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In a flash all those mysterious questions cleared up for Theo- bald Smith.

It was not the old, blood-stuffed tick but its child, the baby tick, who sneaked the assassin into the northern cows; it was this little five- or ten-day-old bug who carried the murderer.

Now he saw why it was that fields took so long to become dangerous—the mother ticks have to drop off the southern cattle; it takes them some days to lay their eggs; these eggs take twenty days or more to hatch; the tick babies have to scamper about to find a cow's leg to crawl up on—all that takes many days, weeks. Never was there a simpler answer to a problem which, without this strange chance, might not yet be solved. . . .

So soon as he could hatch out other thousands of ticks in warm glass dishes, Theobald Smith proceeded to confirm his marvelous discovery; he proved it clean. For every northern cow, on whom he stuck his regiments of incubator ticks, came down with Texas fever. But he was a glutton for proofs, as you have seen, and when the summer of 1890 waned and it grew cold, he installed a coal-stove in a stable, hatched the ticks in a heated place, put a cow in the hot stable, stuck the little ticks diligently onto the hide of the cow, the stove instead of the sun made them grow as they should—and the cow got Texas fever in the winter, a thing which never happens in nature!

For two more summers Smith and Kilborne tramped about their fields, caulking up every seam in the ship of their research, answering every argument, devising astounding simple but admirably adequate answers to every objection the savant horse doctors might make—before these critics ever had a chance to make objections. They found strange facts about immunity. They saw northern calves get mild attacks of Texas fever, a couple of attacks in one summer maybe, and then next year, more or less grown up, graze unconcerned on fields absolutely murderous to a non-immune northern cow. . . . So they explained why southern cattle never die of