The belief that under certain conditions living creatures can be produced directly from matter. The concept is well attested in classical sources, notably Aristotle, and is probably based on even more ancient observations of the emergence of organisms such as fly larvae from dead animal tissue or worms form mud. General considerations have distinguished between Abiogenesis generation from inorganic matter and Heterogenesis generation from living or formerly living tissue. At various times and places, potential candidates for spontaneous generation have included algae, fungi, insects, worms, bacteria, and even small mammals.
Owing to the high regard for Aristotelian science and the ease with which the process could be confirmed by rough observation, belief in spontaneous generation was little doubted by Europeans for centuries. In the 16th and 17th centuries, however, the increasing prestige of experimental science and the development of better instrumentation (particularly microscopes) initiated a complex debate among scientists over the status of spontaneous generation that would last until the 20th century. Notably, the concept continued to elicit support late into the 19th century, even after a great deal of experimental data had been arrayed against it.