worlds, not used to similar conditions, wouldn't have the same resistance. Space travel bears this out—Martian plagues spreading on Earth—Venusians dying of the common cold. Even an interchange of germs between the terrestrial continents was dangerous, according to history. Tuberculosis ravaging the American Indians. Eskimoes killed by the measles. Terrestrial germ diseases don't bother the Callistans, it is true, because their blood is at too high a temperature for Earthly bacteria to survive. But there's another thing—a weak point. The cargo Ron and I brought from Mars in the Barbarian, was the answer."

"Then you guessed, too, what that cargo was, Anna," Ron burst out. "Seeds of some kind—plants. They're growing elsewhere now. Out there in the fields, and on the hillsides. But that's all so crazy! Where can there be any danger in simple, everyday Earth-weeds? Poison ivy is bad, of course; but even it couldn't kill off thousands of Callistans—certainly not in a few minutes!"

"Yes, I guessed what the Barbarian's cargo consisted of, Ron," Anna returned. "I was working in the fields all the time, seeing those plants, which had never been on Titan before. Not even many of the slaves remembered them, though, since we've all been a long time away from home, and from some of the familiar things, there. But I'm a school teacher, and I know a little about biology, and the common afflictions of humankind. But I kept still, because secrecy might be important. Well, those plants grew like wild-fire, under the stimulating rays of the sun-towers. And I was praying that they'd hurry up and blossom. Callisto's a flowerless world, Ron. Probably that's the big point. With an equal start in their growing, the plants blossomed all at once. And the winds blew, and the plague came. And now we colonists are masters of Titan once more. The Acharians can never threaten us again. Not even if they find a way to face the pestilence with filter-masks and so forth. For we've got the major part of their space fleet to protect us. Do you know what I'm talking about now, Ron? Everybody?"

There was an awed quiet in the listening crowd. Then Bart Mallory whooped suddenly. "I get it!" he shouted in triumph. "Of course! Callistan lungs are huge and delicate and entirely unacclimated to one Earthly condition! Naturally they'd react to it far more violently even than we do! And now Terra is mistress of this section of space! My sun-towers must have helped some, by increasing the normal virulence of the plants. But most of the thanks go to Arne Reynaud, and to you, Anna, and to you, Ron."

Mallory, 

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