MYTH 15
THAT LOUIS PASTEUR DISPROVED SPONTANEOUS GENERATION ON THE BASIS OF SCIENTIFIC OBJECTIVITY
Garland E. Allen
[Pasteur] did not permit his religious convictions to influence his scientific conclusions. His canon as a scientist, he firmly stated, was to stand aloof from religion, philosophy, atheism, materialism and spiritualism.… It was Pasteur’s point that the material conditions of life had momentous implications for religion, but that these implications must not interfere with the objective interpretation of experimental results.
—Thomas S. Hall, Ideas of Life and Matter (1969)
There is no question that Louis Pasteur (1823–1895) was a towering figure in nineteenth-century science: trained as a chemist and crystallographer, Pasteur is credited with discoveries in fields as diverse as organic chemistry (determining the basis for chirality, or optical rotation of polarized light), the microbial basis of wine and beer brewing, immunology (the basis for vaccination), and the biology of the phylloxeran pests that were destroying the French vineyards in the nineteenth century. However, so much that has been written about Pasteur has taken on the dimension of myth. He has been revered as an almost superhuman scientist in the vein of Isaac Newton (see Myth 6). For example, it was said of him in 1887: “In France, one can be an anarchist, a communist or a nihilist, but not an anti-Pastorian [sic]. A simple question of science has been made into a question of patriotism.”1
Nowhere is the mythologizing more apparent than in discussions of the famous controversy into which Pasteur entered between 1861 and 1864 with his countryman, the naturalist Félix Archimède Pouchet (1800–1872), on the highly controversial issue of spontaneous generation. Pasteur opposed spontaneous generation while Pouchet defended it, both putting forward experimental evidence to support their respective positions. The myth has centered on Pasteur as a model of the unbiased scientist who follows the truth wherever it may lead, with no preconceived ideas, no philosophical or political influences, and complete impartiality in the gathering and interpretation of data. The myth has two dimensions that cloud our understanding of the nature of science as a process: one is personal, the image of Pasteur as the supreme (and unrealistic) exemplar of objective scientific practice; the other is an exaggerated, idealized view of what has been called the “scientific method” (see Myth 26). Using the tools of the historian, scholars such as John Farley (b. 1936) and the late Gerald L. Geison (1943–2001) have enabled us to debunk the myth and use the case study itself to present a more accurate picture of science as a human, social process.
In an evening lecture in April 1864 at the Sorbonne, Pasteur summarized the experiments that he expected would deliver the final blow to any serious scientific account of the spontaneous generation of microbial life. To a packed auditorium that included such luminaries as Princess Mathilde Bonaparte (1820–1904), niece of Napoleon I, and the writers Alexandre Dumas (1802–1870) and George Sand (1804–1876), Pasteur admitted that the issue of spontaneous generation smacked of serious philosophical and religious questions—namely, those of materialism, evolution, and atheism. However, he went on to deny that any of these issues made any difference in his judgment of the experimental differences obtained over the past four years between himself and Pouchet:
Neither religion nor philosophy, nor atheism, nor materialism, nor spiritualism has any place here. I may even add: as a scientist I don’t much care. It is a question of fact. I have approached it without preconceived idea, equally ready to declare—if experiment had imposed the view on me—that spontaneous generations exist as I am now persuaded that those who affirm them have a blindfold over their eyes.2
This is the stereotype that has characterized much of the popular representations of science over the past century or more. The debate between Pasteur and Pouchet is important to see in its true context as one in which, despite Pasteur’s assertion, political, philosophical, and religious ideology played a major, if unacknowledged, role in designing, carrying out, and interpreting experimental results.
