"Emmer, a Neanderthal type and our archeologist, is around the beginning of the scale. I'm from Earth, near the middle, though on Emmer's side. Meredith, linguist, is on the other side of the middle. And beyond her, toward the far end, is Kelburn, mathematician. There's a corresponding span of fertility. Emmer just misses being able to breed with my kind, but there's a fair chance that I'd be fertile with Meredith and a similar though lesser chance that her fertility may extend to Kelburn."

Taphetta rustled his speech ribbons quizzically. "But I thought it was proved that some humans did originate on one planet, that there was an unbroken line of evolution that could be traced back a billion years."

"You're thinking of Earth," said Halden. "Humans require a certain kind of planet. It's reasonable to assume that, if men were set down on a hundred such worlds, they'd seem to fit in with native life-forms on a few of them. That's what happened on Earth; when Man arrived, there was actually a manlike creature there. Naturally our early evolutionists stretched their theories to cover the facts they had.

"But there are other worlds in which humans who were there before the Stone Age aren't related to anything else there. We have to conclude that Man didn't originate on any of the planets on which he is now found. Instead, he evolved elsewhere and later was scattered throughout this section of the Milky Way."

"And so, to account for the unique race that can interbreed across thousands of light-years, you've brought in the big ancestor," commented Taphetta dryly. "It seems an unnecessary simplification."

"Can you think of a better explanation?" asked Kelburn.

"Something had to distribute one species so widely and it's not the result of parallel evolution—not when a hundred human races are involved, and only the human race."

"I can't think of a better explanation." Taphetta rearranged his ribbons. "Frankly, no one else is much interested in Man's theories about himself."

It was easy to understand the attitude. Man was the most numerous though not always the most advanced—Ribboneers had a civilization as high as anything in the known section of the Milky Way, and there were others—and humans were more than a little feared. If they ever got together—but they hadn't except in agreement as to their common origin.

Still, Taphetta the Ribboneer was an experienced pilot and could be very useful. A clear statement of their position was essential in helping him make up his mind. "You've heard of the adjacency mating principle?" asked Sam Halden.

"Vaguely. Most people have if they've been around men."

"We've got new data and are able to interpret it better. The theory is that humans who can mate with each other were once physically close. We've got a list of all our races arranged in sequence. If planetary race F can mate with race E back to A and forward to M, and race G is fertile only back to B, but forward to O, then we assume that whatever their positions are now, at once time G was actually adjacent to F, but was a little further along. When we project back into time those star systems on which humans existed prior to space travel, we get a certain pattern. Kelburn can explain it to you."

The normally pink body of the Ribboneer flushed slightly. The color change was almost imperceptible, but it was enough to indicate that he was interested.

Kelburn went to the projector. "It would be easier if we knew all the stars in the Milky Way, but though we've explored only a small portion of it, we can reconstruct a fairly accurate representation of the past."

He pressed the controls and stars twinkled on the screen. "We're looking down on the plane of the Galaxy. This is one arm of it as it is today and here are the human systems." He pressed another control and, for purposes of identification, certain stars became more brilliant. There was no pattern, merely a scattering of stars. "The whole Milky Way is rotating. And while stars in a given region tend to remain together, there's also a random motion. Here's what happens when we calculate the positions of stars in the past."

Flecks of light shifted and flowed across the screen. Kelburn stopped the motion.

"Two hundred thousand years ago," he said.

There was a pattern of the identified stars. They were spaced at fairly equal intervals along a regular curve, a horseshoe loop that didn't close, though if the ends were extended, the lines would have crossed.

Taphetta rustled. "The math is accurate?"

"As accurate as it can be with a million-plus body problem."

"And that's the hypothetical route of the unknown ancestor?"

"To the best of our knowledge," said Kelburn. "And whereas there are humans who are relatively near and not fertile, they can always mate with those they were adjacent to two hundred thousand years ago!"

"The adjacency mating principle. I've never seen it demonstrated," murmured Taphetta, flexing his ribbons. "Is that the only era that satisfies the calculations?"

"Plus or minus a hundred thousand years, we can still get something that might be the path of a spaceship attempting to cover a representative section of territory," said Kelburn. "However, we have other ways of dating it. On some worlds on which there are no other mammals, we're able to place the first human fossils chronologically. The evidence is sometimes contradictory, but we believe we've got the time right."

Taphetta waved a ribbon at the chart. "And you think that where the two ends of the curve cross is your original home?"

