page_title: Spontaneous generation
section_title: Modern tests
hierarchical_section_title: Spontaneous generation / Modern tests
caption_attribution_description: Italiano: Esperimento condotto da Francesco Redi sull'abiogenesi (origine della vita).
context_page_description: Spontaneous generation is a body of thought on the ordinary formation of living organisms without descent from similar organisms. The theory of spontaneous generation held that living creatures could arise from nonliving matter and that such processes were commonplace and regular. It was hypothesized that certain forms such as fleas could arise from inanimate matter such as dust, or that maggots could arise from dead flesh. 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. The idea of univocal generation, by contrast, refers to effectively exclusive reproduction from genetically related parent, generally of the same species.
  The doctrine of spontaneous generation was coherently synthesized by Aristotle, who compiled and expanded the work of earlier natural philosophers and the various ancient explanations for the appearance of organisms, and was taken as scientific fact for two millennia.
context_section_description: Jan Baptist van Helmont (1580–1644) used experimental techniques, such as growing a willow for five years and showing it increased mass while the soil showed a trivial decrease in comparison.  As the process of photosynthesis was not understood, he attributed the increase of mass to the absorption of water. His notes also describe a recipe for mice (a piece of soiled cloth plus wheat for 21 days) and scorpions (basil, placed between two bricks and left in sunlight).  His notes suggest he may even have done these things.
  Where Aristotle held that the embryo was formed by a coagulation in the uterus, William Harvey (1578–1657) by way of dissection of deer, showed that there was no visible embryo during the first month.  Although his work predated the microscope, this led him to suggest that life came from invisible eggs.  In the frontispiece of his book Exercitationes de Generatione Animalium (Essays on the Generation of Animals), he made an expression of biogenesis: "omnia ex ovo" (everything from eggs).
  The ancient beliefs were subjected to testing.  In 1668, Francesco Redi challenged the idea that maggots arose spontaneously from rotting meat.  In the first major experiment to challenge spontaneous generation, he placed meat in a variety of sealed, open, and partially covered containers.  Realizing that the sealed containers were deprived of air, he used "fine Naples veil", and observed no worm on the meat, but they appeared on the cloth. Redi used his experiments to support the preexistence theory put forth by the Church at that time, which maintained that living things originated from parents. In scientific circles Redi's work very soon had great influence, as evidenced in a letter from John Ray in 1671 to members of the Royal Society of London:
  Whether there be any spontaneous or anomalous generation of animals, as has been the constant opinion of naturalists heretofore, I think there is good reason to question. It seems to me at present most probable, that there is no such thing; but that even all insects are the natural issue of parents of the same species with themselves. F. Redi has gone a good way in proving this, having cleared the point concerning generation ex materia putrida. But still there remain two great difficulties. The first is, to give an account of the production of insects bred in the by-fruits and excrescencies of vegetables, which the said Redi doubts not to ascribe to the vegetative soul of the plant that yields those excrescencies. But for this I refer you to Mr. Lister. The second, to render an account of insects bred in the bodies of other animals. I hope shortly to be able to give you an account of the generation of some of those insects which have been thought to be spontaneous, and which seem as unlikely as any to be after the ordinary and usual way.
  Pier Antonio Micheli, around 1729, observed that when fungal spores were placed on slices of melon the same type of fungi were produced that the spores came from, and from this observation he noted that fungi did not arise from spontaneous generation.
  In 1745, John Needham performed a series of experiments on boiled broths.  Believing that boiling would kill all living things, he showed that when sealed right after boiling, the broths would cloud, allowing the belief in spontaneous generation to persist.  His studies were rigorously scrutinized by his peers and many of them agreed.
  Lazzaro Spallanzani modified the Needham experiment in 1768, attempting to exclude the possibility of introducing a contaminating factor between boiling and sealing. His technique involved boiling the broth in a sealed container with the air partially evacuated to prevent explosions.  Although he did not see growth, the exclusion of air left the question of whether air was an essential factor in spontaneous generation. However, by that time there was already widespread scepticism among major scientists, to the principle of spontaneous generation. Observation was increasingly demonstrating that whenever there was sufficiently careful investigation of mechanisms of bi
caption: A modern rendering of Francesco Redi's 1668 experiment on abiogenesis. Flies form on the meat in the open jar (left) but not in the closed jar (right).
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