“It'll take us about forty more years to stop,” Jao said. “But we should end up in the approximate volume of space that Original Man used to inhabit."

People were starting to bunch around the optical boundary of the gigantic holo projection that dominated what, by convention, was the forward end of the observation lounge. More spectators were trickling into the fan-shaped chamber. Yggdrasil had just burst out of one of the globular star clusters above the plane of the Milky Way, and word was going around that there was a magnificent view to be seen.

Bram stood a little aside with the rest of the astronomy and physics group, so as not to block the view. He'd been here about a half hour now, but he still couldn't take his eyes off the image.

The central bulge of the galaxy loomed head on out of darkness: a spectacular incandescent yolk with the flattened disk spread out around it. The fires of the nucleus glowed yellow, those of the disk shading to the paler blues of older stars. Nearer, a hail of intervening stars streamed by, Yggdrasil's tremendous speed putting them in visible motion.

The holo vista was startlingly realistic—much more so than the kind of flat computer display Jun Davd had made do with at the beginning of the voyage. But that was hardly surprising. He'd had centuries to improve on it and five generations of brilliant engineers to assist him.

This blazing vision was everything Bram had dreamed of all his life. A knot of indescribable longing formed in his chest. It was easy to forget that this was illusion—that in deceleration mode, with the tethered tree straining forward and a torrent of hadronic photons aimed along the probe's axis through the focus of the sheltering ramscoop fields, the scene rushing toward him was actually beneath his feet.

Jao was taking some good-natured heckling from the crowd of onlookers. “Forty years is a long time to wait,” someone said.

“We could do it faster,” Jao said. “The limiting factor is the number, of g's we want to take over an extended period of time."

“I don't understand how we'll be able to brake at all,” somebody else said, “when we're not approaching edge-on through the spiral arms."

“We've already shed some velocity diving through the globular clusters,” Jao said. “And there's plenty of mass in the central bulge, even though it's not as concentrated as the one that sent us on our way.” His hands sketched the glowing egg in the holo behind him. “We dive straight down through the nice, nourishing H-II regions we're going to find in there, we make a tight demiorbit around the central three parsecs, a safe distance outside the core that contains the black hole, the central star cluster, and the rotating inner ring of ionized gas. The normal orbit at that radius—if we wanted to stay there—takes ten thousand years, so we apply vector to turn it into a powered orbit. Nevertheless, we start to spiral out in spite of ourselves, in flatter and flatter spirals that bring us into the plane of the galaxy. And the spirals are retrograde, of course, so we're using the magnetic brake of the nucleus itself. Nothing to it! It's as easy as tomato pie! Isn't that so, Bram?"

Bram, startled out of his reverie, said, “Uh, certainly. The Milky Way turns out to have a very strong magnetic field at the center from what certain of our observations seem to indicate, and we ought to be able to use its rotation to slow us down considerably—just the opposite of the magnetic assist we got at departure."

He glanced at the peculiar arc that could be seen growing out of the center of the galaxy's golden yolk like a spray caused by a pinprick. It wasn't a very large scale feature—only a few thousand light-years in extent—and its meaning was still being debated by the astronomy department; there was no need to go further into it at this moment.

“The galaxy's made a quarter rotation in the last seventy-four million years,” Jun Davd interposed smoothly. “Or, rather, Original Man's neighborhood in one of the spiral arms has. We had to allow for that, too. I hope everyone appreciates the brilliant job of orbital plotting that Jao's department did."

Jun Davd was as imposing as a young man as he had been when he was old. He'd become even taller with the reversal of calcium loss in his spine and carried his slim, smooth-muscled body with an easy grace that his older incarnation had only hinted at. Bram could never quite get used to seeing him without white hair, though.

“Well, I suppose if we've come this far, we can wait another forty years,” Jao's heckler conceded grudgingly. Bram recognized him as one of the younger passengers who had been born on the trip. He was probably under a century old, and forty years would seem like forever to him.

“Oho, hold on minute,” Jao said with a malicious glint in his eye. “After we get to the volume of space we think Original Man's message came from seventy-four million years ago, we still have to find the suns he used. That could take us another century or two if we don't get lucky right away. We can do some preliminary sifting from a distance, but every time we want to stop and examine a G-type star firsthand, we've got to spend a year boosting down from relativistic speeds, a year boosting up to get to the next one, and at the low gamma we build up for these short hops, at least a couple of years in between. Unless you don't mind gaining a lot of weight, that is!"

“Why do we have to find one of Original Man's worlds at all?” grumbled the slender youth who had once been Doc Pol. Young he had become, but he hadn't lost his grumpiness. “We're here, we're safe in a galaxy that isn't exploding—why not settle down in the first G-type system we find? Put down roots. Let our descendants go gallivanting around looking for Original Man's world if they've a mind to."