"We think so," said Kelburn. "We've narrowed it down to several cubic light-years—then. Now it's far more. And, of course, if it were a fast-moving star, it might be completely out of the field of our exploration. But we're certain we've got a good chance of finding it this trip."

"It seems I must decide quickly." The Ribboneer glanced out the visionport, where another ship hung motionless in space beside them. "Do you mind if I ask other questions?"

"Go ahead," Kelburn invited sardonically. "But if it's not math, you'd better ask Halden. He's the leader of the expedition."

Halden flushed; the sarcasm wasn't necessary. It was true that Kelburn was the most advanced human type present, but while there were differences, biological and in the scale of intelligence, it wasn't as great as once was thought. Anyway, non-humans weren't trained in the fine distinctions that men made among themselves. And, higher or lower, he was as good a biologist as the other was a mathematician. And there was the matter of training; he'd been on several expeditions and this was Kelburn's first trip. Damn it, he thought, that rated some respect.

The Ribboneer shifted his attention. "Aside from the sudden illness of your pilot, why did you ask for me?"

"We didn't. The man became sick and required treatment we can't give him. Luckily, a ship was passing and we hailed it because it's four months to the nearest planet. They consented to take him back and told us that there was a passenger on board who was an experienced pilot. We have men who could do the job in a makeshift fashion, but the region we're heading for, while mapped, is largely unknown. We'd prefer to have an expert—and Ribboneers are famous for their navigational ability."

Taphetta crinkled politely at the reference to his skill. "I had other plans, but I can't evade professional obligations, and an emergency such as this should cancel out any previous agreements. Still, what are the incentives?"

Sam Halden coughed. "The usual, plus a little extra. We've copied the Ribboneer's standard nature, simplifying it a little and adding a per cent here and there for the crew pilot and scientist's share of the profits from any discoveries we may make."

"I'm complimented that you like our contract so well," said Taphetta, "but I really must have our own unsimplified version. If you want me, you'll take my contract. I came prepared." He extended a tightly bound roll that he had kept somewhere on his person.

They glanced at one another as Halden took it.

"You can read it if you want," offered Taphetta. "But it will take you all day—it's micro-printing. However, you needn't be afraid that I'm defrauding you. It's honored everywhere we go and we go nearly everywhere in this sector—places men have never been."

There was no choice if they wanted him, and they did. Besides, the integrity of Ribboneers was not to be questioned. Halden signed.

"Good." Taphetta crinkled. "Send it to the ship; they'll forward it for me. And you can tell the ship to go on without me." He rubbed his ribbons together. "Now if you'll get me the charts, I'll examine the region toward which we're heading."

Firmon of hydroponics slouched in, a tall man with scanty hair and an equal lack of grace. He seemed to have difficulty in taking his eyes off Meredith, though, since he was a notch or so above her in the mating scale, he shouldn't have been so interested. But his planet had been inexplicably slow in developing and he wasn't completely aware of his place in the human hierarchy.

Disdainfully, Meredith adjusted a skirt that, a few inches shorter, wouldn't have been a skirt at all, revealing, while doing so, just how long and beautiful a woman's legs could be. Her people had never given much thought to physical modesty and, with legs like that, it was easy to see why.

Muttering something about primitive women, Firmon turned to the biologist. "The pilot doesn't like our air."

"Then change it to suit him. He's in charge of the ship and knows more about these things than I do."

"More than a man?" Firmon leered at Meredith and, when she failed to smile, added plaintively, "I did try to change it, but he still complains."

Halden took a deep breath. "Seems all right to me."

"To everybody else, too, but the tapeworm hasn't got lungs. He breathes through a million tubes scattered over his body."

It would do no good to explain that Taphetta wasn't a worm, that his evolution had taken a different course, but that he was in no sense less complex than Man. It was a paradox that some biologically higher humans hadn't developed as much as lower races and actually weren't prepared for the multitude of life-forms they'd meet in space. Firmon's reaction was quite typical.

"If he asks for cleaner air, it's because his system needs it," said Halden. "Do anything you can to give it to him."

"Can't. This is as good as I can get it. Taphetta thought you could do something about it."

"Hydroponics is your job. There's nothing I can do." Halden paused thoughtfully. "Is there something wrong with the plants?"

"In a way, I guess, and yet not really."

"What is it, some kind of toxic condition?"

"The plants are healthy enough, but something's chewing them down as fast as they grow."

"Insects? There shouldn't be any, but if there are, we've got sprays. Use them."

