And only from animals sick or dying of tuberculosis, could he grow them. For months he nursed these wee murderers along, planting them from one tube to another--with marvelous watchfulness he kept all other chance microbes away from them. “Now I must shoot these bacilli--these pure cultivations of my bacilli--into healthy guinea-pigs, into all kinds of healthy animals. If then these creatures get tuberculosis, I shall know that my bacilli are necessarily and beyond all doubt the cause!” That man with the terrible single-mindedness of a maniac driven by a fixed idea changed his laboratory into the weirdest kind of zoo. He became grouchy to every one--to curious visitors he was a sarcastic, spiteful little German ogre. Alone he sterilized batteries of shining syringes and shot the crinkly masses of microbes from the cultivations in his serum-jelly tubes--he injected these bacilli ground up in a little pure water into guinea-pigs and rabbits and hens and rats and mice and monkeys. “That’s not enough!” he growled, “I’ll try some animals that never are known to have tuberculosis naturally.” So he ranged abroad and gathered to his laboratory and injected his beloved terrible bacilli into tortoises, sparrows, five frogs and three eels. Insanely Koch completed this most fantastic test by sticking his microbes from the serum cultivation into--a goldfish! Days dragged by, weeks passed, and every day Koch walked into his workshop in the morning and made straight for the cages and jars that held these momentous animals. The goldfish continued to open and shut his mouth and swim placidly about in his round-bellied bowl. The frogs croaked unconcernedly and the eels kept all of their slippery liveliness; the tortoise now and then stuck his head out of his shell and seemed to wink an eye at Koch as if to say: “Your tubercle bugs are food for me--give me some more.” But while his injections worked no harm to these creatures, that do not in the course of nature get consumption anyway--at the same time the guinea-pigs began to droop, to lie pitifully on their sides, gasping. One by one they died, their bodies wasting terribly into tubercles.... Now Koch had forged the last link of the chain of his experiments and was ready to give his news to the world: The bacillus, the true cause of tuberculosis, has been trapped, discovered! When suddenly he decided there was one more thing to do. “Human beings surely must catch these bacilli by inhaling them, in dust, or from the coughing of people sick with consumption. I wonder, will healthy animals be infected that way too?” At once Koch began to devise ways of doing this experiment--it was a nasty job. “I’ll have to _spray_ the bacilli from my cultivations at the animals,” he pondered. But this was a more serious business than turning ten thousand murderers out of jail....