There was a chorus of opposition to squelch that idea. Nobody wanted to give up on the search now that they were so close. Doc Pol hunched his faunlike head between his shoulders and looked stubborn.

“We've waited this long—we can wait a little while longer,” said a woman wearing one of the leaf tabards that had been popular five hundred years back. The costume placed her as one of the old hands—now a minority.

“That's right,” another person said authoritatively. “Man's worlds are our best bet, anyway. There's bound to be some sort of a surviving DNA-based ecology we can adapt to instead of having to start an ecology from scratch from our vats and gardens here. In the meantime, Yggdrasil is a fine world to live on."

“Too fine,” someone else said. “We have folk aboard who've never known any other world. There may be some who'll prefer to stay in orbit with Yggdrasil. It makes a more benign environment than most planetary surfaces. I know I'd be tempted to stay myself."

“Until one day you found yourself sailing off to the cometary halo or heading for a nearby star that Yggdrasil thought had the right absorption lines,” some wag said, and everybody laughed.

Jao's granddaughter, Enyd, claimed attention with a frown and a gesture. She was still chief tree systems officer after all these years, still smooth, cool, and unapproachable. She picked a lover briefly every twenty years or so, but most of the time she remained seemingly wedded to Yggdrasil. The unattached men who had wooed her to no avail called her “the dryad” behind her back.

“Whatever we do, we'll have to slow down enough to let Yggdrasil absorb a comet or two before too long,” she said. “It's been showing the effects of drought these last few years."

The current year-captain, a colorless hydroponicist named Ploz who'd had the support of the sociometrics clique, said with quick concern, “How critical is it?"

“We're not losing leaves or anything like that. But the reproductive cycle is on hold. And the more recent xylem rings are getting extremely narrow—Yggdrasil's pretty much shut down its growth these last few years."

“How much longer before it gets critical?” Ploz persisted.

“I wouldn't want to wait more than another century or so,” Enyd said severely.

There was a general sigh of relief. Jun Davd said, “Yggdrasil will have its comet long before then."

Bram hung around until the press of people got too thick for comfort. Then, with a last regretful look at the galaxy's swollen heart of fire, he elbowed his way out to make room for someone else.

“Don't forget the meeting tomorrow,” Jao called after him. Bram waved acknowledgment.

When he emerged from the crowd, he found a delegation of four waiting to intercept him—three members of the glib younger set who seemed to be running things these days and one of the former oldsters who'd been around at the start of the journey.

Silv Jaks was the group's spokesperson, and that was Bram's clue that the sociometricians had taken a hand in whatever this would turn out to be. Bram wasn't quite sure what sociometrics was, other than the fact that it had something to do with people in groups—it was one of the jargon-packed new sciences that had sprung up during the trip's later generations—but its disciples had definite ideas about the running of the tree, and they wielded a lot of influence among the new people.

“Bram—we hoped we'd find you here,” she said briskly. “Do you have a minute to talk?"

“Sure, Silv.” Bram nodded to the group's senior member. “Hello, Torm."

“I'll come to the point,” Silv said. “We represent a committee that wants you to run for year-captain this term."

“That's very flattering,” Bram said warily, “but I haven't been year-captain for over two hundred years. just a working biologist and part-time astronomy assistant these days. There are plenty of qualified people around.

She set her jaw and drew a breath. “We believe you're the candidate who's needed at this time."

“I thought you were going to throw your weight behind Ploz again. Everyone says he's doing a good job."

“He was the sort of year-captain we needed while we were still coasting. With deceleration, we've become a migratory society again. And there will be further upheavals as the populations of the outlying branches are assimilated into the annual branch once more. We're going to undergo a lot of societal stress in the coming years. More important, our goal is in sight now.” She gestured past the crowds toward the overpowering panorama of the Milky Way's bright center. “We need a renewal of our sense of purpose as a society."

“What's it got to do with me? It seems to me that we've hung on to our sense of purpose all right—even though the Father World is only a legend to most of the folk who were born between the galaxies. As to adjusting to an annual migration again, I wouldn't worry. People manage.” He smiled reassuringly. “It might even breathe some life into All-Level Eve and Bobbing Day again."

Silv was having none of his levity. “You're a symbol of the old days for a lot of the people who weren't around then. And for the original embarkees, you're the leader who started them on this voyage. You were the first year-captain—I looked that up! We've run profiles of all the old leaders through the computer, and we believe you're the one who's best suited to bring the two moieties together."

“The two what?"

It sounded like more of the jargon that Silv's colleagues in the Theoretical Anthropology group were always culling out of the primal Inglex dictionary. They were always creating computer models of imaginary human societies and drawing fanciful conclusions from them. They claimed to use rigorous statistical methods, too, but from what Bram had seen, some of the more overweening practitioners of the arcane speciality had a rather shaky grasp of real math.

