The views of Redi prevailed for a century, and the hypothesis of spontaneous generation had become completely discredited. But meantime the microscope had been improved, and a new world of life revealed. When animal or vegetable substances are infused for a time in water, swarms of creatures are produced in it, called infusorial animalculæ, and which are so small that they can only be seen with a powerful magnifier. This was a new aspect of the production of life, and favored the view of its spontaneous origin. In the middle of the eighteenth century, an Englishman named Needham took the ground that, although putrefying meat may not engender insects, it may yet give rise to animalculæ. "If," said Needham, "these animalculæ come
A. Plastide-particles; B. Bacteria; C. Torula; D. Vibriones; E. Spirilla; F. Leptothrix.
from germs, these germs must exist, either in the infused substance, the water, or the adjacent air. All germs are killed by heat; if, therefore, I boil the infusion, seal it up, and then heat the whole vessel, I shall destroy the germs." He did this; but, after waiting for a time, the animalculæ still appeared in the closed vessel. The experiment seemed conclusive in favor of spontaneous generation, or life without germs; but, again, a learned Italian appeared and attacked the hypothesis. Spallanzani repeated Needham's experiments with more vigilant precautions. He closed the tubes more effectually, and exposed them to a greater heat, after which the animalculæ failed to appear. The real issue in the case was thus fairly reached, and the question became one of the existence of atmospheric germs, and of their power of resisting heat. The results of Spallanzani were generally held conclusive against the hypothesis of spontaneous generation; but, toward the middle of the present century, the question was again opened, and it has been assiduously investigated and hotly discussed by men of science for the last forty years. We cannot even mention the numerous contributions to it that have been made by eminent scientists, but