Historic Dispute : Are infusoria (microscopic forms of life) produced by spontaneous generation

views updated

LIFE SCIENCE

Historic Dispute : Are infusoria (microscopic forms of life) produced by spontaneous generation?

Viewpoint: Yes, prior to the nineteenth century, many scientists believed that infusoria are produced by spontaneous generation.

Viewpoint: No, experiments by Louis Pasteur in the nineteenth century confirmed that infusoria are not produced by spontaneous generation.

As suggested by the term "Mother Earth," many ancient cultures shared the assumption that the earth is a nurturing organism, capable of giving birth to living creatures. From the earliest systems of religion and philosophy up to the seventeenth century, belief in the spontaneous generation of living beings from nonliving matter was almost universal. The doctrine of spontaneous generation generally was applied to the lowest creatures, parasites, and vermin of all sorts, which often appeared suddenly from no known parents.

Since the time of the Greek philosopher Aristotle in the fourth century b.c., philosophers and scientists have seen the question of spontaneous generation as an essential element in the study of the natural world. Generally, the ancients assumed the existence of spontaneous generation, that is the appearance of living beings from nonliving materials, without doubt or question. For Aristotle, examining the existence of spontaneous generation was part of his attempt to establish a natural scheme for the classification of living beings. An ideal system of classification would recognize the many characteristics displayed by all living beings and analyze them in order to reveal the natural affinities beneath the bewildering variety of structures and functions they presented to the natural philosopher. In evaluating the characteristics of animals, such as structure, behavior, habitat, means of locomotion, and means of reproduction, Aristotle concluded that means of reproduction was the most significant factor. Animals could be arranged hierarchically by examining their level of development at birth.

Aristotle described three major means of generation: sexual reproduction, asexual reproduction, and spontaneous generation. Heat, according to Aristotle, was necessary for sexual, asexual, and spontaneous generation. The animals now known as mammals were at the top of Aristotle's hierarchy because they reproduced sexually, and the young were born alive and complete. At the bottom of his scheme were creatures like fleas, mosquitoes, and various kinds of vermin that were produced by spontaneous generation from slime and mud in conjunction with rain, air, and the heat of the sun. According to Aristotle's observations, certain kinds of mud or slime gave rise to specific kinds of insects and vermin. For example, a combination of morning dew with slime or manure produced fireflies, worms, bees, or wasp larvae, while moist soil gave rise to mice.

Although many seventeenth-century naturalists continued to accept the doctrine of spontaneous generation, the Italian physician Francesco Redi, a member of the Academy of Experiments of Florence, initiated a well-known experimental attack on the question that also helped to clarify the life cycle of insects. Noting the way that different flies behaved when attracted to various forms of rotting flesh, Redi suggested that maggots might develop from the objects deposited on the meat by adult flies. Published in 1668 as Experiments on the Generation of Insects, Redi's experiments changed the nature of the debate about spontaneous generation. The introduction of the microscope in the seventeenth century, moreover, proved that the so-called lower creatures were composed of complex parts and were, therefore, unlikely to arise spontaneously from mud and slime. Arguments about the generation of macroscopic creatures were largely abandoned. Seventeenth-century microscopists, however, discovered a new world teeming with previously invisible entities, including protozoa, molds, yeasts, and bacteria, which were referred to as infusoria or animalcules.

Antoni van Leeuwenhoek, a Dutch naturalist and the most ingenious of the pioneering microscopists, was quite sure that the "little animals" he discovered with his microscopes were produced by parents like themselves, but other naturalists took exception to this conclusion. Indeed, questions concerning the nature, origin, and activities of the infusoria were still in dispute well into the late nineteenth century. Studies of the infusoria suggested new experimental tests of the doctrine of spontaneous generation. In 1718 Louis Joblot published an illustrated treatise on the construction of microscopes that described the tiny animals found in various infusions. Following the precedent established by Redi, Joblot attempted to answer questions about the spontaneous generation of infusoria by a series of experiments. Joblot compared flasks of nutrient broth that had been covered or uncovered after boiling. When he found infusoria in the open flask, but not in the sealed vessel, he proved that the broth in the sealed flask could still support the growth of infusoria by exposing it to the air. Supporters of the doctrine of spontaneous generation, such as the French naturalist Georges-Louis Leclerc, comte de Buffon, and the English microscopist John Turberville Needham, attacked Joblot's methods and conclusions. In their hands, flasks of nutrient broth produced infusoria under essentially all experimental conditions.

