Redi believed that maggots developed from eggs laid by flies. To test his hypothesis, he set out meat in a variety of flasks, some open to the air, some sealed completely, and others covered with gauze. As he had expected, maggots appeared only in the open flasks in which the flies could reach the meat and lay their eggs. This was one of the first examples of an experiment in the modern sense, in which controls are used. In spite of his well-executed experiment, the belief in spontaneous generation remained strong, and even Redi continued to believe it occurred under some circumstances.
The invention of the microscope only served to enhance this belief. Microscopy revealed a whole new world of organisms that appeared to arise spontaneously.
It was quickly learned that to create "animalcules," as the organisms were called, you needed only to place hay in water and wait a few days before examining your new creations under the microscope. Maggots were derived from rotting meat, aphids and beetles supposedly sprang from wheat, and mice were generated from soiled clothing mixed with wheat grains. While these theories seem quite ludicrous, at the time they were thought to be reasonable explanations for how certain bugs and other animals seemed to appear from no other living matter.
While a popular theory throughout history, spontaneous generation was not without its critics. Several scientists set out to refute this theory through scientific experimentation. At the same time, other scientists tried to find evidence in support of spontaneous generation.
This debate would last for centuries. In , the Italian scientist and physician Francesco Redi set out to disprove the hypothesis that maggots were spontaneously generated from rotting meat. He contended that the maggots were the result of flies laying eggs on exposed meat. In his experiment, Redi placed meat in several jars. Some jars were left uncovered, some were covered with gauze, and some were sealed with a lid.
Over time, the meat in the uncovered jars and the jars covered with gauze became infested with maggots. However, the meat in the sealed jars did not have maggots. Since only the meat that was accessible to flies had maggots, Redi concluded that maggots do not spontaneously arise from meat.
In , English biologist and priest John Needham set out to demonstrate that microbes, such as bacteria , were the result of spontaneous generation. Thanks to the invention of the microscope in the s and increased improvements to its usage, scientists were able to view microscopic organisms such as fungi , bacteria, and protists. In his experiment, Needham heated chicken broth in a flask in order to kill any living organisms within the broth. He allowed the broth to cool and placed it in a sealed flask.
Needham also placed unheated broth in another container. Over time, both the heated broth and unheated broth contained microbes. Needham was convinced that his experiment had proven spontaneous generation in microbes.
A variant idea was that of equivocal generation, in which species such as tapeworms arose from unrelated living organisms, now understood to be their hosts. Doctrines held that these processes were commonplace and regular.
Such ideas were in contradiction to that of univocal generation: effectively exclusive reproduction from genetically related parent s , generally of the same species. The doctrine of spontaneous generation was coherently synthesized by Aristotle, who compiled and expanded the work of prior natural philosophers and the various ancient explanations of the appearance of organisms; it held sway for two millennia.
Today spontaneous generation is generally accepted to have been decisively dispelled during the 19 th century by the experiments of Louis Pasteur. He expanded upon the investigations of predecessors, such as Francesco Redi who, in the 17 th century, had performed experiments based on the same principles. In summary, Pasteur boiled a meat broth in a flask that had a long neck that curved downward, like a goose.
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