THÉOTIME. It is necessary, Theodore, that I tell you about an experiment I made [une expérience que j'ai faite]. One day in summer I took a large piece of meat that I enclosed in a bottle, and I covered it with a bit of gauze. I noted that various flies came to lay their eggs or larvae on this gauze, and that when they hatched they ate the gauze and fell onto the meat, which they devoured in a short time. But as that smelled too bad, I threw it all out.
THÉODORE. That is how flies come from putrefaction. They put their larvae on the meat and hurriedly fly away. These larvae eat and the flesh putrefies. After those larvae have eaten well, they enclose themselves in their cocoons and leave them as flies; and because of this common men believe insects come from putrefaction.
THÉOTIME. What you say is certain. For many times I have enclosed flesh in a hermetically sealed bottle where no flies have been, and I have never found larvae there.
ARISTE. But how then can it be that one finds very large ones in all sorts of fruits?
THÉODORE. One finds them large, but they entered the fruits small. Search well, you will discover on the skin either a small hole or its scar. But let us not dwell, I pray you, on the proofs that people give that there are animals that come from putrefaction. For these are proofs so weak that they do not merit any reply. One finds mice in a newly constructed vessel, or in a place where there were none. Therefore, it must be that this animal has been engendered from some putrefaction. As if these animals were prevented from seeking out their needs at night, from moving on planks and and on the ropes onto small boats and from there onto the large ships, or as if one could construct vessels elsewhere than on the shore. I am unable to comprehend how such a great number of people of good sense have been able to enter into such a blatant and palpable error on similar reasons. For what is there that is more incomprehensible than an animal being formed naturally out of a little putrified meat? It is infinitely easier to conceive of a bit of rusty iron being changed into a perfectly good watch; for there are infinitely more parts of greater delicacy in a mouse than in the most complex clock.
Nicolas Malebranche, Dialogues on Metaphysics and on Religion (1688), XI.viii. This is about 20 years after Redi's experiment against spontaneous generation, of which Theotimus describes a simple version; this convinced most people who were aware of it that larger animals like flies and mice did not arise spontaneously. Due to the discovery of microbes (by way of the microscope) it was still common for people to hold that microbes spontaneously generated; but, given Malebranche's view of all life as extremely complex machinery, he would hold that the same reasoning applies to microbes that applies to flies and mice. It took Pasteur, however, to provide (or finish providing, since he built on previous work) solid experimental evidence that microbes were not spontaneously generated in things like spoiled broth.
[As a side note, in case I have any readers who are interested in this sort of thing, Tara Smith of the weblog Aetiology is starting a new blog carnival, Animalcules, devoted to microbes. (HT: Science and Politics)]