Many seventeenth-and eighteenth-century naturalists regarded spontaneous generation as a dangerous, even blasphemous, materialistic theory and challenged the claims made by Needham and Buffon. A series of experiments conducted by the Italian physiologist Lazzaro Spallanzani raised questions about Needham's experimental methods. According to Spallanzani, the infusoria that appeared under various experimental conditions entered the vessels through the air. Advocates of spontaneous generation argued that Spallanzani's attempts to sterilize his flasks had destroyed the "vital force" ordinarily present in broths containing organic matter. Reflecting on the status of biology in the 1860s, the German embryologist Karl Ernst von Baer suggested that his studies of the stages in the development of the mammalian embryo from the egg were an important factor in diminishing support for the doctrine of spontaneous generation. Nevertheless, although he regarded spontaneous generation as "highly problematic," he did not think the question had been unequivocally settled.

During the nineteenth century, the design of experiments for and against spontaneous generation became increasingly sophisticated, as proponents of the doctrine challenged the universality of negative experiments. The debate about spontaneous generation became part of the battle over evolutionary theory and the question of biogenesis, the origin of life. Microorganisms were also at the center of great debates about medicine, surgery, and the origin and dissemination of disease and infection. The great French chemist Louis Pasteur entered the battle over spontaneous generation through his studies of fermentation and the stereochemistry of organic crystals. Pasteur often argued that microbiology and medicine could only progress when the idea of spontaneous generation was totally vanquished. Many of Pasteur's experiments were designed to refute the work of Félix-Archiméde Pouchet, a respected French botanist and zoologist, the champion of a doctrine of spontaneous generation called heterogenesis. Pasteur also challenged the work of Henry Charlton Bastian, an English pathological anatomist, who claimed to have evidence for archebiosis, the production of life from inanimate matter.

The spontaneous generation debate was important to Pasteur for professional and political reasons, although he knew that it was logically impossible to demonstrate a universal negative; that is, one cannot prove that spontaneous generation never occurred, never occurs, or will never occur. Indeed, advocates of spontaneous generation have argued that some form of the doctrine is necessarily true in the sense that if life did not always exist on Earth, it must have been spontaneously generated when the planet was very young. In practice, Pasteur's experiments were designed to demonstrate that microbes do not spontaneously arise in properly sterilized media under conditions prevailing today. Nevertheless, Pasteur and his disciples regarded the spontaneous generation doctrine as one of the greatest scientific debates of the nineteenth century.

—LOIS N. MAGNER

Viewpoint: Yes, prior to the nineteenth century, many scientists believed that infusoria are produced by spontaneous generation.

The belief that life can spontaneously generate from nonliving matter has had a great deal of intellectual appeal for many centuries. Certain philosophical approaches and scientific theories have encouraged support for spontaneous generation. This is particularly true of theories that have emphasized the existence of vital forces within nature, perceiving the whole of nature, living and nonliving, as having a basic unity. Depending on the strength or weakness of such approaches to the understanding of nature, support for spontaneous generation has fluctuated over the last few centuries. In the nineteenth century, against the background of scientific, religious, and social transformation, debate over spontaneous generation reached intense levels, as the leading scientists of the age used the issue to establish their own scientific authority.