For background, it is important to recognize that France had been in a state of political, social, and economic turmoil since the February Revolution of 1848, the resignation of King Louis-Philippe, and the establishment of the short-lived Second Republic in December 1848, under the presidency of Prince Louis Napoleon Bonaparte (nephew of Napoleon I). A resurgence of Republicanism brought with it claims for greater liberalization and extension of the franchise, while simultaneously curtailing the powers of the Catholic Church and increasing those of the Legislative Assembly. Under these circumstances, church and state joined forces in the face of their common enemies: republicanism and revolution. Thus, by the time Napoleon initiated his coup d’état in December 1851, proclaiming the Second Empire (and himself emperor), the union of church and state had been greatly strengthened, and the new emperor relied heavily on the support of the church to maintain his power. The polarization between the church–state coalition and the more radical Republicans only deepened in the following decade. Republicans promoted science, materialism, and atheism as means of progress, and rejected the obscurantism associated with the Catholic Church. In this context, spontaneous generation became a focal point of controversy because of its apparent association with the formation of life without divine intervention. Pasteur, as a well-known scientist, devout Catholic, political conservative, and opponent of spontaneous generation, was thus well placed to present an “impartial” but necessary refutation of spontaneous generation.3
Although the experiments of Francesco Redi (1636–1697) had shown that one of the best-known examples of supposed spontaneous generation—the appearance of maggots in putrefying meat—could be rejected by preventing contact by flies with the meat, the issue had continued to surface in the later eighteenth and early nineteenth centuries, particularly in France in the 1820s and 1830s in the debates between anatomist and paleontologist Georges Cuvier (1769–1832) and his opponents Jean-Baptiste Lamarck (1744–1829) and Étienne Geoffroy Saint-Hilaire (1772–1844).4 Both Lamarck and Geoffroy had endorsed the possibility of spontaneous generation, the process being especially integral to Lamarck’s broader transformist, or evolutionary, views (see Myth 10). The same association of transformism, spontaneous generation, and materialism reemerged in the political atmosphere of the Second Empire in even starker relief with the 1862 publication of the French translation of Charles Darwin’s (1809–1882) On the Origin of Species by Clémence Royer (1830–1902).5 Royer voiced her support not only for a naturalistic theory of the origin of species but also for materialism, atheism, and republicanism. Further fueling the fears of Catholics and other conservatives, a movement known as “higher criticism” emerged in the 1860s, which aimed to treat biblical texts as historical documents free from any notions of revelation or supernaturalism.6 The revival of spontaneous generation in this social and intellectual climate predisposed major figures of the scientific establishment, such as Pasteur, against any proponents of such heretical views.
Pasteur’s work in the 1850s on the souring of wine confronting the French wine industry had convinced him of the ubiquity of microbes in the environment and of their likely implication in all fermentations, the spoiling of foods, and the spread of human disease. An important component of Pasteur’s germ theory was his opposition to spontaneous generation. All bacteria, he claimed, came from the reproduction of previously existing bacteria and were not spontaneously generated from nonliving organic matter. Admitting to spontaneous generation would have seriously undermined the integrity of the germ theory.
Pouchet walked into this charged political and scientific arena in 1859 (the same year that Darwin’s Origin of Species appeared) with the publication of his book Hétérogénie; ou Traité de la Génération Spontanée (Heterogenesis; or, Treatise on Spontaneous Generation).7 He must have been well informed on the inflammatory nature of the topic because he devoted considerable space (the first 137 pages) to denying any materialistic or atheistic implications for his belief in spontaneous generation. Indeed, he argued that it was to the greater glory of God that the creation of life should be a continuing process. He denied that spontaneous generation could occur with purely nonorganic materials, even with the same elements involved. Despite these caveats and precautionary statements, Pouchet nevertheless found his work inevitably associated with materialism, atheism, and republicanism, thus attracting the opposition of both church and state.
Beginning in 1858, Pouchet had performed a simple set of experiments that appeared to support his belief in spontaneous generation. He set up a series of flasks of hay infusion heated to about 100°C so as to sterilize their contents. He then introduced into the flasks artificially produced air, or oxygen passed through mercury (as a kind of filter), sealed the flasks, and left them at room temperature. After a few days, they were seen under the microscope to be teeming with bacteria. To counter the argument that he had not boiled the hay infusions enough, he boiled them at 200°C and 300°C, even close to the point of carbonization, and still bacteria appeared.
When Pasteur read Pouchet’s paper, he responded by pointing out that since bacteria exist everywhere around us, it was likely that Pouchet had either not fully sterilized the original broth or not plugged up the flasks quickly or carefully enough, thus allowing bacterial contamination of the liquid. This contention led Pouchet to repeat his experiments several times, always with the same results. Pasteur had to admit that Pouchet’s experiments were the most meticulously of any on spontaneous generation carried out to date. But there was still one possible source of error: Pouchet’s method of passing heated air through the mercury trough before admitting it to the flasks of sterile broth. In a simple experiment, Pasteur took a drop of mercury from the surface of the trough and introduced it into flasks containing a solution of sterilized yeast and sugar water. Within 24–36 hours, the flask was seen to be teeming with microbial life. Conversely, if the mercury were also sterilized before being introduced into the flasks, no microbial life appeared. The source of Pouchet’s spontaneous generation was the contaminated mercury itself.
The two scientists proceeded to exchange comments and letters in scientific publications. So intense did the discussion become, and so important was the resolution seen to be, that in 1862 the French Academy of Sciences announced a contest, with a cash prize to be awarded by a jury of scientists to the best presentation. Pasteur and Pouchet were the main contenders.