Silv metered her words to him carefully. “One-third of the population of the tree had been born within the last hundred and fifty years. Approximately one-fourth consists of the original embarkees and the generation immediately following, who share their values to a large degree. In between is an amorphous group who tend to be polarized in one direction or the other. Thus there has been an evolution toward endogamous moieties which—"

Torm interrupted with a twinkle. “What she's trying to say is that the older and younger voyagers tend to divide into two groups as defined by their cross-mating practices.” He winked. “Not that I've noticed it myself."

Bram laughed. “Tell that to my great-great-great-granddaughter Ame,” he said. “She's been trying to fight off Smeth's attentions for years."

“Naturally, in an evolving culture the lines aren't yet rigidly drawn,” Silv said with Signs of annoyance, “but we've drawn up charts and applied statistical methods, and we believe there's a developing pattern of custom and taboo."

Torm allowed his eyes to glaze, though not enough to be impolite. The lively old fogy of the Bachelors’ Lodge had again become the smallish, dapper young blade he must once have been. Bram would not have put it past him to have cast an eye on Silv herself for a little endogamous tumble.

“What it sublimates down to,” Torm said, “is that Silv's crowd think I have enough influence with the old-timers to be worth cultivating and that if we can get together behind a candidate with across-the-moieties appeal, we'll have the votes to win."

“And that's me?"

“You're the only one we can agree on. If you say yes, the votes are guaranteed."

“Will you accept?” Silv asked.

Bram looked past them at the fiery egg in the holo projection. “I'll think it over,” he promised.

“A symbol of the old days,” Bram repeated with relish. “That's what they're calling me now. They want to dust me off and set me up as year-captain again."

Mim paused in her work. She was feeding the score of a new string quartet by one of her protégés into her computer and punching in the program that would separate it into players’ parts for next Tenday's performance. “That might not be such a bad idea,” she said.

“Oh, Mim, you know the job's nothing but a headache,” he protested.

His daughter, Lydis, said, “What are you going to tell them?"

Lydis had hardly changed at all in the last five hundred years. She was still the same slender, dark-haired edition of her mother that she had been when she reached her final age, without an added ounce, worry line, or shift of body mass. Even her teeth seemed always to come in as identical replicas, not changing the shape of her mouth an iota. Lydis had not inherited Mim's talent for music, though; she was a gifted and relentlessly practical engineer who had designed the hardware for some of the tree's most important industrial biosynthesis plants. Currently she had taken an interest in piloting and was spending a lot of her time exploring the hangars where Yggdrasil's fleet of landers was stored and practicing in the simulators.

“I haven't decided,” Bram said. He went to the coldall and helped himself to a gourd of sapbrew. “The next few years are going to be very exciting ones for astronomy. We have the chance to study the galaxy we're going to live in from the inside out, over a real-time frame of tens of thousands of years, so we can observe processes There's that peculiar feature in the nucleus—the gas arc I mentioned. It suggests a powerful magnetic field, but at right angles to where one ought to be. We may find an answer in the inner parsec of the galaxy. We'll never have the chance for a close-up view again. Jun Davd's offered me the chance to be an important part of it. Being year-captain is time-consuming."

He took a sip from the gourd. “And then there's my own bioengineering project. Genesis Two. I'd have to shut that down entirely for a while."

Mim put the string quarter score aside. “The observations will get made whether you're there or not,” she said. “It will take years—decades—to sort them out and draw conclusions from them. You have all the time in the world. There are still tons of data from the explosion of the old galaxy that are lying there waiting for someone to go through them. Jun Davd can wait. And as for your genetics project, I know it means a lot to you, but that can wait, too. Your assistants can keep it warm. In any case, you couldn't implement the project until we're resettled."

“I don't know, Mim,” he temporized.

“I'll tell you this,” she said. “For the last two hundred years we've had year-captains who immersed themselves more and more in the housekeeping details of the job. When they weren't officious busybodies who tried to interfere with people's lives, that is. Because for at least that long, a majority of the population have been people who think that the human past is a sort of fairy tale. They've never lived on a planet, never had a Nar touch brother—never even seen a real Nar, for that matter. They think that babies have always been made by two people in a nest together—not constructed out of nucleotides. They don't have any real comprehension of the fact that the human species ceased to exist for thirty-seven million years, and that we're here now by the grace of a wonderful race that's ceased to exist itself, and that we escaped by the skin of our teeth, and that Yggdrasil is just a temporary habitat to get us back to the original seedbed of humanity.” She paused for breath. “Bram, do you realize that most of the people on this tree have never in their lives seen a real star—only holo projections? Maybe Silv is right. Maybe we do need to remind ourselves of what this trip is all about!"