Definition

The term "spontaneous generation" refers to the theory that certain forms of life are generated from other, nonliving materials, rather than being reproduced from living members of their own species. As with any scientific debate, the definition of different terms was important to the people involved in the controversy. Spontaneous generation covered a variety of different beliefs about the way in which life forms come into being. For example, "heterogenesis" referred to the belief that life could spontaneously generate from organic matter, and "abiogenesis" referred to the more radical belief that life could be formed from inorganic matter. In addition, the definition of life itself and the nature of the living entities that were supposedly being generated was an issue of intense discussion. As the world of microscopic life was opened to human observation for the first time, words such as "bacteria," "molecules," "infusoria," "animalcules," and "germs" were all used to describe the new forms of life being discovered. The definition of these terms was not clear, and meanings differed from theory to theory and from scientist to scientist. As some historians have pointed out, it was the very authority to define such terms that was often at stake in debates over the nature of life and its generation.

From Aristotle to the Eighteenth Century

Beliefs about the spontaneous generation of life have existed for centuries. In Greek philosophy, Aristotle claimed that certain lower forms of life such as worms, bees, wasps, and even mice were produced as a result of heat acting on mud, slime, and manure. Maggots on meat were believed to have spontaneously generated from the rotting flesh. Such beliefs persisted from antiquity into the seventeenth century, when the Flemish physician and chemist Jan Baptista van Helmont claimed to have produced the spontaneous generation of mice from a mixture of dirty rags and wheat. However, spontaneous generation fell out of favor toward the end of the seventeenth century, as religious and philosophical ideas changed. The Italian natural philosopher Francesco Redi showed through a series of experiments that maggots were not spontaneously generated, but were hatched from eggs laid on meat by flies. New ideas about the generation of life developed, and those who advocated these ideas attacked spontaneous generation in order to defend their own theories.

The area of contention moved from larger forms of life, such as mice and insects, to the new dimensions of microscopic life that had been recently discovered under the microscope of the Dutch naturalist Antoni van Leeuwenhoek. The teeming world of infusoria and animalcules, so new and mysterious, proved amenable to ideas about the creation of life through spontaneous generation. Through the microscopes of eighteenth-and nineteenth-century scientists, these forms of life appeared so simple that the distinction between them and nonliving matter was blurred. It appeared reasonable to believe that such forms of life could be spontaneously generated, as it seemed unlikely that they themselves would have the complexity required to reproduce.

Other philosophical developments helped to encourage support for spontaneous generation. The growing influence in the late eighteenth century of Newtonian physics, with its emphasis upon dynamic forces in nature, also encouraged belief in spontaneous generation. Such vitalist approaches supported the contention that it was possible for particles of non-living matter to be rearranged so that life could be generated. The naturalists John Needham and Georges-Louis Leclerc, comte de Buffon, were the most notable advocates of spontaneous generation in the eighteenth century. Needham conducted several experiments in which he heated meat gravy in sealed containers to kill off any microorganisms, and then observed the regrowth of microscopic life in the gravy. If, as opponents of spontaneous generation claimed, all life came from life, then how did one explain the appearance of life where none existed? For Needham and others, the answer was obviously spontaneous generation. However, those who supported rival theories about the formation of life, such as Lazzaro Spallanzani, conducted similar experiments and obtained different results. Spallanzani claimed to have been able to prevent life from growing in sealed sterilized vessels, indicating that spontaneous generation did not occur and that life could only be produced from other living entities. Scientists on both sides of the debate made accusations about their opponents' experimental methods, but at this stage none of the experiments was generally accepted as providing conclusive evidence and the matter remained unresolved.

The Nineteenth-Century Debates

During the nineteenth century, debate over the issue of spontaneous generation took place against a variety of local philosophical and religious circumstances. In Germany in the early 1800s, spontaneous generation found favor among those who were part of the philosophical movement known as Naturphilosophie. Within this worldview, the spontaneous generation of life was an expression of the essential unity of all things living and nonliving. In mid-nineteenth-century France, theories involving spontaneous generation came to be associated with materialism, atheism, and radical social and political thought. If life could be spontaneously generated, then there appeared to be no need for God as the Creator of all living things. As a result, support for spontaneous generation was regarded by many people as a rejection of orthodox religion, and therefore an attack on all forms of social and political authority in France.

