"Still watering your ale, barman?"

Jame scowled. "You took a hell of a chance, tree-pilot. We should have let you drop…"

"But you didn't." Pallis smiled and relaxed.

During the short journey with the miners to the Belt Pallis remembered his wonder on hearing Rees's tale for the first time. In his role as a friend of the returned exile, he had sat with Rees, Decker and Hollerbach in the old Scientist's office, eyes transfixed by the simple hand movements Rees used to emphasize aspects of his adventures.

It was so fantastic, the stuff of legends: the Bon-eys, the hollow world, the whale, the song… but Rees's tone was dry, factual and utterly convincing, and he had responded to all Hollerbach's questions with poise.

At last Rees reached his description of the whales' great migration. "But of course," Hollerbach breathed. "Hah! It's so obvious." And he banged his old fist into his desk top,

Decker jumped, startled out of his enthrallment. "You silly old fart," he growled. "What's obvious?"

"So many pieces fit into place. Internebular migrations…! Of course; we should have deduced it." Hollerbach got out of his chair and began to pace the room, thumping a bony fist into the palm of his hand.

"Enough histrionics, Scientist," Decker said, "Explain yourself."

"First of all, the whales' songs: these old speculations which our hero has now confirmed. Tell me this: why should the whales have such sizeable brains, such significant intelligence, such sophisticated communication? If you think it through they're basically just grazing creatures, and — by virtue of their sheer size — they are reasonably immune from the attentions of predators, as Rees testifies. Surely they need do little more than cruise through the atmosphere, munching airborne titbits, needing barely more sense than, say, a tree — avoid this shadow, swim around that gravity well…"

Pallis rubbed the bridge of his nose. "But a tree would never fly into the Core — not by choice anyway. Is that what you're saying?"

"Exactly, tree-pilot. To submit oneself to such a regime of tidal stress and hazardous radiation clearly calls for a higher brain function, a far-sighted imperative to override the more elemental instincts, a high degree of communication — telepathic, perhaps — so that the correct behavior may be instilled in each generation."

Rees smiled. "Also a whale needs to select its trajectory around the Core quite precisely."

"Of course, of course."

Decker's face was a cloud of baffled anger. "Wait… Let's take it one step at a time." He scratched his beard. "What advantage do the whales gain by diving into the Core? Don't they just get trapped down there?"

"Not if they get the trajectory right," said Hol-lerbach, a little impatiently. "That's the whole point…Do you see? It's a gravitational slingshot." He held up a gaunt fist, mimed rotation by twisting it. "Here's the Core, spinning away. And—" The other hand was held flat; it swooped in toward the Core. "Here comes a whale." The model whale swooped past the Core, not quite touching, its hyperbolic path twisting in the same direction as the Core's rotation. "For a brief interval whale and Core are coupled by gravity. The whale picks up a little of the Core's angular momentum… It actually gains some energy from its encounter with the Core."

Pallis shook his head. "I'm glad I don't have to do that every time I fly a tree."

"It's quite elementary. After all, the whales manage it… And the reason they go through all this is to pick up enough energy to reach the Nebula's escape velocity."

Decker thumped a fist onto the desk top. "Enough of your babbling. What is the relevance of all this?"

Hollerbach sighed; his fingers reached for the bridge of his nose, searching for long-vanished spectacles. "The relevance is this. By reaching escape velocity the whales can leave the Nebula."

"They migrate," Rees said eagerly. "They travel to another nebula… A new one, with plenty of fresh stars, and a blue sky."

"We're talking about a grand transmission of life among the nebulae," Hollerbach said. "No doubt the whales aren't the only species which swim between the clouds… but even if they were they would probably carry across enough spores and seedlings in their digestive systems to allow life to gain a new foothold."

"It's all very exciting." Rees seemed almost intoxicated. "You see, the fact of migration solves another long-standing puzzle: the origin of life here. The Nebula is only a few million shifts old. There simply hasn't been time for life to evolve here in anything like the fashion we understand it did so on Earth."

"And the answer to this puzzle," Hollerbach said, "turns out to be that it probably didn't evolve here after all."

"It migrated to the Nebula from somewhere else?"

"That's right, tree-pilot; from some other, exhausted cloud. And now this Nebula is finished; the whales know it is time to move on. There may have been other nebulae before the predecessor of our Nebula: a whole chain of migrations, reaching back in time as far as we can see."

"It's a marvellous picture," Rees said dreamily. "Once life was established somewhere in this universe it must have radiated out rapidly; perhaps all the nebulae are already populated in some way, with unimaginable species endlessly crossing empty space—"

Decker stared from one Scientist to the other. He said quietly, "Rees, if you don't come to the point — in simple words, and right now — so help me I'll throw you over the bloody Rim with my own bare hands, And the old fart. Got that?"

Rees spread his hands flat on the desk top, and again Pallis saw in his face that new, peculiar certainty. "Decker, the point is — just as the whales can escape the death of the Nebula, so can we."

Decker's frown deepened. "Explain."

"We have two choices." Rees chopped the edge of his hand into the table. "One. We stay here, watch the stars go out, squabble over the remaining scraps of food. Or—" Another chop. "Two. We emulate the whales. We fall around the Core, use the slingshot effect. We migrate to a new nebula."

"And how, precisely, do we do that?"

"I don't, precisely, know," Rees said acidly. "Maybe we cut away the trees, let the Raft fall into the Core.»

Pallis tried to imagine that. "How would you keep the crew from being blown off?"

Rees laughed. "I don't know, Pallis. That's just a sketch; I'm sure there are better ways."

Decker sat back, his scarred face a mask of intense concentration.

Hollerbach held up a crooked finger. "Of course you almost made the trip involuntarily, Rees. If you hadn't found a way to deflect that whale, even row you'd be travelling among the star clouds with it."

"Maybe that's the way to do it," Pallis said. "Cut our way into the whales, carry in food and water, and let them take us to our new home."

Rees shook his head. "I don't think that would work, pilot. The interior of a whale isn't designed to support human life."

Once more Pallis struggled with the strange ideas. "So we'll have to take the Raft… but the Raft will lose all its air, won't it, outside the Nebula? So we'll have to build some sort of shell to keep in the atmosphere…"

Hollerbach nodded, evidently pleased. "That's good thinking, Pallis. Maybe we'll make a Scientist of you yet."

"Patronizing old bugger," Pallis murmured affectionately.

Again the fire burned in Rees. He turned his intense gaze on Decker. "Decker, somewhere buried in all this bullshit is a way for the race to survive. That's what's at stake here. We can do it; have no doubt about that. But we need your support." Rees fell silent.

Pallis held his breath. He sensed that he was at a momentous event, a turning point in the history of his species, and somehow it all hinged on Rees. Pallis studied the young Scientist closely, thought he observed a slight tremble of his cheeks; but Rees's determination showed in his eyes. At length Decker said quietly, "How do we