He grinned at her. “I think you just like the privileges that go with being married to a year-captain. Like always having a meal or your sleep interrupted by some problem, and never having any privacy, and sitting through long boring meetings where everybody pushes their point of view on you for the thousandth time, and you smile and nod for the thousandth time so that nobody thinks their views are being slighted."

She grinned back at him. “Shall I tell you why Lydis is here?"

“To check up on the old folks and make sure their synapses haven't gotten all stuck together?"

“Oh, Bram!” Lydis said.

Mim said, “So I could ask her how she'd feel about being older sister to a new sibling. A five-hundred-year-older sister."

’”I think it's a wonderful idea,” Lydis said. “It's about time. The new people hardly wait till one child is grown up before having another one. They breed like yeast. I certainly think the demographics of the tree entitle you to another baby. And I'd enjoy having a sibling."

Both of them looked Bram's way. He said with feeling, “Lyd is right, Mim. The end of our journey's in sight. Yggdrasil could handle five or ten times our current population without any strain—but I doubt that even the new people can breed fast enough to fill it up before we reach home territory. You're certainly as eligible as any centenarian to have a second child. But are you sure you don't want to wait until we find a planet?"

“No,” she said. “Call it an act of faith."

Bram embraced her. “Careful, oldsters,” Lydis said. “You might tempt me to have a second child, too, like my silly granddaughter."

Mim, her dark eyes shining, said, “Think of the exciting younghood the child's going to have. A plunge through the heart of the galaxy. The strange sights at the core. A trip to the spiral arm that was the cradle of humanity. And—and the stars coming back!” Her voice took on fervor. “A curtain of stars drawing back little by little as we slow down, stars spreading till they fill the sky again, shifting back to their true colors ... until finally one day our destination star lies before us, big as a sun."

“Yes,” Bram said, caught by the vision. It was what another child had dreamed thirty-seven million years ago in another galaxy.

Mim, warm in his arms, felt what he was thinking. “It's come true, hasn't it, Bram?” she said softly.

“I wonder what we'll find there,” he said.

“You're seeing the husk of a quasar,” Jun Davd said, “left over from the quasar epoch of the cosmos."

He gestured toward the dizzying scene behind the observatory's holo wall. The fabric of the universe seemed to writhe as stars were wrenched from their true positions and yanked into the illusory halo surrounding the black hole's event horizon. Then, as Yggdrasil continued its headlong dive past the hole, the stars smeared around the heart of darkness pulled free and popped back into place. The queer stellar mirage was visible because Yggdrasil, on its first pass, was in a polar orbit—dropping down from above the nuclear bulge, where the hole's accretion disk did not obscure the view.

You could see the inferno of light representing the disk if you stepped close to the transparent shield and looked down into the imaginary space behind the wall. But most people took a look and stepped right back again. You knew it wasn't real, but something about it made you feel as if you were falling.

Smeth, working hard to impress Ame, cleared his throat to get noticed. “It might be a husk, but if so, it's a husk of a hundred million solar masses. Not in the same league as the binary holes at the center of the Father World's galaxy, but massive enough to have caused periodic core explosions of its own. We've detected a sort of smoke ring ten thousand light-years out that seems to be a remnant of the last explosion.” Then he made the mistake of condescending to her. “Of course,” he said importantly, “you weren't born yet when we made the passage between Scylla and Charybdis—that's what we old hands call the binary hole maneuver, from an old legend of Original Man—but those were the great days! You had to have been there!"

“Yes, I've played back the sequence many times,” Ame said offhandedly. “Jun Davd, is there any danger of the next core explosion wiping out life in this galaxy, as it did in the old one?"

Bram watched Ame with a pride of authorship he felt for all his descendants—though in Ame's case he could claim only one thirty-second of the credit. His great-great-great-granddaughter had turned out well, he thought. She was a pert, direct, lively girl with wide green eyes and corn-colored hair. Though she was scarcely forty years old, she was a complete person with good sense and integrated views.

Her interest was in something she called reconstructive paleontology, and with a small group of similar-minded young people who styled themselves such things as comparative geologists and theoretical terralogists, she was attempting to come up with a self-consistent picture of the bygone planet, Earth, that had spawned Original Man. The store of data they had to go on was skimpy—the highly condensed primers of various descriptive sciences that had been included in the Message plus whatever clues they could gather from literary works, dictionary line drawings, parallel processes on the Father World, and similar sources. But she and her friends had been ingenious and had gone surprisingly far with their small database.

To Smeth's chagrin, Ame had brought along two colleagues from the paleoearth department: a woman named Abiga, whose specialty was comparative geology, and a young man named Jorv, who was only in his twenties and who bubbled over with enthusiasm for something he called “deductive zoology."

Bram felt sorry for Smeth, watching him hover and fuss around Ame. Youth had not been kind to the gawky physicist; it had robbed him of a certain acquired gravity and left him awkward and abrasive again. When Smeth had invited Ame to watch the hole approach with the astronomy and physics group, he had expected to monopolize her—and now she could not be pried loose from her chums. Smeth still was trying to figure out if Jorv was attached somehow to Abriga or whether he represented sexual competition.