Pasteur first demonstrated that boiling yeast and sugar water in a flask and then immediately sealing it by melting the glass at the top so that the contents were not allowed contact with air prevented the growth of bacteria and hence decay of the broth. However, Pouchet countered with a perfectly reasonable argument: he pointed out that boiling might have changed the chemical composition of the air inside the flask, rendering it unsuitable for spontaneous generation. Pasteur then designed an elaborate apparatus that would answer this objection while supporting his contention that all supposedly spontaneous generations were the result of contact with microbe-laden particles from atmospheric air. He attached a flask of sterile broth to a glass tube with two stopcocks leading to a suction pump. By means of the suction pump, he passed air through a red-hot tube in a furnace and introduced it into a flask of sterile sugared yeast water, then sealed off the neck of the flask with heat. These flasks sat for months and showed no microbial growth. He then repeated this basic procedure: using normal (unheated) atmospheric air passed through a sterile wad of guncotton placed in the tube, he got the same results: no microbial growth. However, when he then allowed the piece of guncotton to be drawn into the flask, after a few days it teemed with microbes.
To counter any complaint that the guncotton filter may have altered the air passed through it, Pasteur performed another simple but elegant set of experiments with specially designed swan-necked flasks that allowed untreated atmospheric air to diffuse back and forth between the inside and outside with no filtration. He introduced sugared yeast water into these flasks and boiled them to kill any microorganisms and then let them sit for various periods of time. The reasoning behind the S-shaped design for the flask necks was that the lower portion of the neck would serve as a trap for the heavier dust particles and bacteria carried in the air that diffused in and out. Using this apparatus, Pasteur reasoned, would allow air to come in contact with the broth but no airborne bacteria would make it beyond the trap. If spontaneous generation could occur, then it ought to do so under these circumstances. The results of Pasteur’s experiment were quite dramatic: even after several months there was no decay in the flask. Moreover, he made a bold prediction: if he tilted the flask so that some of the sugared yeast water got into the trap and was then returned to the main receptacle (Figure 15.1), the liquid in the receptacle should show microbial growth. When he carried out this experiment, bacteria appeared in the broth in just a few days. The French Academy of Sciences Committee awarded the prize to Pasteur.
In the course of the debate, Pasteur seems to have been driven by the sheer force of logic and ingenious experimental design. But the story is not quite as simple as it might at first appear. It is certainly the case that Pasteur’s experimental work was exemplary. Yet Pasteur himself enjoyed the French government’s financial support, was a loyal supporter of Napoleon III, and, on several occasions, was an invited guest of the emperor at one of his country estates. Pasteur was also a member of the French Academy of Sciences, at the time composed of the most pro-government, scientific elite of France. Pouchet, living in Rouen, was only a corresponding member and was not among the inner circle of elites that included Pasteur. On the religious side, Pasteur was a devout Roman Catholic who, as his Sorbonne speech in 1864 explicitly noted, understood all too well the relationship between the theory of spontaneous generation and liberal, materialistic, and “atheistic” ideas. This is important, as virtually all of the committee’s members were strong Catholics and anti-Republicans who recognized that public affirmation of spontaneous generation was politically sensitive. Indeed, Pouchet, recognizing how stacked the committee was against his views, withdrew from the contest in 1862, only to revive his participation a year later at the urging of friends and associates.8
Perhaps the greatest irony in this story is that both Pouchet and Pasteur were right. The two men were using different sources of organic broth: Pasteur’s was sugared yeast water, while Pouchet’s was a hay infusion, a liquid prepared by soaking hay in water. Because he did not repeat Pouchet’s experiments using hay, Pasteur did not discover that the natural bacteria found in hay infusions include some species that can form spores, which enable them to survive severe conditions, such as drought, cold, or heat. The spores present in Pouchet’s preparation were heat resistant enough to survive the short boiling time and even the higher temperatures to which he subjected them. They were thus able to emerge from their dormant stage and start reproducing once the flask cooled down.
Figure 15.1. Diagram showing how tipping the swan-necked flask, so that unspoiled broth was brought into contact with the curved “dust trap” and then returned to the main receptacle, brought about the appearance of bacterial infestation. (Source: Allen and Baker, Biology: Scientific Process, 63.)
Several major conclusions come out of the analysis of Pasteur’s work on spontaneous generation. One is that Pasteur used the image of the detached scientist to further solidify his position within the French academic establishment. The germ theory of disease had become an important component of both French colonial medicine and the campaigns for public health at home and, thus, among other things, a source of financial and political support for Pasteur and his Institut Pasteur.9 A second conclusion from this study is that there is no such thing as the idealized “scientific method” to which Pasteur and others of the late nineteenth and early twentieth centuries have appealed. This view, which we have all seen promoted in textbook after textbook, has little to do with the way science is practiced in the laboratory on a day-to-day level. One of the most important outcomes of current work in the history, philosophy, and sociology of science is that it demonstrates that science is a very human activity, bound up with human ideals and aspirations.