Carefully Koch smeared a wee bit of his hanging-drop that swarmed with the microbes of the eighth generation--this drop was murky, even to his naked eye, with countless bacilli--he smeared a part of this drop on to a little splinter of wood. Then, with that guardian angel who cares for daring stumbling imprudent searchers of nature standing by him, Koch deftly slid this splinter under the skin of a healthy mouse. The next day Koch was bending near-sightedly over the body of this little creature pinned on his dissecting board; giddy with hope, he was carefully flaming his knives.... Not three minutes later Koch is seated before his microscope, a bit of the dead creature’s spleen between two thin bits of glass. “I’ve proved it,” he whispers, “here are the threads, the rods--those little bacilli from my hanging-drop were just as murderous as the ones right out of the spleen of a dead sheep.” So it was that Koch found in this last mouse exactly the same kind of microbe that he had spied long before--having no idea it was alive--in the blood of the first dead cow he had peered at when his hands were fumbling and his microscope was new. It was precisely the same kind of bacillus that he had nursed so carefully, through long successions of mice, through I do not know how many hanging-drops. First of all searchers, of all men that ever lived, ahead of the prophet Pasteur who blazed the trail for him, Koch had really made sure that one certain kind of microbe causes one definite kind of disease, that miserably small bacilli may be the assassins of formidable animals. He had angled for these impossibly tiny fish, and spied on them without knowing anything at all of their habits, their lurking places, of how hardy they might be or how vicious, of how easy it might be for them to leap upon him from the perfect ambush their invisibility gave them. III Cool and stolid, Koch, now that he had come through these perils, never thought himself a hero; he did not even think of publishing his experiments! To-day it would be inconceivable for a man to do such magnificent work and discover such momentous secrets, and keep his mouth shut about it. But Koch plugged on, and it is doubtful whether this hesitating, entirely modest genius of a German country doctor realized the beauty or the importance of his lonely experiments. He plugged on. He must know more! He went pell-mell at the inoculating of guinea-pigs and rabbits, and at last even sheep, with the innocent looking but fatal fluid from the hanging-drops; and in each one of these beasts, in the sheep just as quickly and horribly as the mouse, the few thousands of microbes on the splinter multiplied into billions in the animals, in a few hours they teemed poisonously in what had been robust tissues, choking the little veins and arteries with their myriads, turning to a sinister black the red blood--so killing the sheep, the guinea-pigs, and the rabbits.