He did curious tests that waited three years to be completed. He took flasks and filled them part way full with milk or urine. He doused them in boiling water and sealed their slender necks shut in a blast flame--then for years he guarded them. At last he opened them, to show that the urine and the milk were perfectly preserved, that the air above the fluid in the bottles still had almost all of its oxygen; no microbes, no destruction of the milk! He allowed germs to grow their silent swarms in other flasks of urine and milk that he had left unboiled, and when he tested these for oxygen he found that the oxygen had been completely used up--the microbes had used it to burn up, to destroy the stuff on which they fed. Then like a great bird Pasteur spread his wings of fancy and soared up to fearsome speculations--he imagined a weird world without microbes, a world whose air had plenty of oxygen, but this oxygen would be of no use, alas, to destroy dead plants and animals, because there were no microbes to do the oxidations. His hearers had nightmare glimpses of vast heaps of carcasses choking deserted lifeless streets--without microbes life would not be possible! Now Pasteur ran hard up against a question that was bound to pop up and look him in the face sooner or later. It was an old question. Adam had without doubt asked it of God, while he wondered where the ten thousand living beings of the garden of Eden came from. It was the question that had all thinkers by the ears for a hundred centuries, that had given Spallanzani so much exciting fun a hundred years before. It was the simple but absolutely insoluble question: Where do microbes come from? “How is it,” Pasteur’s opponents asked him, “how is it that yeasts appear from nowhere every year of every century in every corner of the earth, to turn grape juice into wine? Where do the little animals come from, these little animals that turn milk sour in every can and butter rancid in every jar, from Greenland to Timbuctoo?” Like Spallanzani, Pasteur could not believe that the microbes rose from the dead stuff of the milk or butter. Surely microbes have to have parents! He was, you see, a good Catholic. It is true that he lived among the brainy skeptics on the left bank of the Seine in Paris, where God is as popular as a Soviet would be in Wall Street, but the doubts of his colleagues didn’t touch Pasteur. It was beginning to be the fashion of the doubters to believe in Evolution: the majestic poem that tells of life, starting as a formless stuff stirring in a steamy ooze of a million years ago, unfolding through a stately procession of living beings until it gets to monkeys and at last--triumphantly--to men. There doesn’t have to be a God to start that parade or to run it--it just happened, said the new philosophers with an air of science.