That walking, swimming, and flying represent integral parts of the same problem is proved by the fact that most quadrupeds swim as well as walk, and some even fly; while many marine animals walk as well as swim, and birds and insects walk, swim, and fly indiscriminately. When the land animals, properly so called, are in the habit of taking to the water or the air; or the inhabitants of the water are constantly taking to the land or the air; or the insects and birds which are more peculiarly organized for flight, spend much of their time on the land and in the water; their organs of locomotion must possess those peculiarities of structure which characterize, as a class, those animals which live on the land, in the water, or in the air respectively. In this we have an explanation of the gossamer wing of the insect,--the curiously modified hand of the bat and bird,--the webbed hands and feet of the Otter, Ornithorhynchus, Seal, and Walrus,--the expanded tail of the Whale, Porpoise, Dugong, and Manatee,--the feet of the Ostrich, Apteryx, and Dodo, exclusively designed for running,--the feet of the Ducks, Gulls, and Petrels, specially adapted for swimming,--and the wings and feet of the Penguins, Auks, and Guillemots, especially designed for diving. Other and intermediate modifications occur in the Flying-fish, Flying Lizard, and Flying Squirrel; and some animals, as the Frog, Newt, and several of the aquatic insects (the Ephemera or May-fly for example[1]) which begin their career by swimming, come ultimately to walk, leap, and even fly.[2] [1] The Ephemeræ in the larva and pupa state reside in the water concealed during the day under stones or in horizontal burrows which they form in the banks. Although resembling the perfect insect in several respects, they differ materially in having longer antennæ, in wanting ocelli, and in possessing horn-like mandibles; the abdomen has, moreover, on each side a row of plates, mostly in pairs, which are a kind of false branchiæ, and which are employed not only in respiration, _but also as paddles_.--Cuvier’s Animal Kingdom, p. 576. London, 1840. [2] Kirby and Spence observe that some insects which are not naturally aquatic, do, nevertheless, swim very well if they fall into the water. They instance a kind of grasshopper (_Acrydium_), which can paddle itself across a stream with great rapidity by the powerful strokes of its hind legs.--(Introduction to Entomology, 5th edit., 1828, p. 360.) Nor should the remarkable discovery by Sir John Lubbock of a swimming insect (_Polynema natans_), which uses its wings _exclusively as fins_, be overlooked.--Linn. Trans. vol. xxiv. p. 135. Every degree and variety of motion, which is peculiar to the land, and to the water- and air-navigating animals as such, is imitated by others which take to the elements in question secondarily or at intervals.