As if that weren't bad enough, Jun Davd was being courtly.

“No, we believe the Milky Way is reasonably tame now,” Jun Davd said in answer to Ame's question. “The quasar epoch used up the tremendous quantity of material within the core that might fuel an event on that titanic a scale and stored it conveniently in the form of the black hole we're orbiting now. The subsequent explosions—like the one that caused the ‘smoke ring’ Smeth mentioned—obviously could not have been violent enough to wipe out life in the Milky Way ... though they might have had some minor effect on species."

Ame and Jorv exchanged a peculiar glance.

“When did the last core explosion take place?” Ame asked. “Or is there any way of estimating it?"

“Yes, indeed, there is,” Jun Davd said. “We've been observing the so-called smoke ring over a period of more than fifteen thousand objective years during our dive into the galactic bulge. It keeps expanding and contracting to strike a balance between its rotantional velocity and the gravitational attraction of the center. From the rate of oscillation, we calculate that the last core explosion took place approximately one hundred and forty-one million years ago.” He smiled. “And I gather that Original Man evolved after that event, since he broadcast his Message only seventy-four million years ago."

“Jun Davd,” Ame said, hesitating, “when is the next core explosion due?"

Smeth opened his mouth, but Jun Davd beat him to it.

“Theoretically, there shouldn't be one. The last explosion should have depleted the galactic center of the necessary mass. The smoke ring's velocity and distance suggest an ejected mass of one hundred million solar masses—and an explosion powered by converting the equivalent of ten thousand suns completely into energy. Now a black hole of one hundred million solar masses sits in the center, and except for the observed stars around it, the center should pretty well have been swept clean.” He frowned.

“But?"

“By generating artificial profiles of the twenty-one centimeter line, we've determined the amount of invisible matter that must be rotating around the galactic center.” He paused, decided to add to his explanation. “You see, that gave us the Doppler shift of the neutral hydrogen present."

She seemed to know what he was talking about. “The faster the rotation, the greater the mass?"

He brightened. “Precisely. And the figure we get is two hundred million solar masses."

“Twice what ought to be there?"

He nodded. “And we don't know where it came from."

“Jao has a theory about that, though,” Bram put in.

“I've heard about Jao's theories, Bram-tsu.” Ame laughed. She was a dutiful descendant, always giving him an ancestral honorific in the abbreviated Chin-pin-yin form. He had told her over and over again to simply call him Bram, but like so many of the young people, she was a stickler for convention; it was as if the newest generation were trying to revive a structure of human tradition all by themselves.

Jao, hearing his name, twisted his shaggy red head around from the console he had been working. “Yah,” he said, “there has to be some kind of mechanism for renewing matter in the core of the galaxy. It doesn't have to amount to much—about seven-tenths of a solar mass per year."

“The problem is,” Jun Davd said indulgently, “that this hypothetical flow of matter isn't coming from the galactic plane, and it isn't coming from outside the galaxy, as when the Whirlpool cannibalized the Bonfire."

“So that leaves one place, right?” Jao continued. “The nucleus of the galaxy itself. Matter just appears there."

Smeth found his voice. “That's preposterous!” he said. “It's nothing more than a rehash of the old discredited theory of the continuous origin of matter!"

“No, listen, this is a new idea based on the heavy-neutrino model of the universe,” Jao insisted. “If neutrinos have mass, then they could account for ninety percent of the mass of the universe, and ordinary matter is a film wrapped around great clumps of neutrinos. And where do the clumps come from? I'm glad you asked that. They're simply the walls of a great spongy cellular structure, one of whose bubbles is our own dear old universe. This all takes place in eleven-dimensional space-time, needless to say. The different domains are a necessary consequence of the first moments of creation—and you'll notice that the domains would be nonregular in shape, and that fits in well with the observed fibrous structure of the universe. So how do we create matter in the nuclei of galaxies without violating baryon conservation?"

He glared around at everybody.

“I have a feeling he's going to tell us,” Bram said.

“We don't!” Jao proclaimed triumphantly. “We have an exchange of neutrinos and un-neutrinos through the walls of the domains. The walls leak. Why are the leaks located in the centers of galaxies? Easy. Because of the hypermasses there—the super black holes sinking deep into the plenum and stretching the warp and woof of space-time to its limits. You may well ask why no right-handed neutrinos have ever been observed, despite the predictions of theory! Because the scales are balanced in other domains, that's why! So symmetry is preserved in the larger sense. Baryons—like protons and neutrons—can't cross the domain walls without instantly decaying. But un-neutrinos exhibit antidecay and assemble themselves into elementary particles, in the reverse of beta decay."

