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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