The Spontaneous-Generation Debate

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The Spontaneous-Generation Debate

Overview

According to the ancient theory of spontaneous generation, living organisms could originate from nonliving matter. During the seventeenth and eighteenth centuries, however, naturalists began to conduct experiments that challenged the doctrine of spontaneous generation. After Francesco Redi published his Experiments on the Generation of Insects, the spontaneous-generation debate was essentially limited to microscopic forms of life. During the eighteenth century, the French naturalist Georges Buffon and the English microscopist John Turbeville Needham carried out a series of experiments that seemed to support the doctrine of spontaneous generation, but their conclusions and experimental methods were challenged by the Italian physiologist Lazzaro Spallanzani. After conducting a series of rigorous experiments on the growth of microorganisms, Spallanzani vigorously challenged the belief in spontaneous generation. Spallanzani claimed that Needham had not heated his tubes enough and that he had not sealed them properly.

Background

According to the theory of spontaneous generation, living organisms can originate from nonliving matter. Belief in the spontaneous generation of life was almost universal from the earliest times up to the seventeenth century. Small, lowly creatures and all sorts of vermin, which often appeared suddenly from no known parents, seemed to arise from lifeless materials. Insects, frogs, and even mice were thought to arise from slime, mud, and manure in conjunction with moisture and heat. In ancient times, this belief seemed to conform to common observations about the sudden appearance of insects, small animals, and parasites. Spontaneous generation also provided an answer to philosophical and religious questions about the origin of life.

During the seventeenth and eighteenth centuries, however, some naturalists began to conduct experiments that tested and challenged the doctrine of spontaneous generation. Italian physician and poet Francesco Redi (1626-1698) was one of the first to question the spontaneous origin of living things. In 1668 Redi initiated a now-famous experimental attack on the question of spontaneous generation. Redi discovered that when adult flies were excluded from rotting meat, maggots did not develop. If flies were not excluded, they laid eggs on the meat and the eggs developed into maggots. Thus, Redi demonstrated that maggots and flies were not spontaneously generated from rotting meats but instead developed from eggs that were deposited by adult flies. Although Redi's Experiments on the Generation of Insects (1668) did not totally discredit the doctrine of spontaneous generation, the eighteenth-century spontaneous-generation debate was essentially limited to microscopic forms of life.

Seventeenth-century microscopists were able to see a new world teeming with previously invisible entities, including protozoa, molds, yeasts, and bacteria. Many naturalists thought that the new world of microscopic "animalcules" discovered by the great microscopist Anton van Leeuwenhoek (1632-1723) provided proof that minute plants and animals were spontaneously generated in pond water or similar media. Some naturalists thought that these minute entities might even be the living molecules, or "monads," postulated by the mathematician and philosopher Gottfried Wilhelm Liebniz (1646-1716). While Leeuwenhoek was quite sure that he had discovered "little animals" that must have descended from parents like themselves, others took exception to this conclusion. Indeed, questions concerning the nature, origin, and activities of microorganisms were not clarified until the late nineteenth century.

Several interesting accounts of "infusoria" were, however, published in the eighteenth century. Louis Joblot (1645-1723), for example, confirmed the existence of some of Leeuwenhoek's animalcules. In 1718 Joblot published an illustrated treatise on the construction of microscopes that described his observations of the animalcules that could be found in various infusions. Joblot is now remembered primarily for his opposition to the doctrine of spontaneous generation. To prove that infusoria were not spontaneously generated, Joblot boiled his growth medium and divided it into two portions. A flask containing one portion was sealed off and the other sample was left uncovered. The open flask was soon teeming with microbial life, but the sealed vessel was free of infusoria. To prove that the medium was still susceptible to putrefaction, Joblot exposed it to the air and showed that infusoria were soon actively growing. Joblot concluded that something from the air had to enter the medium to produce microorganisms.

Impact

Joblot's experiments were repeated with many variations by other naturalists, but the results obtained were not consistent. Among the most notable eighteenth-century advocates of the doctrine of the spontaneous generation of microorganisms were the French naturalist Georges Buffon (1707-1788) and the English microscopist John Turbeville Needham (1713-1781). Together as well as separately, Needham and Buffon carried out a series of experiments to disprove the work of Joblot.

John Turberville Needham was a naturalist as well as a teacher and a clergyman. He was the first Roman Catholic to become a member of the Royal Society of London. In 1767 Needham retired to the English seminary in Paris. He devoted the rest of his life to his studies and experiments. Needham had decided to study natural history after reading accounts of "animalcules" and "infusoria" and philosophical speculations about microorganisms, spontaneous generation, and the origin of life. Having rejected mechanistic theories of physiology, Needham adopted vitalism (the idea that life processes cannot be explained by the laws of chemistry and physics) and the doctrine of spontaneous generation. In 1745 he published a book entitled An Account of Some New Microscopical Discoveries, in which he presented his experimental evidence for the theory of spontaneous generation.

