expire day and night, is as essential as oxygen is to the animal economy, and thus, without introducing a single scrap of any living plant, the balance is sustained, and death seems to be kept at a distance. If at first I threw in a tuft of callitriche or anacharis, or any other true aquatic vegetable, oxygen would be supplied abundantly; and in practice it might be well to begin so, because some little time elapses ere the seeds of the microscopic forest, the tops of whose trees present to the eye but a felt-like coating of superficial greenness, are developed into true plants; though with a fair amount of indirect daylight, and at certain seasons of the year, a few hours suffice to set the vegetative process, with all its proper consequences, in full action. Many of the readers of this paper will call to mind the aquarium that stands in my entrance-hall. It contains twenty fishes, large and small, and not a single scrap of vegetation except what has been developed in situ by spontaneous generation. It is three years since that was fitted and stocked, and committed to the management of Nature, with the sole exception of the external aid afforded by regular supplies of food for its inmates, which need not be taken account of, now that we are considering it as a world in which the balance of life and death is sustained by the operation of principles ordained by the Creator.
It is when we leave the principles and attempt to classify the details of the scheme that we become bewildered. The smooth revolution of the fly-wheel and the noiseless oscillation of the piston convince the unprofessional observer of a great engine that mechanical motions are possessed of poetry; but, if he would analyze the relations of the cog-wheels, the indications of the "governor," the "gauge," and the pressure-valve, he must descend to hard facts, and forget for a while the sublime suggestions of a system of mechanism that throbs like a living creature. Admit a full glare of summer sun to the aquarium, and forth-with the water loses its pellucid fluidity, and becomes deeply tinged throughout of a dull green, as if some pigment had been dissolved in it. Instead of plants attached to stones and glass only, and animals that float unseen, the whole of the water is occupied by visible masses of animal and vegetable life; and, if the fishes suffer, it will be from undue heat, not from the addition to the element in which they live of this new mass of being. Shut out the sunshine, let the fresh air play over the surface of the water, let moderate daylight stream through it as before, and speedily the green fog clears away, the water again becomes transparent, and the balance is restored. Monas, euglena, uvella, cryptomonas, gonium, and other wondrous infusoria, may be detected as constituents of the cloudy mass while it lasts, called into being because the conditions of the tank were such as they required, as if life in embryo were everywhere locked up until the moment came for its liberation, and some particular circumstance was the talisman to set it free or, if we consider created forms to be marshalled in grand
