Spallanzani was among the first to show experimentally that living organisms—such as maggots in rotting meat—could not simply appear out of nowhere. Though his work was not considered conclusive on the subject, it represented the beginnings of a modern view of biology.
Naturalists before the eighteenth century had observed many instances of what seemed to them to be the “spontaneous generation” of life. Meat left out would sprout maggots, and frogs could emerge from apparently simple mud. There was no mystery associated with these seeming miracles, however. Indeed, spontaneous generation made perfect sense to those who believed in a Creator who had the ability to produce life from abiotic matter. If life had first originated in this manner, the reasoning went, life could appear again through similar means. There also were nonreligious explanations for the phenomenon. Greek philosopher Aristotle, in the fourth century
The first significant experiments to address the subject of spontaneous generation were carried out by Franceso Redi in 1668. An Italian physician and member of the Accademia del Cimento (Academy of Experiment)
The debate over spontaneous generation continued for more than a century, and the development of the microscope,
Needham later went to Paris, where he met and began a collaboration with the comte de Buffon. Buffon was well noted for his contributions to the growing field of comparative anatomy in the massive work Histoire naturelle, générale et particulière (1749-1789; Natural History, General and Particular,
Lazzaro Spallanzani had a differing interpretation of Needham’s results, however: He believed that the broth had been contaminated before being sealed. His criticism of Needham’s techniques formed the basis for a 1765 dissertation on the subject. He also, more important, devised a set of practical tests to confirm his hypothesis that Needham’s samples must have been contaminated. Spallanzani’s experimental procedure was relatively simple: He boiled his samples for varying periods to ensure that nothing survived and then sealed the mixture in an airtight container. Beginning with a duration of forty-five minutes, Spallanzani tested various boiling periods, observing whether anything would still grow in the broth after each test.
Spallanzani determined that extensive boiling prevented microorganisms from growing, resulting in the medium remaining sterile. To refute the potential counterargument that boiling destroyed a “life force” that had existed in the broth and that was necessary for spontaneous generation to occur, Spallanzani sealed his vessels with semipermeable barriers. He created seals with pores of various sizes, and he observed that the number of organisms that returned to the boiled broth was a function of the pore size. The implication of this result was that contamination from air had been the source of growth in Needham’s experiments.
Spallanzani also observed that there existed several classes of organisms that differed in their sensitivity to heat. One class, probably protozoa, was highly sensitive to heat, and Spallanzani labeled this class “superior animalcula.” On the other hand, the class he named “lower class animalcula,” probably bacteria, was less sensitive. Thus, a nonrigorous application of heat in an experiment might kill only the more sensitive microscopic organisms, leaving the less sensitive ones to “appear spontaneously” afterward.
Despite these results, Spallanzani’s experiments did not resolve the issue of spontaneous generation in the minds of all scientists. The experiments’ results admitted of different interpretations, especially after Joseph Priestley
The theory explained quite efficiently an otherwise mysterious phenomenon that, in the days before preservatives and refrigeration, was extremely common—the sudden appearance of maggots, flies, or other biological contaminants on seemingly clean foods. The advances in experimental design in the sciences during the sixteenth and seventeenth centuries allowed naturalists to test the theory. Among the first to do so was Francesco Redi. However, despite Redi’s initial results, which seemed to demonstrate that infestation of maggots in meat resulted from flies, not decay, the belief in spontaneous generation continued for years. Even Redi himself was not completely convinced that spontaneous generation was impossible.
Spallanzani’s work, following from Redi’s, was carried out during a period in the eighteenth century in which science
Where Redi’s experiments, important in their use of biological controls, had demonstrated that relatively large forms of life would not spontaneously appear, Spallanzani showed that even microscopic organisms could be eliminated through the use of heat. In the process, he brought an experimental approach to the study of such organisms, complementing the scientific approach to the macroscopic world developed by the comte de Buffon. Nicolas Appert, a French cook, several decades later would apply Spallanzani’s approach in developing a method of canning
In the immediate wake of Spallanzani’s experiments, however, the debate concerning spontaneous generation was not yet settled. Criticism of Spallanzani’s experiments centered on the lack of a formal, scientifically rigorous understanding of the nature of life. Various vitalist arguments about a “life force” that made generation possible complicated matters tremendously: If this hypothetical force was itself intangible, it would be difficult or impossible to prove that it had not been altered by heat, the lack of air, or other effects of Spallanzani’s experiments.
Moreover, because of the state of experimental design in the 1760’s, Spallanzani himself never came to the conclusion that microorganisms were present in the air. The fact that there was a gas called oxygen in the air, indeed the fact that the air was composed of a mixture of different gases, was new information added to the debate in the 1770’s. As a result, one could still not rule out the possibility that it was the presence of oxygen that was required for spontaneous generation to occur.
It would be nearly one hundred years before Louis Pasteur
Spallanzani would continue to provide commonsense solutions to experimental problems in other areas of biology for the rest of his career. Addressing the question of how fertilization of frogs’ eggs is carried out, Spallanzani dressed male frogs in “pants,” noting that fertilization did not occur under such circumstances. He concluded that an external process must account for fertilization. He demonstrated the role of sperm by filtering frog semen, again noting the absence of fertilization once the semen had been filtered.
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Brock, Thomas. Milestones in Microbiology, 1546-1940. Washington, D.C.: ASM Press, 1999. Compilation of original historic papers, including one by Spallanzani (1799), in various areas of microbiology. Commentary by the editor discusses the significance of each work. -
Buffon, Georges-Louis Leclerc, Comte de. Natural History: General and Particular. Translated by William Smellie. Bristol, Avon, England: Thoemmes Press, 2001. Translation of Buffon’s work on comparative anatomy and evolution, which represented an attempt to present a systematic view of nature. -
Geison, Gerald. The Private Science of Louis Pasteur. Princeton, N.J.: Princeton University Press, 1995. Pasteur’s “swan-neck flask” experiment effectively ended the debate over the validity of spontaneous generation. -
Lechevalier, Hubert, and Morris Solotorovsky. Three Centuries of Microbiology. New York: Dover, 1974. Describes the history of the subject from Girolamo Fracastoro (sixteenth century) through geneticists of the twentieth century. Emphasis is on the leading figures during these centuries. -
Lennox, J. “Teleology, Chance, and Aristotle’s Theory of Spontaneous Generation.” Journal of the History of Philosophy 19 (1981): 219-238. Summarizes Aristotle’s viewpoint on the subject, significant because his was one of the earliest theories. -
Strick, James. Sparks of Life: Darwinism and the Victorian Debates over Spontaneous Generation. Cambridge, Mass.: Harvard University Press, 2000. Analysis of the scientific debates concerning spontaneous generation during the mid- to late nineteenth century.
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Comte de Buffon; Joseph Priestley; Lazzaro Spallanzani.