This was the background to the mid-century debates between Félix-Archiméde Pouchet and Louis Pasteur. In 1858 Pouchet began to publish the results of experiments that he claimed proved spontaneous generation could occur. Pouchet was not a social or political radical, and tried to show that belief in spontaneous generation was in accord with orthodox religious belief. However, the conservative opponents of spontaneous generation in France denounced his beliefs as heretical and atheist, and it was important to members of the conservative establishment that Pouchet's conclusions be proved wrong.

Again, the experiments focused on whether microscopic forms of life would grow in solutions in sealed containers, heated to temperatures that would kill all forms of life, and then left to cool. Pouchet was able to consistently produce life in his sealed and sterilized infusions, and his conclusion was that they must have been spontaneously generated from the nonliving organic matter in the vessels. It was generally believed that no forms of life could survive the temperatures to which Pouchet heated his infusions. Pouchet was also sure his experimental technique ensured that the solutions were not contaminated. Therefore, there seemed no other way to interpret the results other than to allow the existence of spontaneous generation.

At this stage, Louis Pasteur, the greatest experimental scientist of his era, entered the debate on spontaneous generation from his work on fermentation. Pasteur had come to the conclusion that fermentation and putrefaction in substances were the result of microbes that were present in the air. Therefore, Pasteur was sure the life forms that had grown in Pouchet's infusions were not spontaneously generated there, but were caused by germ-carrying dust that had contaminated the experiments. Life could only be produced from life after all. For Pasteur, his debate with Pouchet over spontaneous generation was an ideal opportunity to convince the public of his theory about the presence of germs in the air, and their role in the process of fermentation and putrefaction. The issue of spontaneous generation inevitably became caught up with support or opposition to Pasteur's germ theory. To many observers, the idea that life could be spontaneously generated appeared to be less fantastic than Pasteur's theory that the air was filled with microscopic living entities. As Pouchet and others pointed out, if that were the case, surely the air would be so foggy with these life forms as to be impenetrable!

Darwinism and Spontaneous Generation

At the same time as the Pouchet-Pasteur debate, the issue of spontaneous generation came to the fore in England, against the background of controversy over Darwinian evolution. The publication of Charles Darwin's On the Origin of Species by Means of Natural Selection (1859) generated an enormous amount of public debate and conflict. His theory that current species were not created as they now exist, but evolved from earlier life forms over thousands of years, was enormously controversial. It was particularly significant because many people, both supporters and opponents, interpreted Darwinism as providing support for spontaneous generation. Indeed, spontaneous generation appeared necessary to Darwinism, if one was to maintain a purely naturalistic explanation of the beginnings of life. If all species had evolved from a few early life forms, then from what had the very earliest forms of life themselves evolved? Unless one believed that life on Earth had always existed, or invoked some kind of Divine Creation, then it was necessary to believe that life had been spontaneously generated from nonliving matter at least once in the far distant past. To allow this belief was to then admit the possibility that spontaneous generation could still occur, given the right conditions. In the swirl of debate and ideas that surrounded Darwinism, one of the clearest outcomes was that there was no longer an unbridgeable gap between living and nonliving matter in the minds of many people. Spontaneous generation seemed to be necessary in light of evolution's unified concept of nature, a necessary part of the chain that bound together the lifeless and the living.

Henry Charlton Bastian was the most prominent and determined advocate of spontaneous generation in England. Bastian was a supporter of Darwinian evolutionary theory, and one of those who interpreted the theory as lending support to spontaneous generation. During the 1860s and 1870s, Bastian carried out many experiments that he felt showed spontaneous generation did occur. Once again, his proof hinged on the growth of microorganisms in infusions that had been sealed and heated to high temperatures. When they cooled, if signs of life did appear in the infusion, then Bastian interpreted this as the result of the nonliving matter rearranging its most basic components to create new life. Bastian also argued from analogy, comparing the appearance of specks of life in suitable fluids where no life had previously existed to the formation of specks of crystals in other fluids. Opponents attacked Bastian's experimental method, claiming that his sloppy techniques must have led to the contamination of his containers with microorganisms from the outside.

