This singular race of beings has given rise to the erroneous hypothesis of equivocal or spontaneous generation, that is to say, the production of living animalcules by a chemical or even fortuitous combination of the elements of inert matter. That question has been decided by direct experiment, for Professor Schultz kept boiled infusions of animal and vegetable matter for weeks in air which had passed through a red-hot tube, and no animalcules were formed, but they appeared in a few hours when the same infusions were freely exposed to the atmosphere, which shows clearly that the germs of the lowest grade of animal life float in the air, waiting as spores do, till they find a nidus fit for their development. M. Pasteur, Director of the Normal School in Paris, in a series of lectures published in the ‘Comptes Rendus,’ has not only proved that the atmosphere abounds in the spores of cryptogamic fungi and moulds, but with infusoria of the form of globular monads, the Bacteria, and vibrios, which are like little rods round at their extremities and extremely active. The Bacteria mona and especially the Bacteria terma, are exceedingly numerous. These minute beings are the principal agents in the decomposition of organic matter. They are more numerous in dry than in wet weather, in towns than in the country, on plains than on mountains. In a memoir read at the Academy of Sciences, Paris, Mr. J. Samuelson mentions that he had received rags from Alexandria, Japan, Melbourne, Tunis, Trieste and Peru. He sifted dust from the rags from each of these localities respectively through fine muslin into vases of distilled water. Life was most abundant in the vases containing dust from Egypt, Japan, Melbourne, and Trieste. The development of the different forms was very rapid, and consisted of protophytes, Rhizopods and true Infusoriæ. In most of the vases monads and vibrios appeared first, and from these Mr. Samuelson traced a change first into one then into another species of infusoria. In the dust from Japan he followed the development of a monad into what appeared to be a minute Paramœcium, then into Lexodes cucullus, and finally into Colpoda cucullus. From these and other experiments it is proved that many infusoria now classed as distinct types are really one and the same animal in different states of development. That appears to be the case also with the Amœbæ. In the dust from Egypt Mr. Samuelson found a new Amœba whose motions were very rapid; as to shape and mode of motion he compared it to soap bubbles blown with a pipe. He traced the gradual changes of the globular form of this Amœba until its pseudopodia were in full action, its increase by conjugation, and other circumstances of its life. In the same dust and in that only, the development of the Protococcus viridis was seen, and that in such abundance that at last the water was tinged green by that plant. In the dust from Egypt a vibrio was changed into a vermiform