"They were still poisonous and they were well adapted," Hunt said.

"Quite so."

"What happened then?"

"Then a very interesting thing must have happened. The Ganymean race appeared and went through all the stages you would expect of a primitive culture beginning to grope its way toward civilization--making tools, growing food, building houses and so on. Well, by this time, as you might imagine, the ancient self-defense that they had inherited from their remote marine ancestors for protecting them against carnivores was turning out to be more of a damned nuisance than a help. There were no carnivores to be protected from and it was soon obvious that none were likely to appear. On the other hand, the acute accident-proneness that resulted from self-poisoning was proving to be a severe handicap." Danchekker held up a finger to show a small band of adhesive plaster around the second joint. "I nicked myself with a scalpel yesterday," he commented. "Had I been one of those early Ganymeans, I would most probably have been dead within the hour."

"Okay, point taken," Hunt conceded. "But what could they do about it?"

"Somewhere around the time that I was describing--the early beginnings of civilization--the ancients discovered that the poisons in the secondary system could be neutralized by including certain plants and molds in their diet. They discovered this by observing the habits of some animals whose immunity to damage that should have meant certain death was well known. That simple step was probably their biggest single leap forward. Coupled with their intelligence it virtually insured dominance over all forms of Minervan life. It opened up the whole of medical science, for example. With their self-poisoning mechanism defused, surgery became possible. At a later stage in their history they developed a simple surgical method of neutralizing the secondary system permanently without having to rely on drugs. It became standard practice for every Ganymean to be treated in this way soon after birth. Even later still, when they had progressed to a level beyond ours, they isolated the gene that caused the secondary system to develop in the fetus in the first place and eradicated it completely. They literally bred this trait out of themselves. None of the Ganymeans we've met was born with a secondary system at all, and neither were quite a few generations before them. Rather an elegant solution, don't you think?"

"Incredible," Hunt agreed. "I've never had a chance to talk about that kind of thing with them. . . not yet anyway."

"Oh, yes." Danchekker nodded. "They were extremely proficient genetic engineers, were our Ganymean friends. . . very proficient."

Hunt thought for a second and then snapped his fingers in sudden comprehension.

"But of course," he said. "In doing that they buggered their CO2 tolerance too."

"Precisely, Vic. All the other animals on Minerva retained the high natural tolerance. Only the Ganymeans were different; they sacrificed it in exchange for accident-resistance."

"But I don't see how they could," Hunt said, frowning again. "I mean, I can see how they did it, but I don't see how they could get away with it. They must have needed the CO2 tolerance, otherwise they wouldn't have evolved it in the first place. They must have known that too. Surely they weren't stupid."

Danchekker nodded as if he already knew what Hunt was going to say.

"That probably wasn't so obvious at the time," he said. "You see, the composition of the Minervan atmosphere fluctuated through the ages much the same as that of Earth has. From various researches the Ganymeans established that at the time land life first emerged, volcanic activity was at a peak and the level of CO2 was very high; naturally, therefore, the earliest species developed a high resistance. But as time went on the level decreased progressively and appeared to have stabilized itself by the time of the Ganymeans. They came to regard their tolerance mechanism as an ancient relic of conditions that no longer existed and their experiences showed that they could get by without it. The margin was small--the CO2 level was still high by our standards--but they could manage. So, they decided to do away with it permanently."

"Ah, but then the level started going up again," Hunt guessed.

"Suddenly and catastrophically," Danchekker confirmed. "On a geological time scale anyway. They were in no immediate danger, but all their measurements and calculations indicated that if the rate of increase went on, they--or their descendants one day anyway--would be in trouble. They would be unable to survive without their ancient tolerance mechanism, but they had eliminated that mechanism from their race. All the other animals would have no difficulty in adapting, but the Ganymeans were somewhat stuck."

The full magnitude of the problem that had confronted the Ganymeans dawned on Hunt at last. They had bought a one-way ticket out of the hard-labor camp only to find that it led to the death cell.

"What could they do?" Danchekker asked, and then went on to answer the question for himself. "First--use their technology to hold the CO2 level down by artificial means. They thought of that but their models couldn't guarantee them a tight enough measure of control over the process. There was a high risk that they'd end up freezing the whole planet solid and, being the cautious breed that they were, they elected not to try it--at least not until it was a last-resort measure.

"Second--they could reduce the CO2 as before, but have ready at hand a method for warming up the Sun to compensate for the loss of the greenhouse effect if the atmospheric engineering got out of control. They tried that on Iscaris but it went wrong, as the scientists on Minerva learned when they received a message from the Shapieron that was sent just before the ship itself got away."

Hunt made no move to interrupt, so Danchekker continued. "Third--they could migrate to Earth. They tried doing so on a pilot scale, but that went wrong too." Danchekker shrugged and held the posture, his arms extended to indicate that he had run out of possibilities. Hunt waited for a moment longer, but the professor evidently had nothing more to say.

"So what the devil did they do?" Hunt asked.

"I don't know. The Ganymeans don't know either, since whatever else may have been thought of was thought of after they had left Minerva. They are as curious as we are--more so I would imagine. It was their world."

"But the animals from Earth," Hunt insisted. "They were all imported later on. Couldn't they have had something to do with the solution?"

"They could have, certainly, but what exactly, I've no idea. Neither have the Ganymeans. We're satisfied, though, that it would not have been anything to do with using a terrestrial type of ecology to absorb the CO2 . That simply wouldn't have worked."

"That idea's gone right out the window, eh?"

"Right out," Danchekker said decisively. "Why they brought the animals there and whether or not it had anything to do with their atmospheric problem is still all a mystery. . . ." The professor paused and peered intently over the top of his spectacles. "There's another mystery too now--a new one--from what we've just been talking about."

"Another one?" Hunt returned his stare curiously. "What?"

"All the other Minervan animals," Danchekker replied slowly. "You see, if they all possessed a perfectly adequate mechanism for dealing with CO2 , it couldn't have been the changing atmosphere of Minerva that wiped them all out after all. If that didn't, then what did?"