Trevize said skeptically, "If that's what's happening, it doesn't happen quickly. This world may have been free of human beings for twenty thousand years and yet most of it still seems to be very much a going concern."
"Surely," said Bliss, "that depends on how well the ecological balance was set up in the first place. If it is a fairly good balance to begin with, it might last for a long time without human beings. After all, twenty thousand years, though very long in terms of human affairs, is just overnight when compared to a planetary lifetime."
"I suppose," said Pelorat, staring intently at the planetary vista, "that if the planet is degenerating, we can be sure that the human beings are gone."
Bliss said, "I still detect no mental activity at the human level and I am willing to suppose that the planet is safely free of humanity. There is the steady hum and buzz of lower levels of consciousness, however, levels high enough to represent birds and mammals. Just the same, I'm not sure that unterraforming is enough to show human beings are gone. A planet might deteriorate even if human beings existed upon it, if the society were itself abnormal and did not understand the importance of preserving the environment."
"Surely," said Pelorat, "such a society would quickly be destroyed. I don't think it would be possible for human beings to fail to understand the importance of retaining the very factors that are keeping them alive."
Bliss said, "I don't have your pleasant faith in human reason, Pel. It seems to me to be quite conceivable that when a planetary society consists only of Isolates, local and even individual concerns might easily be allowed to overcome planetary concerns."
"I don't think that's conceivable," said Trevize, "anymore than Pelorat does. In fact, since human-occupied worlds exist by the million and none of them have deteriorated in an unterraforming fashion, your fear of Isolatism may be exaggerated, Bliss."
The ship now moved out of the daylit hemisphere into the night. The effect was that of a rapidly deepening twilight, and then utter darkness outside, except for starlight where the sky was clear.
The ship maintained its height by accurately monitoring the atmospheric pressure and gravitational intensity. They were at a height too great to encounter any upthrusting mountainous massif, for the planet was at a stage when mountain-building had not recently taken place. Still, the computer felt its way forward with its microwave finger-tips, just in case.
Trevize regarded the velvety darkness and said, thoughtfully, "Somehow what I find most convincing as the sign of a deserted planet is the absence of visible light on the dark side. No technological society could possibly endure darkness. As soon as we get into the dayside, we'll go lower."
"What would be the use of that?" said Pelorat. "There's nothing there."
"Who said there's nothing there?"
"Bliss did. And you did."
"No, Janov. I said there's no radiation of technological origin and Bliss said there's no sign of human mental activity, but that doesn't mean there's nothing there. Even if there are no human beings on the planet, there would surely be relics of some sort. I'm after information, Janov, and the remainders of a technology may have its uses in that direction."
"After twenty thousand years?" Pelorat's voice climbed in pitch. "What do you think can survive twenty thousand years? There will be no films, no paper, no print; metal will have rusted, wood will have decayed, plastic will be in shattered grains. Even stone will have crumbled and eroded."
"It may not be twenty thousand years," said Trevize patiently. "I mentioned that time as the longest period the planet may have been left empty of human beings because Comporellian legend has this world flourishing at that time. But suppose the last human beings had died or vanished or fled only a thousand years ago."
They arrived at the other end of the nightside and the dawn came and brightened into sunlight almost instantaneously.
The Far Star sank downward and slowed its progress until the details of the land surface were clearly visible. The small islands that dotted the continental shores could now be clearly seen. Most were green with vegetation.
Trevize said, "It's my idea that we ought to study the spoiled areas particularly. It seems to me that those places where human beings were most concentrated would be where the ecological balance was most lacking. Those areas might be the nucleus of the spreading blight of unterraforming. What do you think, Bliss?"
"It's possible. In any case, in the absence of definite knowledge, we might as well look where it's easiest to see. The grasslands and forest would have swallowed most signs of human habitation so that looking there might prove a waste of time."
"It strikes me," said Pelorat, "that a world might eventually establish a balance with what it has; that new species might develop; and that the bad areas might be recolonized on a new basis."
"Possibly, Pel," said Bliss. "It depends on how badly out of balance the world was in the first place. And for a world to heal itself and achieve a new balance through evolution would take far more than twenty thousand years. We'd be talking millions of years."
The Far Star was no longer circling the world. It was drifting slowly across a five-hundred-kilometer-wide stretch of scattered heath and furze, with occasional clumps of trees.
"What do you think of that?" said Trevize suddenly, pointing. The ship came to a drifting halt and hovered in mid-air. There was a low, but persistent, hum as the gravitic engines shifted into high, neutralizing the planetary gravitational field almost entirely.