In addition to the connection between Darwinism and spontaneous generation, opposition to Pasteur's germ theory was also behind Bastian's determination to prove that spontaneous generation could occur. Many of those who supported Bastian in England were doctors, and Bastian himself was a neurologist and a professor of pathological anatomy. They were opposed to Pasteur's germ theory because of the enormous consequences it would have for medical theory and practice. They were more willing to believe that the life forms in Bastian's infusions were spontaneously generated because to admit that they were the product of airborne germs would lead to an understanding of disease and its treatment that they believed was misguided and unacceptable. It was also a matter of professional territorial protection. Many physicians resented the intrusion of the new upstart bacteriologists and experimental scientists into the domain of medicine and the understanding of disease.

Therefore, support for spontaneous generation had its source in a variety of different approaches and motivations. The belief that spontaneous generation could occur was an expression of an approach to the world that emphasized the essential unity between all things in nature, living and nonliving. It also required the belief that nonliving matter contained some kind of vital force or energy that could lead to the creation of living entities. In the nineteenth century, the issue of spontaneous generation also became caught up in debate over two of the most significant theories in the history of science—Darwinism and Pasteur's germ theory. For very different reasons, both encouraged the support of the belief that the spontaneous generation of microscopic life forms could occur. Thus, the theory of spontaneous generation was entangled in the most crucial scientific debates of the nineteenth century, which helps to explain why it was such a burning issue.

—KATRINA FORD

Viewpoint: No, experiments by Louis Pasteur in the nineteenth century confirmed that infusoria are not produced by spontaneous generation.

Pre-Nineteenth-Century Background

Although the concept of spontaneous generation was an old one dating back to the ancient Greeks, by the late seventeenth and early eighteenth centuries few European naturalists still believed that plants and animals were created from dead or inorganic matter. Observations and experiments had thoroughly discredited the idea. For example, Francesco Redi demonstrated in 1668 that maggots were not spontaneously generated by rotting meat as most people assumed, but rather developed from eggs laid by flies on the meat. Experiments such as Redi's and those of Marcello Malpighi on plant galls, which were also believed to be spontaneously generated, led to an acceptance that larger life forms could only arise from other living things.

However, with Antoni van Leeuwenhoek's development of the microscope, defenders of spontaneous generation received a boost to their position. Using the microscope, Leeuwenhoek and others immediately discovered the existence of countless hitherto unknown and excessively small creatures that seemed to appear out of nowhere. Originally called animalcules, they were especially likely to be found in infusions of hay and other organic substances. Was it not possible that these infusoria were spontaneously generated in these infusions, particularly in fermenting or putrefying fluids? Did spontaneous generation occur with such primitive life forms?

During the eighteenth century, the controversy became intense. The opponents of spontaneous generation demonstrated that filtering, boiling, or chemically altering a medium could often prevent the appearance of infusoria. On the other hand, defenders of the concept could produce evidence based on experiments where precautions were taken to prevent outside contamination, and yet the organisms still appeared. The culmination of these eighteenth-century debates was the one between John Needham and Lazzaro Spallanzani, a debate that, in its essentials, foreshadowed those between Louis Pasteur and his opponents a century later.

Needham, who was supported in his arguments by the famous zoologist Georges-Louis Leclerc, comte de Buffon, claimed that microscopic organisms developed in infusions that had previously been sterilized by heat. Spallanzani repeated Needham's experiments, but sealed his flasks before heating the infusions. When no organisms developed, he correctly concluded that those observed by Needham had come from the air. Spallanzani argued that this demonstrated that every organism had to have a parent, even the tiny animalcules. There was no spontaneous generation. Needham challenged this conclusion, however, by asserting that Spallanzani's prolonged heating had altered the "vegetative force" in the infusion and had destroyed the air in the flask so that no life could be spontaneously generated in such conditions. In essence, the debate remained mired at this point for the next century.

