Spontaneous generation is the
scientific fallacy that holds that some forms of
life arise spontaneously from non-living matter. This generation was observed primarily in
decaying matter, for example, a slab of dead meat would give rise to
maggots. Or take a
seventeenth century recipe for the spontaneous production of
mice: place sweaty underwear and husks of wheat in an open-mouthed jar, then wait for about 21 days, and the sweat from the underwear will turn the husks of wheat into mice. The invention of the microscope only strengthened this theory, as a whole world of "animacules" could now be observed to arise from, for example, hay left in water.
The theory of spontaneous generation was put to death by experiments by
Francesco Redi,
Lazzaro Spallanzani, and
Louis Pasteur, in the 17th through 19th centuries.