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.