procession, may we not expect that every tribe will hurry to its appointed place the instant that a door is opened?
Microscopists have long been at war, but without bloodshed, as to the place to be assigned to certain organic forms which are hidden from our common eyesight. While the war goes on as to whether desmidiaeæ and diatomaceæ be animal or vegetable, or both, let facts suffice us here in the study of the aquarium. Does an animal exhale carbonic acid? Yes. Well, here are plants or animals, concerned in keeping up the balance, which exhale oxygen, and their name is legion. Volvox globator and the bacillariæ labor as hard to supply the fishes with the life-sustaining gas as do the silken threads of verdure that line the glass like a carpet. Is the possession of starch a distinctive feature of the vegetable? Perhaps so. Truly here are desmidiaeæ that contain starch, and, if I make the possession of cilia the test for assigning certain forms to the animal kingdom, I find in the aquarium spores of algae furnished with them. Motion I know to be no test, because algæ-spores dance through the water gayly till they find a resting-place, and, when the aquarium was first filled, it was by dancing they at last found where to pitch their tents, and cease their nomad wanderings. But they all work together to sustain the balance, and the law of "give and take" prevails among them—the stentor devours the oscillatorise, rotatoria, and monads, and the hydras swallow all; every darting speck is a tomb wherein some smaller speck of life is to be buried, and life thus prospers on the decay it is itself undergoing.
But all this while a fine deposit slowly settles among the pebbles, which form the lower stratum of this watery world. Between the stones a fine alluvial silt collects and thickens. The first frost, sufficiently severe to touch the tank, causes the whole green coating to peel off from the glass and rock, and, while this subsides, to add to the thickness of the alluvium—how slightly, and yet how sufficiently for an example of Nature's working!—a new growth commences, and that balance is restored. Do you not see that the chief teaching of geology—the piling of stratum upon stratum, the conversion of disrupted rock and decayed plant and animal into rock again—is here exemplified in the history of a domestic toy, which contains already one example of stratification in the silence of watery submergence? A tank which has been fitted with loam, pebbles, and plants of the brook and river, will, if left undisturbed for three years, be in this state. Those plants will all have decayed, but there will be an abundant spontaneous vegetation. The accumulations of that short period will have settled into a close mass, almost as hard as stone; and if fishes have died in the mean time, and have not been removed, their bones will be found overlaid with hardened mud, just as we find them in the old red sandstone, or the chalk, or the carboniferous rocks, and shall we not call them our own fossils? See again in this case in which
