COLOURS OF ANIMALS AND SEXUAL

SELECTION.

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—Typical Colours—The Nature of Colour—Colour a normal product of Organization—Theory of Protective Colours—Theory of Warning Colours—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 Insects—Theory of display of Ornaments by Males-Natural Selection as neutralizing Sexual Selection-Theory of Typical Colours—Colourdevelopment as illustrated by Humming-birds—Local causes of Colourdevelopment-Summary on Colour-development in Animals.

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 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 beauti

ful 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,

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

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