The Pasteur-Pouchet Debate

In the middle third of the nineteenth century, a series of experiments by Franz Schülze (1836), Theodor Schwann (1837), Heinrich Schröder (1854), and Theodor von Dusch (1859) further demonstrated that what were now called the germs of the microbial life that caused fermentation and putrefaction in infusions were introduced from the air. Collectively, these experiments strongly suggested that the germs of microorganisms already existed, were airborne, and were not spontaneously generated by the infusions themselves. But occasionally, in substances such as milk and egg yolks, these experimenters could not prevent the formation of infusoria, thus encouraging the adherents of spontaneous generation.

The culminating debates over spontaneous generation began in 1858 when Félix-Archiméde Pouchet presented a paper to the Académie des Sciences, France's highest scientific body, in which he claimed to have produced spontaneous generation under carefully controlled conditions that allowed no chance of outside contamination. As the director of the Muséum d'Histoire Naturelle at Rouen, Pouchet was well known to the public through his books of science popularization. He was also a respected naturalist at the height of his career, a man who had made several valuable contributions to science. Thus, his defense of spontaneous generation drew considerable interest with both the scientific community and the public, especially after the publication in 1859 of his lengthy book Hétérogénie ou traité de la génération spontanée basé sur de nouvelles expériences (Heterogenesis: A treatise on spontaneous generation based on new experiments), in which he repeated his claim that life could originate spontaneously from lifeless infusions.

Pouchet's activities caused the Académie des Sciences to offer a prize for the best experiments that could help resolve this controversy over spontaneous generation. Interest in the question was extraordinarily high not only for purely scientific reasons, but also because there were political and religious overtones to the debate. France in this period was controlled by the extremely conservative dictatorship of Napoleon III, who came to power after bitter social warfare in 1848. He was strongly supported by the Catholic Church and by all individuals who feared another social revolution. Those defending spontaneous generation seemed to imply that acts of creation could and did occur without God's intervention. Since the Catholic Church and its teachings were regarded as a bulwark against materialism, socialism, atheism, and revolution, anyone attacking the concept of spontaneous generation could count on favorable backing from the church, social leaders, and the government.

Pouchet's claim was challenged by Louis Pasteur, a chemist who had just completed several years of work on fermentation, demonstrating that the process was caused by microorganisms. Since some scientists argued to the contrary that the microorganisms were the product of fermentation rather than its cause, Pasteur's interest in the debate over spontaneous generation was a natural one. He assumed that just as microorganisms caused fermentation, germs of microorganisms caused what appeared to be spontaneous generation. The Pouchet-Pasteur debate was especially bitter. Pasteur was a relatively young and extremely ambitious man who craved the recognition and support of the public, as well as of the French scientific establishment. Attacking spontaneous generation would garner him that support. Extremely pugnacious by nature, Pasteur never tolerated any questions about the validity of his hypotheses. In addition, he was one of the most brilliant and careful experimenters in the history of science. Pouchet was clearly overmatched in this controversy.

In the period from February 1860 to January 1861, Pasteur presented five short papers to the Académie des Sciences detailing his experiments on the spontaneous generation question. These papers were expanded into his 1861 essay "Mémoire sur les corpuscles organisés qui existent dans l'atmosphére" (Report on the organic corpuscles which exist in the air), which won him the Académie's prize. His purpose in these experiments was to demonstrate that ordinary air contained living organisms ("germs" in his terminology) and that they alone had the ability to produce life in infusions. In other words, there was no spontaneous generation; living things developed from other living things and, in this case, the air itself conveyed the germs. Deprive the infusions of airborne germs, and no microorganisms will form.

In these papers, Pasteur described a number of experiments in which he boiled sugared yeast water to create a sterile but nutrient liquid. He then introduced into the sealed flasks containing this medium air that had also been sterilized, either by various filtration methods or by heating at a high temperature. Under these conditions, no microorganisms formed in the medium. But when he subsequently introduced ordinary air into these flasks, the liquid soon swarmed with microbial life. In a series of control flasks in which the liquid was sterilized but then exposed to the air, microbes almost always developed.

