One of the buildings at the edge of the park suddenly vanished, and was replaced instantly by another of quite dif-ferent design. The transformation was so abrupt that had Arvin been blinking he would have missed it. He stared in amazement at the subtly altered city, but even during the first shock of astonishment his mind was seeking for the answer. He remembered the words that had appeared on the monitor screen-REGRESSION WILL COMMENCE-and he knew at once what was happening.

«That’s, the city as it was thousands of years ago,» he said to Khedrn. «We’re going back in time.»

«A picturesque but hardly accurate way of putting it,» re-plied the Jester. «What is actually happening is that the monitor is remembering the earlier versions of the city. When any modifications were made, the memory circuits were not simply emptied; the information in them was taken to subsidiary storage units, so that it could be recalled whenever needed. I have set the monitor to regress through those units at the rate of a thousand years a second. Already, we’re looking at the Diaspar of half a million years ago. We’ll have to go much farther back than that to see any real changes-I’11 increase the rate.»

He turned back to the control board, and even as he did so, not one building but a whole block whipped out of existence and was replaced by a large oval amphitheater.

«Ali, the Arena!» said Khedron. «I can remember the fuss when we decided to get rid of that. It was hardly ever used, but a great many people felt sentimental about it.»

The monitor was now recalling its memories at a far higher rate; the image of Diaspar was receding into the past at millions of years a minute, and changes were occurring so rapidly that the eye could not keep up with them. Alvin noticed that the alterations to the city appeared to come in cycles; there would be a long period of stasis, then a whole rash of , rebuilding would break out, followed by another pause. It was almost as if Diaspar were a living organism, which had to regain its strength after each explosion of growth.

Through all these changes, the basic design of the city had not altered. Buildings came and went, but the pattern of streets seemed eternal, and the park remained as the green heart of . Diaspar. Alvin wondered how far back the monitor could go. Could it return to the founding of the city, and pass through the veil that sundered known history from the myths and legends of the Dawn?

Already they had gone five hundred million years into the past. Outside the walls of Diaspar, beyond the knowledge of the monitors, it would be a different Earth. Perhaps there might be oceans and forests, even other cities which Man had not yet deserted in the long retreat to his final home.

The minutes drifted past, each minute an aeon in the little universe of the monitors. Soon, thought Alvin, the earliest of all these stored memories must be reached and the regression would end. But fascinating though this lesson was, he q did not see how it could help him to escape from the city as it was here and now.

With a sudden, soundless implosion, Diaspar contracted to a fraction of its former size. The park vanished; the boundary wall of linked, titanic towers instantly evaporated. This city was open to the world, for the radial roads stretched out to , the limits Of the monitor image without obstruction. Here was Diaspar as it had been before the great change came upon mankind. ~~

«We can go no farther,» said Khedron, pointing to the monitor screen. On it had appeared the words: REGRESSION CONCLUDED. «This must be the earliest version of the city that has been preserved in the memory cells. Before that, I doubt if the eternity circuits were used, and the buildings were allowed to wear out naturally.»

For a long time, Alvin stared at this model of the ancient city. He thought of the traffic those roads had borne, as men came and went freely to all the corners of the world-and to other worlds as well. Those men were his ancestors; he felt a closer kinship to them than to the people who now shared his life. He wished that he could see them and share their thoughts, as they moved through the streets of that billionyear-remote Diaspar. Yet those thoughts could not have been happy ones, for they must have been living then beneath the shadow of the Invaders. In a few more centuries, they were to turn their faces from the glory they had won and build a wall against the Universe:

Khedron ran the monitor backward and forward a dozen times through the brief period of history that had wrought the transformation. The change from a small open city to a much larger closed one had taken little more than a thousand years. In that time, the machines that had served Diaspar so faithfully must have been designed and built, and the knowledge that would enable them to carry out their tasks had been fed into their memory circuits. Into the memory circuits, also, must have gone the essential patterns of all the men who were now alive, so that when the right impulse called them forth again they could be clothed in matter and would emerge reborn from the Hall of Creation. In some sense, Alvin realized, he must have existed in that ancient world. It was possible, of course, that he was completely synthetic-that his entire personality had been designed by artist-technicians who had worked with tools of inconceivable complexity toward some clearly envisaged goal. Yet he thought it more likely that he was a composite of men who had once lived and walked on Earth.

Very little of the old Diaspar had remained when the new city was created; the park had obliterated it almost completely. Even before the transformation, there had been a small, grass-covered clearing at the center of Diaspar, surrounding the junction of all the radial streets. Afterward it had expanded tenfold, wiping out streets and buildings alike. The Tomb of Yarlan Zey had been brought into existence at this time, replacing a very large circular structure which had previously stood at the meeting point of all the streets. Alvin had never really believed the legends of the Tomb’s antiquity, but now it seemed that they were true.

«I suppose,» said Alvin, struck by a sudden thought, «that we can explore this image, just as we explored the image of today’s Diaspar?»

Khedron’s fingers flickered over the monitor control board, and the screen answered Alvin’s question. The long-vanished city began to expand before his eyes as his viewpoint moved along the curiously narrow streets. This memory of the Diaspar that once had been was still as sharp and clear as the image of the city he lived in today. For a billion years, the information circuits had held it in ghostly pseudo-existence, waiting for the moment when someone should call it forth again. And it was not, thought Alvin, merely a memory he was seeing now. It was something more complex than that-it was the memory of a memory.

He did not know what he could learn from it, and whether it could help him in his quest. No matter; it was fascinating to look into the past and to see a world that had existed in the days when men still roamed among the stars. He pointed to the low, circular building that stood at the city’s heart.

«Let’s start there,» he told Khedron. «That seems as good a place as any to begin.»

Perhaps it was sheer luck; perhaps it was some ancient memory; perhaps it was elementary logic. It made no dif. ference since he would have arrived at this spot sooner or later-this spot upon which all the radial streets of the city converged.

It took him ten minutes to discover that they did not meet here for reasons of symmetry alone ten minutes to know that his long search had met its reward.