At a depth in the sea which exceeds the height of the loftiest mountain, Humboldt asserts that each bed of water is animated by an innumerable phalanx of inhabitants imperceptible to the human eye. These microscopic creatures are, in short, the smallest and the most numerous creations in Nature. They constitute with human beings one of the wheels of that very complicated machine, the globe. They are in the rank and at the station willed for them, as determined in the great First Thought. Suppress these microscopic beings, and the world would be incomplete. It was said, and wisely said, long, long ago, "there is nothing so small to the view but that it may become great by reflection." The Infusoria, in short, abound everywhere. We find their remains on the loftiest mountain ridges, and in the profoundest depths of the sea. They increase and multiply alike under the Equator, and towards the polar regions. The seas, rivers, ponds--the flower vase which rests upon the casement--even our tissues, and the fluids of our bodies--all contain infusorial animalcules. Whole beds of strata, often many feet thick, and covering a surface of considerable extent, are almost exclusively formed of their accumulated débris. It is to the Infusoria that the mud of the Nile and other fluviatile and lacustrine deposits owe their prodigious fertility. To them also is due the red or green layer of colouring matter found in ponds and tanks at certain seasons. When exposed to great solar heat, in order to extract the salt, as it is in the vast artificial basins hollowed out for the purpose in the salt marshes near the sea-shore in the south of France, the salt water, when it reaches a certain degree of concentration, acquires a fine rose colour, which is due to the presence of innumerable masses of small Infusoria having a reddish shell. Finally, let us add that the solid débris of certain fossil Infusoria, of surprising minuteness, have formed the stone so much used by workers in metal, which is known as _tripoli_. The study of these creatures is intensely interesting to the naturalist, the philosopher, the physician, and the general reader. They have had a great part assigned to them in Nature, as is evident in the formation of certain beds of rock of immense extent, in which the geologist traces their action. Our earliest knowledge of the Infusoria is traceable to the seventeenth century; to the celebrated naturalist, Leuwenhoek, we are indebted for their discovery. On the 24th of April, 1676, this observer saw for the first time some infusorial animalcules. Fifty years later, Baker and Trembley studied them anew. In 1752, Hill essayed the first attempt at their classification. In 1764, Wiesberg gave them the name of Infusoria, because he found them in such great abundance in animal and vegetable _infusions_. Müller published a special book upon them.