Spontaneous generation

Spontaneous generation or anomalous generation is an obsolete body of thought on the ordinary formation of living organisms without descent from similar organisms. Typically, the idea was 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. Doctrines supporting such processes of generation held that these processes are commonplace and regular. Such ideas are 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 it is generally accepted to have been decisively dispelled during the 19th century by the experiments of Louis Pasteur. He expanded upon the investigations of predecessors (such as Francesco Redi who, in the 17th century, had performed experiments based on the same principles). However, some experimental difficulties were still there and objections from persons holding the traditional views persisted. Many of these residual objections were dealt with by the work of John Tyndall, succeeding the work of Pasteur.

Pasteur's experiment is generally known to have refuted the theory of spontaneous generation in 1859. Disproof of the traditional ideas of spontaneous generation is no longer controversial among professional biologists. By the middle of the 19th century, the theory of biogenesis had accumulated so much evidential support, due to the work of Louis Pasteur and others, that the alternative theory of spontaneous generation had been effectively disproven. John Desmond Bernal suggests that earlier theories such as spontaneous generation were based upon an explanation that life was continuously created as a result of chance events.

Spontaneous generation refers both to the supposed processes in which different types of life might repeatedly emerge from specific sources other than seeds, eggs or parents, and also to the theoretical principles which were presented in support of any such phenomena. Crucial to this doctrine is the idea that life comes from non-life, with the conditions, and that no causal agent is needed (i.e. Parent). Such hypothetical processes sometimes are referred to as abiogenesis, in which life routinely emerges from non-living matter on a time scale of anything from minutes to weeks, or perhaps a season or so. An example would be the supposed seasonal generation of mice and other animals from the mud of the Nile. Such ideas have no operative principles in common with the modern hypothesis of abiogenesis, in which life emerged in the early ages of the planet, over a time span of at least millions of years, and subsequently diversified without evidence that there ever has been any subsequent repetition of the event.

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