"It's an animal," said Firmon. "We tried poison and got a few, but now they won't touch the stuff. I had electronics rig up some traps. The animals seem to know what they are and we've never caught one that way."

Halden glowered at the man. "How long has this been going on?"

"About three months. It's not bad; we can keep up with them."

It was probably nothing to become alarmed at, but an animal on the ship was a nuisance, doubly so because of their pilot.

"Tell me what you know about it," said Halden.

"They're little things." Firmon held out his hands to show how small. "I don't know how they got on, but once they did, there were plenty of places to hide." He looked up defensively. "This is an old ship with new equipment and they hide under the machinery. There's nothing we can do except rebuild the ship from the hull inward."

Firmon was right. The new equipment had been installed in any place just to get it in and now there were inaccessible corners and crevices everywhere that couldn't be closed off without rebuilding.

They couldn't set up a continuous watch and shoot the animals down because there weren't that many men to spare. Besides, the use of weapons in hydroponics would cause more damage to the thing they were trying to protect than to the pest. He'd have to devise other ways.

Sam Halden got up. "I'll take a look and see what I can do."

"I'll come along and help," said Meredith, untwining her legs and leaning against him. "Your mistress ought to have some sort of privileges."

Halden started. So she knew that the crew was calling her that! Perhaps it was intended to discourage Firmon, but he wished she hadn't said it. It didn't help the situation at all.

Taphetta sat in a chair designed for humans. With a less flexible body, he wouldn't have fitted. Maybe it wasn't sitting, but his flat legs were folded neatly around the arms and his head rested comfortably on the seat. The head ribbons, which were his hands and voice, were never quite still.

He looked from Halden to Emmer and back again. "The hydroponics tech tells me you're contemplating an experiment. I don't like it."

Halden shrugged. "We've got to have better air. It might work."

"Pests on the ship? It's filthy! My people would never tolerate it!"

"Neither do we."

The Ribboneer's distaste subsided. "What kind of creatures are they?"

"I have a description, though I've never seen one. It's a small four-legged animal with two antennae at the lower base of its skull. A typical pest."

Taphetta rustled. "Have you found out how it got on?"

"It was probably brought in with the supplies," said the biologist. "Considering how far we've come, it may have been any one of a half a dozen planets. Anyway, it hid, and since most of the places it had access to were near the outer hull, it got an extra dose of hard radiation, or it may have nested near the atomic engines; both are possibilities. Either way, it mutated, became a different animal. It's developed a tolerance for the poisons we spray on plants. Other things it detects and avoids, even electronic traps."

"Then you believe it changed mentally as well as physically, that it's smarter?"

"I'd say that, yes. It must be a fairly intelligent creature to be so hard to get rid of. But it can be lured into traps, if the bait's strong enough."

"That's what I don't like," said Taphetta, curling. "Let me think it over while I ask questions." He turned to Emmer. "I'm curious about humans. Is there anything else you can tell me about the hypothetical ancestor?"

Emmer didn't look like the genius he was—a Neanderthal genius, but nonetheless a real one. In his field, he rated very high. He raised a stubble-flecked cheek from a large thick-fingered paw and ran shaggy hands through shaggier hair.

"I can speak with some authority," he rumbled. "I was born on a world with the most extensive relics. As a child, I played in the ruins of their camp."

"I don't question your authority," crinkled Taphetta. "To me, all humans—late or early and male or female—look remarkably alike. If you are an archeologist, that's enough for me." He paused and flicked his speech ribbons. "Camp, did you say?"

Emmer smiled, unsheathing great teeth. "You've never seen any pictures? Impressive, but just a camp, monolithic one-story structures, and we'd give something to know what they're made of. Presumably my world was one of the first they stopped at. They weren't used to roughing it, so they built more elaborately than they did later on. One-story structures and that's how we can guess at their size. The doorways were forty feet high."

"Very large," agreed Taphetta. It was difficult to tell whether he was impressed. "What did you find in the ruins?"

"Nothing," said Emmer. "There were buildings there and that was all, not a scrap of writing or a tool or a single picture. They covered a route estimated at thirty thousand light-years in less than five thousand years—and not one of them died that we have a record of."

"A faster-than-light drive and an extremely long life," mused Taphetta. "But they didn't leave any information for their descendants. Why?"

