"There was more to it than that though, wasn't there?" Carpenter queried. "We're pretty certain that all species of Minervan land dwellers died out pretty rapidly somewhere around twenty-five million years ago . . . all except the Ganymeans themselves maybe. That sounds like just the effect you'd expect if the concentration did rise and all the species there couldn't handle it. It seems to support the hypothesis pretty well."

"I think Paul's got a point," Sandy Holmes chimed in. "Everything adds up. Also, it fits in with the ideas we've been having about why the Ganymeans were shipping all the animals into Minerva." She turned toward Carpenter, as if inviting him to complete the story from there.

As usual, Carpenter didn't need much encouragement. "What the Ganymeans were really trying to do was redress the CO2 imbalance by covering the planet with carbon-dioxide-absorbing, oxygen-producing terrestrial green plants. The animals were brought along to provide a balanced ecology that the plants could survive in. Like Sandy says, it all fits."

"You're trying to fit the evidence to suit the answers that you already want to prove," Danchekker cautioned. "Let's separate once more the evidence that is fact from the evidence which is supposition or mere suggestion." The discussion continued with Danchekker leading an examination of the principles of scientific deduction and the techniques of logical analysis. Throughout, the figure who had been following the proceedings silently from his seat at the end of the table farthest from the screen continued to draw leisurely on his cigarette, taking in every detail.

Dr. Victor Hunt had also accompanied the team of scientists who had come with Jupiter Five more than three months before to study the Ganymean ship. Although nothing truly spectacular had emerged during this time, huge volumes of data on the structure, design and contents of the alien ship had been amassed. Every day, newly removed devices and machinery were examined in the laboratories of the surface bases and in the orbiting J4 and J5 mission command ships. Findings from these tests were as yet fragmentary, but clues were beginning to emerge from which a meaningful picture of the Ganymean civilization and the mysterious events of twenty-five million years before might eventually emerge.

That was Hunt's job. Originally a theoretical physicist specializing in mathematical nucleonics, he had been brought into the UN Space Arm from England to head a small group of UNSA scientists; the group's task was to correlate the findings of the specialists working on the project both on and around Ganymede and back on Earth. The specialists painted the pieces of the puzzle; Hunt's group fitted them together. This arrangement was devised by Hunt's immediate boss, Gregg Caldwell, executive director of the Navigation and Communications Division of UNSA, headquartered in Houston. The scheme had already worked well in enabling them to unravel successfully the existence and fate of Minerva, and first signs were that it promised to work well again.

He listened while the debate between the biologists went full circle to end up focusing on the unfamiliar enzyme that had started the whole thing off.

"No, I'm afraid not," Danchekker said in reply to a question from Rousson. "We have no idea at present what its purpose was. Certain functions in its reaction equations suggest that it could have contributed to the modification or breaking down of some kind of protein molecule, but precisely what molecule or for what purpose we don't know." Danchekker gazed around the room to invite further comment but nobody appeared to have anything to say. The room became quiet. A mild hum from a nearby generator became noticeable for the first time. At length Hunt stubbed his cigarette and sat back to rest his elbows on the arms of his chair. "Sounds as if there's a problem there, all right," he commented. "Enzymes aren't my line. I'm going to have to leave this one completely to you people."

"Ah , nice to see you're still with us, Vic," Danchekker said, raising his eyes to take in the far end of the table. "You haven't said a word since we sat down."

"Listening and learning." Hunt grinned. "Didn't have a lot to contribute."

"That sounds like a philosophical approach to life," Fichter said, shuffling the papers in front of him. "Do you have many philosophies of life. . . maybe a little red book full of them like that Chinese gentleman back in nineteen whatever it was?"

"'Fraid not. Doesn't do to have too many philosophies about anything. You always end up contradicting yourself. Blows your credibility."

Fichter smiled. "You've nothing to say to throw any light on our problem with this wretched enzyme then," he said.

Hunt did not reply immediately but pursed his lips and inclined his head to one side in the manner of somebody with doubts about the advisability of revealing something that he knew. "Well," he finally said, "you've got enough to worry about with that enzyme as things are." The tone was mildly playful, but irresistibly provocative. All heads in the room swung around abruptly to face in his direction.

"Vic, you're holding out on us," Sandy declared. "Give."

Danchekker fixed Hunt with a silent, challenging stare. Hunt nodded and reached down with one hand to operate the keyboard recessed into the edge of the table opposite his chair. Above the far side of Ganymede, computers on board Jupiter Five responded to his request. The display on the conference room wall changed to reveal a densely packed columnar arrangement of numbers.

Hunt allowed some time for the others to study them. "These are the results of a series of quantitative analytical tests that were performed recently in the J5 labs. The tests involved the routine determination of the chemical constituents of cells from selected organs in the animals you've just been talking about--the ones from the ship." He paused for a second, then continued matter-of-factly. "These numbers show that certain combinations of elements turned up over and over again, always in the same fixed ratios. The ratios strongly suggest the decay products of familiar radioactive processes. It's exactly as if radioisotopes were selected in the manufacture of the enzymes."

After a few seconds, one or two puzzled frowns formed in response to his words. Danchekker was the first to reply. "Are you telling us that the enzyme incorporated radioisotopes into its structure. . . selectively?" he asked.

"Exactly."

"That's ridiculous," the professor declared firmly. His tone left no room for dissent. Hunt shrugged.

"It appears to be fact. Look at the numbers."

"But there is no way in which such a process could come about," Danchekker insisted.

"I know, but it did."

"Purely chemical processes cannot distinguish a radioisotope from a normal isotope," Danchekker pointed out impatiently. "Enzymes are manufactured by chemical processes. Such processes are incapable of selecting radioisotopes to use for the manufacture of enzymes." Hunt had half expected that Danchekker's immediate reaction would be one of uncompromising and total rejection of the suggestion he had just made. After working closely with Danchekker for over two years, Hunt had grown used to the professor's tendency to sandbag himself instinctively behind orthodox pronouncements the moment anything alien to his beliefs reared its head. Once he'd been given time to reflect, Hunt knew, Danchekker could be as innovative as any of the younger generation of scientists seated around the room. For the moment, then, Hunt remained silent, whistling tunelessly and nonchalantly to himself as he drummed his fingers absently on the table.

Danchekker waited, growing visibly more irritable as the seconds dragged by. "Chemical processes cannot distinguish a radioisotope," he finally repeated. "Therefore no enzyme could be produced in the way you say it was. And even if it could, there would be no purpose to be served. Chemically the enzyme will behave the same whether it has radioisotopes in it or not. What you're saying is preposterous!"