Those animals and plants which are called parasites furnish another example of the extraordinary profusion with which life is distributed over the earth. Animals and plants which live on other animals or on other plants, and which feed on the substance of their involuntary entertainers, are called _parasites_. Each of the mammals has its parasites, such as fleas, lice, &c., and man has the flea, the louse, and the bug. So each vegetable has its parasite. The oak gives shelter and food to lichens and various cryptogamia, and even on its roots we find particular kinds of cryptogamia, such as the truffle. Thus we see that life plants itself, grafts itself upon life. But, more than this, these parasites in their turn have their smaller parasites, so minute as only to be microscopically discerned. Take a lichen off an oak and examine it with a magnifying glass, and also examine a flea, or a nit, and you will behold the curious spectacle of a parasite attached to another parasitical creature, and living upon its substance. From the great vegetable the alimentary substance passes to the visible parasite, and from that to the invisible. In this little space life is superposed and concentrated. Such a fact proves with what prodigious abundance life is spread over our globe. Thus, then, we see that the surface of the globe, the fresh waters, and the salt seas, and, finally, the atmosphere, are inhabited by immense numbers of living beings. Life abounds on the earth, in the waters, and in the air. Our globe is like an immense vase, in which life is accumulated, pressed down, and running over. But, the earth, the air, and the waters are not the only places at the command of nature. Above the atmosphere there extends another region, with which astronomers and physicists are acquainted, and which they call _ether_ or _planetary ether_. The atmosphere which surrounds our globe, and is drawn with it in its course through space, as it is drawn with it in its rotation upon its own axis, is not very high. It does not extend beyond thirty or forty leagues, and it diminishes in substance in proportion to its elevation above the earth. At three or four leagues in height the air is so rarefied that it becomes impossible for men or animals to breathe it. In aërostatic ascents it is impossible to go beyond seven or eight kilometres, because at that height the air loses so much density, is so highly rarefied, that it no longer serves for purposes of respiration, nor counterbalances the effect of the interior pressure of the body on the exterior. After that height, the density of the air decreases more and more, until there is absolutely no air. At that point begins the fluid which astronomers and physicists call _ether_.