A single 30-day span begat much of modern biology
By the mid-19th century, the debate around spontaneous generation had moved to a more empirical and material plane. Increasingly sophisticated experiments involving the boiling of nutritive broths in sealed flasks were now underway. In part, these more advanced experiments addressed the objections raised by the spontaneists that the air in the flask had been corrupted by virtue of the heat generated by boiling and had thus lost its vegetative force. New experimental designs allowed new air into the sealed flasks after heating. Furthermore, this new air could be filtered, heated, or passed through sulfuric acid or charcoal prior to its introduction into the flasks, thus ridding it of living contaminants. These experiments pointed to the conclusion that any combination of treatments that would destroy germs in the broth (boiling) and in the incoming air (filtering) would result in a lifeless solution.
Still, old theories die hard. Inconsistencies in the experimental outcomes and confusion in the interpretations lingered. In frustration, the French Academy of Sciences threw down the gauntlet, offering the Alhumbert prize to anyone who “would, by virtue of well-conceived experiments, bring about a new day on questions of so-called spontaneous generation.” The prize committee was chaired by Geoffroy St.-Hilaire, an eminent French anatomist who would soon, interestingly, become a ferocious critic of Darwin.
But as far as the controversy with spontaneous generation was concerned, Pasteur was a reluctant theorist. He entered the fray primarily because he thought it might help him strengthen his conclusions about the nature of fermentation. His friends warned him to tread carefully and to set a time limit to his involvement in what appeared to be a potentially unsolvable controversy. But Pasteur could not resist; he was a superb experimentalist and a committed empiricist, and he developed a series of ingenious swan-necked flasks that proved crucial to his demonstrations. This part of the story, along with the flasks themselves, has by now achieved mythical status in biology.
What is less acknowledged, however, is Pasteur’s ecological thinking. He understood that any argument that invoked existing organisms to account for the colonization of sterilized broth depended on the demonstration that organisms existed in the air, and in sufficient numbers to explain the results. Intellectually subtle, Pasteur understood that the concentration of bacteria (or spores, cysts or any other airborne colonist) would likely vary from environment to environment. He undertook experiments to prove just that point. In elegant 19th-century prose, he describes how air drawn in from “a quiet street in Paris, at a height of 3 to 4 meters” (the Rue d’Ulm, as it turns out) after several beautiful summer days, contained “thousands of organized corpuscles.” But, he argued:
… this result will vary depending on the state of the atmosphere, if we are working before or after a rain, in quiet or agitated atmospheric conditions, during the day or at night, and near or far from the ground…. Imagine the thousand and one causes that might increase or reduce the number of solid particles that we have all seen floating in a beam of light that penetrates a dark room … we must [thus] expect differences in our experimental outcomes.
Pasteur would go on to repeat his experiments drawing air into sterilized broths at different times, in varying weather, at varying altitudes, in cities and in towns, in the countryside and on glaciers. And every time, his experiments demonstrated consistent results, which he would summarize in 1862, upon receiving the Alhumbert prize: “Spontaneous generation is a chimera.”