VI So, first of all microbe hunters, Theobald Smith traced out the exact path by which a sub-visible assassin goes from one animal to another. In the field where there were southern cattle and ticks, the northern cattle died of Texas fever; in the field where there were southern cattle _without_ ticks the northern cows grew fat and remained happy; in the field where there were no southern cattle but _only_ ticks--there too, the northern cattle came down with Texas fever. It must be the tick. By such simple, two-plus-two-make-four--but oh! what endlessly careful experiments, Theobald Smith proved those western cowmen to have observed a great new fact of nature.... He chiseled that fact out of folk-shrewdness, just as the anonymous invention of the wheel has been taken out of folk-inventiveness and put to the uses of modern whirring dynamos.... You would think he thought he had proved enough--those experiments were so clear. You would think he would have advised the government to start an exterminating war on ticks, but that was not the kind of searcher Theobald Smith was. Instead, he waited for the heat of the summer of 1890 to come, and then he started doing the same experiments over, and some new ones too, all of them simple tricks, but each of them necessary to nail down the fact that the tick was the real criminal. “How do those bugs carry the disease from a southern cow to a northern one?” he pondered. “We know now one tick lives its whole life on just one cow--it doesn’t flit from beast to beast like a fly....” This was a knotty question--too subtle for the crude science of the ranchers--and Smith set himself to chew that knot.... “It must be,” he meditated, “that ticks, when they have sucked enough blood, and are ripe, drop off, and are crushed, and leave the little pear-shaped microbes on the grass--to be eaten by the northern cattle!” So he took thousands of ticks, sent up in those cans from North Carolina, and mixed them with hay, and fed them to a susceptible northern cow kept carefully in a special stable. But nothing happened; the cow seemed to relish her new food; she got fat. He tried drenching another cow with mashed up ticks made into a soup--but that cow too seemed to enjoy her strange dose. She prospered on it. It was no go--cows didn’t, apparently, get the microbe by eating ticks; he was mixed up for a while. And other plaguey questions kept him awake nights. Why was it that it took thirty days or more, after the southern tick-loaded cows came on the field, for such a field to become dangerous? Stockmen knew this too; they knew they could mix just-arrived southern cows with northern ones, and keep them together twenty days or so, and then if they took the northern ones away--they would never get Texas fever; but if you left them in that field a little longer (even if the southern cows were taken away) bang! would come the fatal epidemic into the herd of northerners. That was a poser!