For the systematic arrangement of the class of birds, the conformation of their feet has been found to afford characters of great importance to the ornithologist, inasmuch as the organisation of these members must obviously be in strict relation with the localities they inhabit. To account for the distribution of the feathered tribes, and to explain the relationships that exist between them and other animals, a great variety of ingenious theories have been broached, all of them more or less fanciful. The different families, sub-families, and minor groups into which they have been divided have been again and again sorted, like a pack of cards, frequently more in accordance with the whim of the player than with the established rules of the game. And yet a little reflection will show that the great principles of zoological classification are so simple, and at the same time so immutable, that we sometimes cannot but admire the ingenuity displayed in going wrong. [Illustration: Fig. 10.--TONGUE OF THE WOODPECKER, SHOWING THE MECHANISM EMPLOYED FOR ITS PROTRUSION. _a_, harpoon-like Tip of the Tongue; _d_, _e_, _f_, _g_, _h_, Framework of the Throat; _i_, _k_, Glands furnishing adhesive secretion.] Few things are more manifest to the student of nature than that, in the distribution of the animal creation, it has been ordained that every locality shall be peopled by forms of life pre-eminently adapted for its occupation. If, therefore, we are asked whether birds ought to be arranged in circles or in squares, in hexagons or in pentagons, in groups of five or in groups of seven, our simple reply would be by inquiring how many localities could be pointed out as requiring appropriate occupants, and to this question it is not difficult to find a satisfactory answer.