Spontaneous generation is the belief that lower forms of life might spontaneously arise from non-living material. It stemmed from everyday (but incomplete) observations such as that insects and worms appeared from rotting meat, frogs from mud, and mice from rotting wheat.
Some of the earliest Western thinkers believed that life forms emerged from a "primordial slime" that was either activated by the sun and air or fertilized by seeds transmitted by them. Aristotle (384–322 BC) was the first figure to argue that some life forms emerge spontaneously by means of an elemental principle similar to a seed. Aristotle stated that reproduction involves an active principle. Represented by the male's semen, which acts upon a passive principal, represented by the material substrate of the potential organism provided by the female. The form of the organism is transmitted by the semen to the material, giving rise to the new organism. In spontaneous generation, however, the passive substrate is a mixture of seawater and earth, not a living organism. The active principle is provided by pneuma or "vital heat," found in all things in varying levels. This pneuma stimulates a sort of fermentation that eventually gives rise to the new organism.
According to Aristotle, only certain organisms are produced in this way – namely, certain fish, oysters, and eels. These organisms provided Aristotle with empirical evidence that spontaneous generation occurs: that oysters are generated spontaneously, for example, is suggested by the fact that they do not multiply during transportation. Furthermore, the "eggs" that they contain never hatch offspring. Eels, according to Aristotle's observations, did not possess the organs required for procreation nor did they produce eggs.
Speculation concerning the possibility of spontaneous generation gave rise to a number of biological theories, including the homunculus (little man). It was espoused by theologians in the Middle Ages, including Thomas Aquinas, and upheld by the likes of William Harvey and Isaac Newton. Only when the hypothesis was properly put to the test by experiments, such as those of Redi (1668) Spallanzani (1765), de La Tour (1837), Schwann and, most decisively, by Pasteur (1862), was it seen to be in error. Any lingering doubts were removed by the work of Tyndall. However, the notion that life can develop from non-life, albeit over many millions of years, has been revived in the modern concept of the origin of life from prebiotic chemicals.
1. Farley, J. The Spontaneous Generation Controversy from Descartes to Oparin. Baltimore: The Johns Hopkins University Press (1977).