To confirm his contention that it was not spontaneous generation but rather airborne germ-laden dust that caused the appearance of the microorganisms, Pasteur devised what became his most famous experiments, those involving the "swan-necked" flasks. These flasks contained sugared yeast water, with long, narrow necks drawn out and bent in several directions, some curving downward and then up. They were not sealed; air could enter the flasks slowly but freely. After boiling the liquid in them, Pasteur placed the flasks in areas without heavy air currents. The air entered the necks slowly enough to allow gravity to trap the germ-laden dust in the curves. No microorganisms developed in the medium. In those swan-necked flasks where no boiling had occurred, microorganisms grew since the germ-laden air was already present before the necks were drawn out. If the curved necks of the sterile flasks were broken off allowing the air to enter quickly, microbial life soon appeared, but if the swan necks were not removed, the liquid remained sterile indefinitely, even though dust-free air was entering the flasks. What all this demonstrated, Pasteur insisted, was that what appeared to be spontaneous generation was really the result of germs carried by airborne dust.

In other papers in this series, Pasteur demonstrated that the density of germs in the air varies with environmental conditions, air movement, and altitude. The latter factor became especially critical in pushing the debate forward. Pasteur claimed that he had opened sealed, sterilized flasks of yeast extract and sugar at various elevations on mountains. The higher he went, the less contamination he encountered. Of the twenty flasks he opened at an elevation of over a mile, only one developed microbial life. In his "Mémoire" of 1861, Pasteur not only elaborated on his own experiments, but also on a flaw he had found in Pouchet's experiment. Pouchet argued in favor of spontaneous generation because he could produce microorganisms by adding sterilized air, under a blanket of mercury, to boiled hay infusions. Pasteur showed, however, that the surface of mercury in laboratory troughs was covered with germ-bearing dust. Thus, Pasteur contended, in conducting his experiments Pouchet had introduced these germs into his infusions.

Pasteur clearly had the upper hand in the debate. He was, however, open to attack because his opponents argued, as had Needham a century earlier, that heating somehow modified or destroyed some basic condition necessary for spontaneous generation to occur. In April 1863 Pasteur announced that he had taken blood and urine (both substances rich in nutrients) from living animals and preserved them free from microbial growths without having heated them, but by simply protecting them from germ-laden air. Although his victory seemed complete, late that same year Pouchet announced that he had duplicated Pasteur's experiments by exposing sterilized hay infusions at high altitudes. Contrary to Pasteur's results, all of Pouchet's flasks quickly developed microorganisms. Admitting oxygen, Pouchet claimed, caused spontaneous generation. Pasteur countered by asserting that Pouchet must have somehow introduced airborne germs into the infusions. In January 1864 the Académie des Sciences appointed a commission, whose members were friends and defenders of Pasteur, to decide the issue. Ironically, both Pasteur and Pouchet were operating under what proved to be two related and false assumptions. Both assumed that boiling water killed all living organisms or, in Pasteur's terminology, their germs. Neither had any idea of the thermal resistance of some microorganisms, such as the hay bacillus endospore. Thus, they both assumed that Pasteur's sugared yeast water and Pouchet's hay infusions were equivalent substances in these experiments.

Pasteur strengthened his case in a brilliant, if hardly objective, public lecture he gave in April 1864 at the Sorbonne to an audience of the scientific, political, social, and cultural elite of France. After dramatically tracing the history of the dispute, he concluded, "The spontaneous generation of microscopic beings is a mere chimera. There is not a single known circumstance in which microscopic beings may be asserted to have entered the world without germs, without parents resembling them." He was also careful to emphasize that the doctrine of spontaneous generation threatened the very foundation of society by attacking the idea of a "Divine Creator." In June 1864 Pouchet, undoubtedly realizing that the commission members were biased in Pasteur's favor, withdrew without duplicating his experiments. Not surprisingly, the commission then announced in Pasteur's favor; that is, against spontaneous generation.

