True flight seems to need a definite mechanism of one kind--namely, a mechanism which shall give rapid and reiterated blows to the air from a point towards the dorsal side, and head end of the body, by structures of considerable superficial extent, and capable of rapid and delicate inclinations of surface. Such structures must be light and therefore delicate, and yet possess very considerable strength to resist the strain of the body's prolonged sustentation, and to effect its occasionally very rapid progress, as in the swift and in dragon-flies. These conditions which we find fulfilled in all existing flying organisms were also fulfilled organisms which have for ages passed away from the surface of this by planet, such as the extinct flying reptiles called _Pterosauria_ or _Pterodactyles_.[82] In all such rapidly flying creatures the form of the body is necessarily modified so as to throw the centre of gravity where it may be best sustained. It is this which packs what are practically a bird's teeth in its belly, and thickens so greatly the muscles on its breast which are formed in such a way as to serve both the usual purposes of breast-muscles, and also that which is effected in most cases by muscles of the back, which in birds are very greatly diminished in volume and extent. But there are living creatures which have relations with two media; which, though they are aquatic, yet by the help of the air rise and float, so as to be partly bathed in the atmosphere; while others carry down a portion of that atmosphere below the surface of water, so as to be sub-aqueously aërial. Examples of the last-mentioned condition are afforded by such spiders as have the habit of enclosing a bubble of air within the meshes of their self-woven network, and going down with it, being thus able there to maintain themselves as in a diving-bell. The reverse condition obtains in such plants as _Valisneria_,[83] which secrete air within expanded bladder-like receptacles, and, thus aided, rise to the surface and float. Another example is that of certain polyp animals, such as the Portuguese man of war, which also rise and swim upon the surface of the sea by the aid of floats in the form of bladders, which are also filled with air by means of their own life processes. The same also is the case in many seaweeds.