Smeth was beside himself. His face was red as a tomato, and he seemed in danger of bursting.

“You can't do that!” he said, his voice cracking. “How can you change leptons into baryons?"

“With mesons as the mediators, naturally,” Jao said. “And while we're at it, what do you think happened to those missing solar neutrinos you tried to detect in that chlorine tank experiment of yours back on the Father World? I'll tell you. They were falling in, not out. They were being funneled through the domain wall to another domain. There must have been a black hole on the other side."

Smeth opened his mouth indignantly to reply, but Ame cut in to get the subject back on track.

“So what it boils down to is that, by whatever mysterious process, there's enough material in the center of the galaxy for another explosion, even though it shouldn't be there?” she asked.

“Admirably put,” Jun Davd said.

“And that presumably this process acted in the past to cause periodic explosions?"

“Yes."

Ame mulled it over. “Could core explosions be a byproduct of some other process? Or vice versa?"

Jun Davd pursed his lips. “That's an interesting question. More interesting than you know. Why do you ask?"

“I'm trying to pin down periodicity. You said that the last core explosion in the Milky Way must have taken place one hundred and forty-one million years ago?"

“Thereabouts."

“It's a very interesting coincidence."

Her zoologist friend, Jorv, nodded energetically.

“Coincidence? How?” Jun Davd asked.

“Because one hundred and thirty-nine million years ago was when the dinosaurs became extinct."

Jorv burst out, “Practically on the heels of the explosion!"

“What is a dinosaur?” Jun Davd inquired.

“It was a very large animal that predated humans on Earth. Though they couldn't have been as large as the Message described them—there must be some error or misinterpretation of scale. We have line sketches of several of the main types. They were the dominant form of life on Earth for about one hundred and thirty million years, then they abruptly vanished."

“Like Original Man,” Jun Davd murmured. “I take your point.'

“How big were they?” Jao asked.

“They were built on the same general plan as human beings,” Ame said. “Four limbs, bilateral symmetry, bony skeleton—so I suppose it's possible that they could have been as much as three or four times human size. We've done computer simulations of them, and with a thicker bone cross section as compared to a human, certain efficiencies in oxygenating tissues, a slower metabolism, and so forth, we think it's possible for them to have weighed as much as a ton."

Jao whistled. “That big?"

“The Message data has some of them attaining a length of ninety feet and weighing as much as fifty tons.” She smiled. “But of course that's nonsense. No creature with an internal skeleton could attain that size. It would have to have some sort of exoskeleton supporting its weight like a scaffolding, or an external shell, like the orthocone creatures on the Father World—and of course, then they couldn't be very active."

Jorv's boyish face had been working, and now he said, “Don't take away my big dinosaurs, Ame! I've been running a different set of assumptions through the computer, and I can get bipeds up to twenty feet tall and quadrupeds twice that scale."

“Bone is bone, and histology is histology,” Ame said tolerantly, “but if you want to postulate more efficient absorptive surfaces than human beings have, and a food intake of a quarter ton a day, and radiators to get rid of body heat, you're welcome to your big dinosaurs."

It was the usual sort of professional banter, but Bram could all but see Smeth's suspicious mind working to make something more of it. Poor Smeth, he thought. The case was hopeless, but it wasn't up to him to tell Smeth that!

“So these ... dinosaurs disappeared at the approximate time of the core explosion?” Jun Davd mused. “Hmmm ... allowing time for some undefined process to spread to Earth's latitude in the galaxy at the quite reasonable rate of about one and one-half percent of the speed of light?”

“Not just the dinosaurs, Jun Davd,” Jorv said. “They disappeared completely—every last one of them—but many other land animals and most marine species disappeared at the same time. If the record that we've been given is correct."

“And about one hundred fifty-five million years before that mass extinction,” Ame put in, “there was another major extinction in which half of all animal families were wiped out. And one hundred and fifty-five million is almost an exact multiple of twenty-six million."

“What's the significance of twenty-six million?” Jun Davd asked.

Ame and Jorv fell over themselves to be the one to tell him. Ame won out. “The geological record we got from Original Man—and you must realize it's only a summary, without the detail we'd like—shows that there seem to be major species die-offs at intervals that work out to an average of about twenty-six million years."

Jorv amended: “In two cases, there seems to have been a follow-up extinction of species at an interval somewhere between thirteen and fifteen million years."

“That's what worries me,” Ame said. “There were two cycles of extinction that followed the dinosaur disappearances that don't appear to have affected the precursors of Homo sapiens, but add another thirteen million years to that and you get a value pretty close to the time when Original Man's Message was cut off."

“Jao,” Jun Davd said, turning around. “What sort of correlation can we get on that major date of one hundred and fifty-five million years prior to the last core explosion?"

