Nor does the vast procession end here. Our very mother-earth is formed of the débris of life. Plants and animals which have been, build up its solid fabric.[22] We dig downwards, thousands of feet below the surface, and discover with surprise the skeletons of strange, uncouth animals, which roamed the fens and struggled through the woods before man was. Our surprise is heightened when we learn that the very quarry itself is mainly composed of the skeletons of microscopic animals; the flints which grate beneath our carriage wheels are but the remains of countless skeletons. The Apennines and Cordilleras, the chalk cliffs so dear to homeward-nearing eyes--these are the pyramids of bygone generations of atomies. Ages ago, these tiny architects secreted the tiny shells, which were their palaces; from the ruins of these palaces we build our Parthenons, our St. Peters, and our Louvres. So revolves the luminous orb of Life! Generations follow generations; and the Present becomes the matrix of the Future, as the Past was of the Present: the Life of one epoch forming the prelude to a higher Life. When we have thus ranged air, earth, and water, finding everywhere a prodigality of living forms, visible and invisible, it might seem as if the survey were complete. And yet it is not so. Life cradles within Life. The bodies of animals are little worlds, having their own animals and plants. A celebrated Frenchman has published a thick octavo volume devoted to the classification and description of “The Plants which grow on Men and Animals;”[23] and many Germans have described the immense variety of animals which grow on and in men and animals; so that science can now boast of a parasitic Flora and Fauna. In the fluids and tissues, in the eye, in the liver, in the stomach, in the brain, in the muscles, parasites are found; and these parasites have often _their_ parasites living in them! We have thus taken a bird’s-eye view of the field in which we may labour. It is truly inexhaustible. We may begin where we please, we shall never come to an end; our curiosity will never slacken. “And whosoe’er in youth Has thro’ ambition of his soul given way To such desires, and grasp’d at such delights, Shall feel congenial stirrings, late and long.” As a beginning, get a microscope. If you cannot borrow, boldly buy one. Few purchases will yield you so much pleasure; and while you are about it, do, if possible, get a good one. Spend as little money as you can on accessory apparatus and expensive fittings, but get a good stand and good glasses. Having got your instrument, bear in mind these two important trifles--work by daylight, seldom or never by lamplight; and keep the unoccupied eye _open_. With these precautions you may work daily for hours without serious fatigue to the eye.