Shilohin seemed somewhat reassured by the absence of any violent emotional response on his part, and so continued. "Not surprisingly, the psychological effects on the colonists were equally disastrous. The whole sorry affair was quietly ended and filed away as one of the shabbier episodes of our history. We prefer to try and forget about it."

A babble of human voices interspersed with laughter came from further along the corridor. As Hunt looked up expectantly Shilobin touched his arm to retain his attention for a moment longer.

"That, Dr. Hunt, is the real reason why we feel too ashamed to talk about the Oligocene Earth and its animals," she said.

The Shapieron was pronounced fully functional once more and the Ganymeans announced their intention to take the ship for a test flight to the outermost fringe of the Solar System. The trip was expected to take about a week.

A mixed gathering of scientists, engineers and UNSA personnel had congregated in the messroom at Pithead to watch the takeoff, the view of which was being relayed from Main Base and shown on the wall screen. Hunt, Carizan and Towers were sharing a table at the back of the room and drinking coffee. As the countdown neared zero, the hubbub of conversation quieted and an air of expectancy descended.

"All UNSA vessels have cleared the area. You're okay to go on schedule." The voice of the controller at Main sounded from the audio grille.

"Acknowledged," the familiar voice of ZORAC replied. "All our prelaunch checks are positive. We're lifting off now. Au revoir until about a week from now, Earthmen."

"Sure. See ya around."

For a few seconds longer the huge, majestic shape, its tail end now retracted and its outer bays closed, remained motionless, towering skyward to dominate the untidy sprawl of the base in the foreground. Then the ship began to lift, slowly and smoothly, sliding up into an unbroken background of stars as the camera followed it and the last ice crest disappeared off the bottom of the picture. Almost at once it started to contract rapidly as the foreshortening increased with the angle at a rate that hinted of the fearsome buildup of speed.

"Man, look at her go!" came the voice from Main. "Do you have radar contact yet, J5? "

"It's going like greased lightning out of hell," another voice answered. "We're starting to lose it. The image is breaking up. They must be on main drive already--their stress field's starting to scramble the echoes. Image on the optical scanners is losing coherence too. . ." And then: "That's it. It's gone. . . like it was never there at all. Fantastic!"

That was that. A few low whistles of surprise broke the silence in the mess room at Pithead, followed by muttered exclamations and murmurings. Gradually the fragments of conversation flowed together and merged into a steady continuum of noise that rose and found its own level. The picture on the screen reverted to the view of Main, now looking somehow empty and incomplete without the ship standing in the background. Even after so short a time, life on Ganymede without the Giants around didn't feel quite right.

"Well, I've got to go," Hunt said, rising from his chair. "Chris wants to talk about something. See you both later." The other two looked up.

"Sure. See you later."

"See you, Vic."

As he moved toward the door, Hunt realized that Pithead didn't seem right either without a single Ganymean in sight. It was strange, he thought, that every one of them should need to go on a test flight; but. . . that was not really something for Earthmen to reason why. He realized also that not having ZORAC around would also take some getting used to. He had come unconsciously to accept the ability to communicate directly with others and to consult with the machine, whatever time of day it was or wherever he happened to be. ZORAC had come to be a guide, mentor, tutor and advisor all rolled into one--an omniscient and omnipresent companion. Hunt suddenly felt very alone and isolated without it. The Ganymeans could have left specialized relay equipment at Ganymede that would have sustained a link to ZORAC, but the mutual slowing down of clocks that the Shapieron's velocity would produce, together with the large distance that its flight would entail, would soon have made any form of meaningful cornmunication impossible. It was, he admitted privately to himself, going to be a long week.

Hunt found Danchekker in his lab fussing over his Minervan plants, which by this time were proliferating in every corner of the room and seemed set to embark on an invasion of the corridor outside. The subject that the professor wanted to discuss was the theory that he and Hunt had formulated jointly, before the arrival of the Ganymeans, concerning the low inherent tolerance of all Minervan land-dwelling species to atmospheric carbon dioxide. This theory held that the trait had been inherited, along with the basic system of chemical metabolism, from some very early, common, marine ancestor. After discussing the matter at some length with various Ganymean scientists through ZORAC, Danchekker now knew that this theory was wrong.

"In fact, when land dwellers eventually appeared on Minerva, they evolved a very efficient method of coping with the planet's high carbon-dioxide level. The way in which they did it was one which, with the benefit of hindsight, was very obvious and very simple." Danchekker stopped rummaging around among the mass of leaves for a moment and half turned his head to allow Hunt time to reflect on the statement. Hunt, perched casually on one of the stools with an elbow resting on the edge of the bench beside him, said nothing and waited.

"They adapted their secondary circulation systems to absorb the excess," Danchekker told him. "Systems that had evolved specifically to remove toxins in the first place. They provided a ready-made mechanism ideal for the job."

Hunt turned the proposition over in his mind and rubbed his chin thoughtfully.

"So . . ." he said after a while. "This idea we had that they all inherited a low tolerance was way off the rails. . . all baloney."

"Baloney."

"And this characteristic stayed, did it? I mean, all the species that came later inherited the mechanism . . . they were all well adapted to their environment?"

"Yes. Perfectly adequately."

"But there's still something I don't see yet," Hunt said, frowning. "If what you've just said was true, the Ganymeans should have inherited an adequate resistance too. If they did, they wouldn't have had a CO2 problem. But they themselves said they did have a CO2 problem. So how come?"

Danchekker turned to face him and wiped his palms on the front of his lab coat. He beamed through his spectacles and showed his teeth.

"They did inherit it . . . the resistance mechanism. They did have a problem too. But, you see, the problem wasn't natural; it was artificial. They brought it upon themselves, far later in their history."

"Chris, you're talking in riddles. Why not start at the beginning?"

"Very well." Danchekker began wiping dry the tools he had been using and replaced them in one of the drawers as he spoke. "As I said a moment ago, when land-dwelling life appeared on Minerva, the secondary circulation systems that all species already possessed--which caused them to be poisonous--adapted to absorb the excess carbon dioxide. Thus, although Minervan air was high in carbon dioxide compared to that of Earth, all the forms of life that emerged there flourished quite happily since they had evolved a perfectly good means of adapting to their surroundings which is the way one would expect Nature to work. When, after hundreds of millions of years, intelligence emerged in the form of primitive Ganymeans, they too possessed the same basic architecture, which had remained essentially unchanged. So far so good?"