Spontaneous generation refers to the idea that living creatures can arise from nonliving matter. Before microscopes enabled people to view forms of life too small to see with the naked eye, the origins of many living things were a mystery. People often wondered why, for example, maggots would appear in decaying meat. Aristotle first expressed the idea of spontaneous generation in the 4th century b.c., but the scientific community of 17th century Europe raised doubts about Aristotle's idea.

Francesco Redi (1626-1697), an Italian physician and poet, was the first to demonstrate the improbability of spontaneous generation with an experiment involving the reproductive methods of flies. Redi set out to test the possibility that life might arise from tiny seeds or eggs that were too small to be seen. He prepared eight different containers of meat; four were sealed and four left open. After a few...