According to Needham, many organisms developed in prepared infusions of various substances even if the infusions had been placed in sealed tubes and heated for 30 minutes. When Needham repeated Joblot's experiments, whether the flasks were open or closed and the medium boiled or not boiled, all vessels soon swarmed with microscopic life. Needham assumed that this heat treatment should have killed any living organisms that might have been in the original medium. According to Needham, a powerful vegetative force remained in every particle of matter that had previously been part of a living being. Therefore, when animals or plants died, they slowly decomposed and released the "common principle," which Needham thought of as a kind of universal semen from which new life arose. He concluded that the growth of microorganisms under his experimental conditions proved that spontaneous generation of microbial life had occurred. Published in the Philosophical Transactions of the Royal Society in 1748, Needham's views were well known. The claims of Needham and Buffon did not, however, stand unchallenged for very long.

When the Italian physiologist Lazzaro Spallanzani (1729-1799) repeated Needham's experiments, he obtained conflicting results. Spallanzani, who had studied philosophy, theology, law, and mathematics, was appointed professor of logic, metaphysics, and Greek at Reggio College in 1754. Six years later, he became professor of physics at the University of Modena. In 1769 he accepted a position at the University of Pavia and remained there until his death. (After attacking Needham and Buffon on the subject of spontaneous generation, Spallanzani investigated regeneration, transplantation, reproduction, generation, artificial insemination, the circulation of the blood, digestion, and the electric organ of the torpedo fish before returning to studies of microscopic plants and animals at the end of his career.) Like his friends Albrecht von Haller (1708-1777) and Charles Bonnet (1720-1793), Spallanzani supported an ovist preformationist view of generation and he attacked Buffon's mechanistic epigenetic theory.

After examining various forms of microscopic life, Spallanzani concluded that Leeuwenhoek had been correct in identifying these minute entities as living organisms. To prove that these entities were alive, he carried out a series of experiments in which he boiled rich growth media for fairly long periods of time. He found that, if he placed media that had been boiled for 30 minutes into phials and immediately sealed them by fusing the glass, no microorganisms were produced. He concluded, therefore, that the infusoria found in pond water and other preparations were actually living organisms.

In another series of experiments, Spallanzani exposed significant errors in the experiments conducted by Needham and Buffon. By heating a series of flasks for different lengths of time, Spallanzani determined that various sorts of microbes differed in their susceptibility to heat. Whereas some of the larger animalcules were destroyed by slight heating, other, very minute, entities seemed to survive in liquids that had been boiled for almost an hour. Further experiments convinced Spallanzani that all these little animals entered the media from the air. Convinced that a great variety of animalcular "eggs" must be disseminated through the air, Spallanzani concluded that the air could either convey the germs to the infusions or assist in the multiplication of those germs already in them.

In 1767 Spallanzani published an account of his research on the growth of microorganisms and his criticism of the theory proposed by Buffon and Needham. According to Buffon and Needham, living things contained special "vital atoms" that were responsible for all physiological activities. They suggested that after the death of an individual these living atoms were released into the soil and water and taken up by plants. They thought that the "infusoria" that could be found in pond water or infusions of plant and animal material were actually evidence of these vital atoms.

Despite Spallanzani's criticisms, Needham and Buffon continued to champion the doctrine of spontaneous generation. Indeed, the debate was not resolved until the nineteenth century, when the great French chemist Louis Pasteur and the English physicist John Tyndall declared war on spontaneous generation. Although Spallanzani's experiments answered many of the questions raised by advocates of spontaneous generation as well as proved the importance of sterilization, his critics claimed that he had tortured the all important "vital force" out of the organic matter by his cruel treatment of his media. The vital force was, by definition, capricious and unstable, rendering it impossible to expect reproducibility in experiments involving organic matter.

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. Because any apparent exception could allow proponents of the theory to maintain that spontaneous generation only occurred under special conditions, however, opponents were always on the defensive. The work of Louis Pasteur (1822-1895) and John Tyndall (1820-1893) effectively proved that the existence of germs in the air was the critical issue in establishing the experimental basis of the debate. Pasteur was convinced that microbiology and medicine could only progress when the idea of spontaneous generation was totally vanquished. Although both knew that it is logically impossible to prove a universal negative, they demonstrated that under present conditions living beings arise from "parents" like themselves. Pasteur and Tyndall proved that the microbes that Needham and Buffon thought arose from the media actually came from microbes carried by particulate matter in the air. Pasteur proved that microorganisms come from the multiplication of parent microorganisms of their own kind. The experiments conducted by Pasteur and Tyndall did not deal with the question of the ultimate origin of life, but they did demonstrate that microbes do not arise de novo in properly sterilized media under the conditions prevailing today. Advocates of the doctrine 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 at some point.

LOIS N. MAGNER