Aftermath of the Debate

This decision essentially ended the debate in France. Pouchet published no new material on the topic and Pasteur moved on to study other problems, such as silk-worm diseases. The dispute, however, was not quite over. It shifted to England, where it became entwined with Darwinian theories. Some evolutionists there argued that spontaneous generation and evolution were in fact linked theories that could strengthen each other. In 1872 a respected physician and naturalist named Henry Charlton Bastian published an immense two-volume work, The Beginnings of Life: Being Some Account of the Nature, Modes of Origin, and Transformation of Lower Organisms. In this book and in other writings, Bastian insisted on the existence of spontaneous generation. Pasteur himself was only briefly involved in a debate with Bastian. Between July 1876 and July 1877, the two fought over Bastian's claim that, under certain circumstances, microbial life originated spontaneously in sterilized urine. Pasteur argued that Bastian had somehow contaminated his experiments and invited Bastian to present the dispute to a commission of the Académie des Sciences. Bastian at first agreed, but like Pouchet before him, eventually withdrew without appearing before the group.

However, Bastian's main opponent turned out to be John Tyndall, an Irish physicist whose Floating Matter in the Air in Relation to Putrefaction and Infection (1881) and earlier works defended the theory that microorganisms were carried on airborne dust. Tyndall also devised glycerine-coated chambers in which scattered light beams revealed the presence of microscopic organic matter in the air. Along with Ferdinand Cohn, Tyndall proved that the hay bacillus could survive many hours in boiling water. Thus, Pouchet's spontaneous generation was really caused by endospores he himself had introduced into his hay infusions. After Tyndall, it was very difficult to espouse the theory of spontaneous generation. Meanwhile, both Joseph Lister and John Burdon Sanderson established that Pasteur's "corpuscles" were not the germs of microorganisms but were rather the fully developed microorganisms themselves. After the work of these men, fewer and fewer supporters of spontaneous generation appeared; with Bastian's death in 1915 they became extraordinarily scarce.

It is interesting to note that neither Pasteur nor Tyndall nor anyone else has ever devised an experiment proving that spontaneous generation is not possible. What they did accomplish was to demonstrate that spontaneous generation had been shown to never have occurred. In terms of formal logic, it is impossible to argue that spontaneous generation can never take place. Nevertheless, scientists today, who operate on the basis of experimental research and not formal logic, all adhere to the germ theory of life. There are at least three reasons for this. First, no claim for a case of spontaneous generation has ever been proven. Secondly, the use of pure cultures in scientific research has demonstrated countless times that the only life that develops in these cultures is that which is placed there. Lastly, the germ theory, not spontaneous generation, has proven to be the basis of all modern microbiology.

—ROBERT HENDRICK

KEY TERMS

ABIOGENESIS:

The theory that life could be formed from inorganic matter, without parents.

ENDOSPORE:

A spore, or reproductive body, developed within bacteria cells.

EVOLUTION:

The theory that all species on Earth were not created as they are now, but evolved from earlier forms of life over millions of years.

GERM:

Until the late 1870s, the word referred to the precursor of a microorganism and not to the microorganism itself, as it does today.

HETEROGENESIS:

The theory that life could be formed, without parents, from organic matter.

INFUSION:

A liquid extract obtained by steeping a substance in water.

INFUSORIA:

Microscopic forms of life, also known as animalcules, molecules, bacteria, microorganisms, etc.

MEDIUM:

A sterilized nutrient substance used to cultivate bacteria, viruses, and other organisms.

PLANT GALLS:

The swelling of plant tissue caused by insect larvae or fungi. For centuries, these galls were thought to be spontaneously generated.

PUTREFACTION:

The decomposition of organic matter caused by bacteria or fungi.

SPONTANEOUS GENERATION:

The theory that living organisms can be spontaneously created from dead or inorganic matter under certain specific circumstances, for example, placing dirty clothes in a container with wheat or cheese could lead to the spontaneous creation of mice. Its adherents were divided into three groups: those who held that inorganic substances could generate living organisms (abiogenesis); those who believed that degenerating organic material was necessary for the process (heterogenesis); and those who hypothesized that both mechanisms could produce life.