Some small organisms are visible to the eye, at least in large numbers. Thousands of years ago, people came up with an explanation for the sudden appearance of mold on bread, maggots on meat, mice in grain: The creatures came to life spontaneously in decaying organic matter.
The theory of spontaneous generation — the belief that under the right circumstances living organisms could come into being without parents — was the target of perhaps the first real scientific experiment, in 1668.
That was the year that the Italian physician Francesco Redi set out to prove his idea that maggots came from eggs laid by flies. This was no fluke: Redi was an intellectual who belonged to prestigious literary societies and undertook many experiments over the course of his life.
Redi set out three groups of jars containing rotting meat. One group he closed completely, one he covered with gauze, and one he left completely open.
As time went on, flies enter the uncovered jars. They landed on the gauze on the partially covered jars. However, there were no flies around the totally covered jars.
Later, many maggots appeared on the meat in uncovered jars. A few maggots appeared on the meat in the partially covered jars. No maggots showed up on the meat in the totally covered jars.
Redi's use of several jars for each situation showed that his results could be replicated, an important aspect of any scientific experiment.
Redi had proved that flies had to be present on or around the meat for maggots to generate. His work began to raise doubts about spontaneous generation, though it was a long time before it was truly put to rest.