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Herr Doktor," the cattle men told Koch, "but how is it that our cows or sheep can be all right in one pasture—perfectly healthy, and then, when we take them into another field, with fine grazing in it, they die like flies?"

Koch knew of this troublesome, mysterious fact too. He knew that in Auvergne in France there were green mountains, horrible mountains where no flock of sheep could go without being picked off, one by one, or in dozens and even hundreds by the black disease, anthrax. And in the country of the Beauce there were fertile fields where sheep grew fat—only to die of anthrax. The peasants shivered at night by their fires: "Our fields are cursed," they whispered.

These things bothered Koch—how could his tiny bacilli live over winter, even for years, in the fields and on the mountains? How could they, indeed, when he had smeared a little bacillus-swarming spleen from a dead mouse on a clean slip of glass, and watched the microbes grow dim, break up, and fade from view? And when he put the nourishing watery fluid of ox-eyes on these bits of glass, the bacilli would no longer grow; when he washed the dried blood off and injected it into mice—these little beasts continued to scamper gayly about in their cages. The microbes, which two days before could have killed a heavy cow, were dead!

"What keeps them alive in the fields, then," muttered Koch, "when they die on my clean glasses in two days?"

Then one day he ran on to a curious sight under his microscope—a strange transformation of his microbes that gave him a clew to his question; and Koch sat down on his stool in his eight-by-ten laboratory in East Prussia and solved the mystery of the cursed fields and mountains of France. He had kept a hanging-drop, in its closed glass well, at the temperature of a mouse's body for twenty-four hours. "Ah, this ought to be full of nice long threads of bacilli," he muttered, and looked down the tube of his microscope—"What's this?" he cried.

The outlines of the threads had grown dim, and each thread