"Who knows? Their mental processes were certainly far different from ours. They may have thought we'd be better off without it. We do know they were looking for a special kind of planet, like Earth, because they visited so many of that type, yet different from it because they never stayed. They were pretty special people themselves, big and long-lived, and maybe they couldn't survive on any planet they found. Perhaps they had ways of determining there wasn't the kind of planet they needed in the entire Milky Way. Their science was tremendously advanced and when they learned that, they may have altered their germ plasm and left us, hoping that some of us would survive. Most of us did."

"This special planet sounds strange," murmured Taphetta.

"Not really," said Emmer. "Fifty human races reached space travel independently and those who did were scattered equally among early and late species. It's well known that individuals among my people are often as bright as any of Halden's or Meredith's, but as a whole we don't have the total capacity that later Man does, and yet we're as advanced in civilization. The difference? It must lie somewhere in the planets we live on and it's hard to say just what it is."

"What happened to those who didn't develop space travel?" asked Taphetta.

"We helped them," said Emmer.

And they had, no matter who or what they were, biologically late or early, in the depths of the bronze age or the threshold of atomic—because they were human. That was sometimes a frightening thing for non-humans, that the race stuck together. They weren't actually aggressive, but their total number was great and they held themselves aloof. The unknown ancestor again. Who else had such an origin and, it was tacitly assumed, such a destiny?

Taphetta changed his questioning. "What do you expect to gain from this discovery of the unknown ancestor?"

It was Halden who answered him. "There's the satisfaction of knowing where we came from."

"Of course," rustled the Ribboneer. "But a lot of money and equipment was required for this expedition. I can't believe that the educational institutions that are backing you did so purely out of intellectual curiosity."

"Cultural discoveries," rumbled Emmer. "How did our ancestors live? When a creature is greatly reduced in size, as we are, more than physiology is changed—the pattern of life itself is altered. Things that were easy for them are impossible for us. Look at their life span."

"No doubt," said Taphetta. "An archeologist would be interested in cultural discoveries."

"Two hundred thousand years ago, they had an extremely advanced civilization," added Halden. "A faster-than-light drive, and we've achieved that only within the last thousand years."

"But I think we have a better one than they did," said the Ribboneer. "There may be things we can learn from them in mechanics or physics, but wouldn't you say they were better biologists than anything else?"

Halden nodded. "Agreed. They couldn't find a suitable planet. So, working directly with their germ plasm, they modified themselves and produced us. They were master biologists."

"I thought so," said Taphetta. "I never paid much attention to your fantastic theories before I signed to pilot this ship, but you've built up a convincing case." He raised his head, speech ribbons curling fractionally and ceaselessly. "I don't like to, but we'll have to risk using bait for your pest."

He'd have done it anyway, but it was better to have the pilot's consent. And there was one question Halden wanted to ask; it had been bothering him vaguely. "What's the difference between the Ribboneer contract and the one we offered you? Our terms are more liberal."

"To the individual, they are, but it won't matter if you discover as much as you think you will. The difference is this: My terms don't permit you to withhold any discovery for the benefit of one race."

Taphetta was wrong; there had been no intention of withholding anything. Halden examined his own attitudes. He hadn't intended, but could he say that was true of the institutions backing the expedition? He couldn't, and it was too late now—whatever knowledge they acquired would have to be shared.

That was what Taphetta had been afraid of—there was one kind of technical advancement that multiplied unceasingly. The race that could improve itself through scientific control of its germ plasm had a start that could never be headed. The Ribboneer needn't worry now.

"Why do we have to watch it on the screen?" asked Meredith, glancing up. "I'd rather be in hydroponics."

Halden shrugged. "They may or may not be smarter than planetbound animals, but they're warier. They don't come out when anyone's near."

Lights dimmed in the distant hydroponic section and the screen with it, until he adjusted the infra-red frequencies. He motioned to the two crew members, each with his own peculiar screen, below which was a miniature keyboard.

"Ready?"

When they nodded, Halden said: "Do as you've rehearsed. Keep noise at a minimum, but when you do use it, be vague. Don't try to imitate them exactly."

At first, nothing happened on the big screen, and then a gray shape crept out. It slid through leaves, listened intently before coming forward. It jumped off one hydroponic section and fled across the open floor to the next. It paused, eyes glittering and antennae twitching.

Looking around once, it leaped up, seizing the ledge and clawing up the side of the tank. Standing on top and rising to its haunches, it began nibbling what it could reach.

Suddenly it whirled. Behind it and hitherto unnoticed was another shape, like it but larger. The newcomer inched forward. The small one retreated, skittering nervously. Without warning, the big one leaped and the small one tried to flee. In a few jumps, the big one caught up and mauled the other unmercifully.