One day, Duncan would realize that all men had such powers, but might use them only once in a lifetime. In Karl, the gift was exceptionally well developed; from that moment onward, Duncan had learned to take

seriously even his most outrageous speculations. That was twenty years ago; whatever had happened to that little set of plastic figures? He could not remember when he had last seen it.

But here it was again, reincarnated in colored minerals-the peculiar rose-tinted granite from the Galileo Hills, the obsidian of the Huygens

Plateau, the pseudo marble of the Herschel Escarpment. And there—it was unbelievable, but doubt was impossible in such a matter-was the rarest and most mysterious of all the gemstones found on this world. The X of the puzzle was made of Titanite itself; no one could ever mistake that blue-black sheen with its fugitive flecks of gold. It was the largest piece that Duncan had ever seen, and he could not even guess at its value.

“I don’t know what to say,” he stammered. “It’s beautiful-I’ve never seen anything like it.”

He put his arms around Grandma’s thin shoulders -and found, to his distress, that they were quivering uncontrollably. He held her gently until the shaking stopped, knowing that there were no words for such moments, and realizing as never before that he was the last love of her empty life, and he was leaving her to her memories.

CHILDREN OF THE CORRIDORS

There was a sense of sadness and finality about almost everything that he did in these last days. Sometimes it puzzled Duncan; he should be excited, anticipating the great adventure that only a handful of men on his world could ever share. And though he had never before been out of touch with his friends and family for more than a few hours, he was certain that a year’s absence would pass swiftly enough, among the

wonders and distractions of Earth. So why this melancholy? If he was saying farewell to the things of his youth, it was only for a little while, and he would appreciate them all the more when he returned…. When he returned. That, of course, was the heart of the problem. In a real sense, the Duncan Makenzie who was now leaving Titan would never return; indeed, that was the purpose of the exercise. Like Colin thirty years ago, and Malcolm forty years before that, he was heading sunward in search of knowledge, of power, of maturity-and, above all, of the successor which his own world could never give him. For, of course, being Malcolm’s duplicate, he too carried in his loins the fatal Makenzie gene.

Sooner than he had expected, he had to prepare his family for the new addition. After the usual number of earlier experiments, he had settled down with Marissa four years ago, and he loved her children as much, he was certain, as if they had been his own flesh and blood. Clyde was now six years old, Caroline three. They in their turn appeared to be as fond of

Duncan as of their real fathers, who were now regarded as honorary members of the Clan Makenzie. Much the same thing had happened in Colin’s generation-he had acquired or adopted three families-and in Malcolm’s.

Grandfather had never gone to the trouble of marrying again after Ellen had left him, but he had never lacked company for long. Only a computer could keep track of the comings and goings on the periphery of the clan; it often seemed that most of Titan was related to it in some way or other. One of

Duncan’s major problems now was deciding who would be mortally offended if he failed to say good-bye.

Quite apart from the time factor, he had other rea sons for making as few farewells as possible. Every one of his friends and relatives-as well as almost complete strangers-seemed to have some request for him, some mission they wanted him to carry out as soon as he reached Earth. Or, worse still, there was some essential item (“It won’t be any

trouble”) they wanted him to bring back. Duncan calculated that he would have to charter a special freighter if he aoquiesced to all these demands.

Every job now had to be divided into one of two categories. There were the things that must be done before he left Titan, and those that could be postponed until he was aboard ship. The latter included his studies of current terrestrial affairs, which kept slipping despite ColWs increasingly frantic attempts to update him

Extricating himself from his official duties was also no easy task, and

Duncan realized that in a few more years it would be well-nigh impossible

He was getting involved in too many things, though that was a matter of deliberate family policy. More than once he had complained that his title of Special Assistant to the Chief Administrator gave him responsibility without power. To this, Chief Administrator Colin had retorted: “Do you know what power means in our society? Giving orders to people who carry them out only it and when they feel like it.”

This was, of course, a gross libel on the Titanian bureaucracy, which functioned surprisingly well and with a minimum of red tape. Because all the key individuals knew each other, an immense amount of business got done through direct personal contact. Everyone who had come to Titan had been carefully selected for intelligence and ability, and knew that survival depended upon cooperation. Those who felt like abandoning their social responsibilities first had to practice breathing methane at a hundred below.

One possible embarrassment he had at least been spared. He could hardly leave Titan without saying good-bye to his once closest friend-but, very fortunately, Karl was off-world. Several months ago he had left on one of the shuttles to join a Terran survey ship working its way through the outer moons. Ironically enough, Duncan had envied Karl his chance of seeing some unknown worlds; now it was Karl who would be envious of him.

He could well imagine Karl’s frustration when he heard that Duncan was on his way to Earth. The thought gave him more sadness than pleasure; the

Makenzies, whatever their faults, were not vindictive Yet Duncan could not help wondering how often Karl’s reveries would now turn sunward, and to the moment long ago when their emotions had been irrevocably linked with the mother world.

Duncan was just sixteen, and Karl twenty-one, when the cruise liner Mentor had made her first, and it was widely hoped only, rendezvous with Titan.

She was a converted fusion-drive freighter-slow but economical, provided adequate supplies of hydrogen could be picked up at strategic points.

Mentor had stopped at Titan for her final refueling, on the last leg of a grand tour that had taken her to Mars, Ganymede, Europa, Pallas, and

Iapetus, and had included flybys of Mercury and Eros. As soon as she had loaded some fifteen thousand tons of hydrogen, her exhausted crew planned to head back to Earth on the fastest orbit they could compute, if possible after marooning all the passengers.

The cruise must have seemed a good idea when a consortium of Terran universities had planned it several years earlier. And so indeed it had turned out, in the long run, for Mentor graduates had since proved their worth throughout the Solar System. But when the ship staggered into her parking orbit, under the command of a prematurely gray captain, the whole enterprise looked like a first-magnitude disaster.

The problems of keeping five hundred young adults entertained and out of mischief on a six-months’ cruise aboard even the largest space liner had not been given sufficient thought; the law professor who had signed on as master-at-arms was later heard to complain bitterly about the complete absence from the ship’s inventory of hypodermic guns and knockout gas. On the other hand, there had been no deaths or serious injuries, only one pregnancy, and everyone had learned a great deal, though not necessarily in the areas that the organizers had intended. The first few weeks, for example, were mostly occupied by experiments in zero-gravity sex, despite warnings that this was an expensive addiction for those compelled to spend most of their lives on planetary