Jao's fingers flew over his board. “It's hard to say, Jun Davd. We got one scenario that could have put another core explosion about there. You know, the one where we factored in the expanding molecular ring at the center. But we don't know enough yet."

Jun Davd turned back to Ame. “There's not enough to go on,” he said. “We have one striking correlation of a core explosion with one of your two major extinctions and one more possible correspondence. But there's been no more explosion since the dinosaurs disappeared and no periodic phenomenon that subdivides the major events into twenty-six-million-year cycles."

“How about this?” Jao said. “Imagine something that sweeps the galaxy like the spokes of a wheel. Imagine eight spokes. Four major spokes, like the arms of a cross. And in between, four minor spokes, also arranged roughly as a cross. As Man's sun orbits the galactic center, it encounters a spoke approximately every twenty-six million years."

“Why eight spokes?” Jun Davd inquired.

“You have to start somewhere. Eight has a nice symmetry. Besides, it works out in lots of ways. You'll see in a minute."

“What do you mean?” Ame asked, leaning forward.

“Look,” Jao said. He did something to his console, and the tremendous scene behind the holo wall vanished, to be replaced by the rough scrawls Jao was tracing on his touch pad with one meaty forefinger.

“Hey, wait a minute!” Smeth protested.

“The universe won't go away,” Jao said. “I'll bring it back in a minute. I want everyone to see this."

A bold vertical line in screaming orange grew in the illusory void behind the wall. “We start with a two-spoke model,” Jao said. “Two opposing arms, like a lot of cosmic phenomena. Like galactic gas jets, for example. Now we add another pair of arms at right angles."

He slashed another orange line across the first. It wasn't quite horizontal—apparently by design, because as everyone watched, the second pair of arms, responding to whatever crude instructions he had punched into his board, slowly turned until the adjustment was made.

“You see, there's some repulsive force holding them equidistant from the two previous arms,” Jao said. “Like magnetism. Now we subdivide one more time."

Two more crude slashes turned the cross into an asterisk. This time the finger-painted lines weren't as thick, and they were a paler orange. They were also slightly out of position, but after a moment the second cross rotated with respect to the first until all the angles were equal.

“These are the baby arms,” Jao said. “They're not full-grown yet. They're weaker. That's why I drew them skinny."

Jorv, with growing excitement, said, “What did you mean when you said it works out?"

“For starts, when you multiply these twenty-six-million-year events of yours by eight, you get a figure of two hundred and eight million years. Which is a pretty good match for the length of time it takes Man's sun to make one complete orbit around the galactic center."

“Very suggestive,” Jun Davd said. “Eight events per solar orbit. Of course, you're assuming either that the spokes are stationary, or the sun is stationary, or the spokes are rotating at twice the sun's speed at the radius of the sun's orbit."

“Like I said, you have to start somewhere,” Jao said. “I like figures that come out even. So you begin with a nice regular series—two, four, eight. And multiples of orbital speed—which incidentally would put the end of a full-grown spoke somewhere at the rim of the galaxy."

“Why not a ten-spoke model?” Smeth interrupted, with a sidelong glance at Ame. “You'd get a multiple of two hundred and sixty million years, which is closer to the two-hundred-and-fifty-million-year galactic year that some of us prefer!"

Jao shrugged. “Suit yourself. You could make that work, too, with a little fiddling with the relative velocities. You can work out the figures if you like. But an eight-spoke model's more elegant."

“I take it there's more,” Jun Davd prompted.

Jao brightened. “Yah. Watch this."

His thick fingers busied themselves on the board, and a bright yellow dot appeared about halfway out on one of the orange spokes. It began tracing a slow circle around the center of the geometric figure. A moment later the eight-armed figure began to revolve, too, at a somewhat faster rate, with the arms continually overtaking the dot.

“Now, let's label the arms so we can keep track of them,” Jao said.

Letters popped up, alphabetizing the arms. Jao did not stop the process quickly enough, and a second round of letters began to subtend the first.

“That's okay, leave it,” Jao said. “If I were laying this out flat, I'd be going past eight into the next cycle, anyway."

Ame and Jorv were intent on the garish sketch hanging in the space before them. Bram could see Ame's lips moving as she counted to herself.

“Now we take those forty-five-degree angles and subdivide them again,” Jao said.

Eight more lines grew within the spoked figure. These were dotted lines, even paler and more tentative than the lines of the second cross. Not all of them were full-length. Several of them still fell short of the yellow dot's circular orbit.

“These are your follow-up extinctions,” Jao said to Jorv, “The ones you say sometimes follow the main events thirteen to fifteen million years later. They subdivide the twenty-six-million-year intervals, and they're growing outward at different rates, so we can't fit all of them into the picture. Yet."

“Very thin,” Smeh said.

“So we start with what we do know. Here's the dinosaurs."

One of the arms of the original cross thickened and darkened to burnt orange. The tenuous dotted line following it began to blink for attention.

“Okay, anything there?” Jao asked.

Ame and Jorv looked at each other. “The Miocene crisis,” Ame said immediately. “A mass wipeout of shellfish, plankton, some land animals. But it's only twelve million years later."

“Close enough,” Jao said. “Now, let's skip back one hundred and fifty-five million years to the other really major crisis. The one that wiped out half of all animal families on Earth. Have we got a secondary extinction associated with it?"

“Yes,” Ame said, catching her breath. “A bigger one this time. A very large extinction of marine organisms that could fall within a ten- to fifteen-million-year period. We couldn't fit it into our data before without stretching it."

“All in knowing how,” Jao said with a grin, while Smeth smoldered.

Jao thickened the axis and shaded it over to include the following arm. “Very interesting,” he said. “The two big ones are at right angles to each other. Forming two arms of the older cross. And it's the arms sprouting in their wake that turn out to be the real killers."

“As if whatever it is was getting stronger,” Ame suggested.

“Yah."

“Jao—"

“Not yet. Let's fill in as many blanks as we can first."

Over the next several minutes they assigned eleven extinction episodes to the rotating spokes. Nine of them fit the pattern of the eight major spokes, and two fell within the secondary following position.

“How do you explain the missing pieces?” Smeth asked.

“How should I know?” Jao rumbled. “Insufficient data. Fluctuations in the strength of the spokes. Maybe factors that we haven't figured yet. The sun catches up with a spiral arm every hundred million years or so and stays inside for ten million years. It bobs up and down through the plane in a thirty-three-million-year cycle, if it behaves like the other stars at that radius. Maybe the dust intensifies the killer effect on some passes. Maybe it does the opposite and acts as a shield. Why don't you try to combine all the cycles and see what you can work out? The important thing is that everything we do have fits the pattern."

His belligerence died. Like everybody else in the observatory he was staring at the one big fact that hung before them in the rotating holo image.

“The ninth extinction and the first extinction are doubled up on the same spoke,” Ame said in a half whisper.

“It came back again for a second swipe,” Jao said.

“The second visitation was the last extinction before human beings evolved."

“The first swipe was the big double event,” Jorv said. “First the trilobites and all that plankton—ninety percent of sea life. Then half of all animal life on Earth."

“And if there was a ... a similar follow-up,” Ame said, “it would have come at just about the same time that Original Man disappeared from the universe."

Smeth's harsh voice grated through the ensuing silence.

“It couldn't be. Man is an intelligent being, not a—a dinosaur! He would have found some way to protect himself. Or flee. After all, it isn't as if the entire Milky Way was sterilized the way the Father World's galaxy was. Some life survived each of these—extinctions and went on to evolve."

“When the dinosaurs disappeared,” Jorv said, his young voice getting away from him, “no species of land animal weighing more than twenty pounds survived. Man's ancestors were very small and primitive. It was the highly evolved species that went. The second time around, that was man."

Jao stared thoughtfully at the rotating orange arms of his holo model. “Original Man had only spread a few hundred light-years. At most, a few thousand. You can't travel faster than the speed of light. You can't outrun something that extends to the galactic rim and sweeps the galaxy laterally. They could only have caught up with the previous killer arm."

He retreated into gloomy contemplation. Nobody else seemed very lively, either. Bram was just about to say something, when Jun Davd did it for him.

“Let's not jump to conclusions,” Jun Davd said. “This is all highly speculative. Jao doesn't have a theory, just a hypothesis. We need more data. We'll set up a long-term computer model and keep feeding our observations into it."

Yah, I'll get on that today,” Jao said.

Jan Davd went on, “Original Man's sector of the galaxy is about thirty thousand light-years from the center, and on our outward spiral, we'll sweep great areas of the disk over a real-time period of tens of millennia. We'll be able to make observations that were not possible for Original Man, no matter how much further advanced than us he was."

Ame and Jorv looked puzzled, and Jun Davd added gently, “The time wasn't there for him, you see."

As it sank in, the tension began to go out of the room. A few tentative smiles made their appearance. Ame brought her chin up and said to Bram, “You've brought us this far, Bramtsu. Nothing can frighten us now. This galaxy is humankind's heritage, and we're here to claim it."

Smeth edged forward, trying to reestablish contact with Ame. “That's right,” he said. “The whole thing may be nothing more than a statistical fluke, anyway."

“There's just one thing.” Jao's voice brought their heads around again. “If the eight-arm model is correct, our sector of the galaxy is due for another brush soon."

His hand swept the board, and the rotating orange lines snapped out of existence. The universe came terrifyingly back into sight, a raw torrent of light that slammed them across the eyes and turned the human figures into stark silhouettes. Yggdrasil's plunge toward periastron had carried it past the black hole's equator, and the accretion disk had risen out of the floor.