“On a parasite, probably. This kind seems to be infecting grass. We need to find out.”

So they gathered samples, and drove back down to Sabishii.

Nirgal sat on a futon on the floor of Tariki’s living room, feeling sick. Tariki and Sax talked long after dinner, discussing the situation. Other viroids had been appearing in a rapid dispersal from Tharsis; apparently they had made it across the cordon sanitaire of space, arriving on a world that had been previously innocent of them. They were smaller than viruses, much smaller, and quite a bit simpler. Nothing but strands of RNA, Tariki said, about fifty nanometers long. Individuals had a molecular weight of about 130,000, while the smallest known viruses had molecular weights of over a million. They were so small that they had to be centrifuged at over 100,000 g in order to be pulled out of suspension.

The potato-spindle-tuber viroid was well understood, Tariki told them, tapping around on his screen and pointing at the schematics called up. A chain of merely 359 nucleo-tides, lined out in a closed single strand with short double-strand regions braiding it. Viroids like this one caused several plant diseases, including pale cucumber disease, chrysanthemum stunt, chlorotic mottle, cadang-cadang, citrus exocortis. Viroids had also been confirmed as the agent in some animal brain diseases, like scrapie, and kuru, and Creutzfeldt-Jakob disease in humans. The viroids used host enzymes to reproduce, and then were taken to be-regulatory molecules in the nuclei of infected cells, disturbing growth-hormone production in particular.

The particular viroid in Nirgal’s basin, Tariki said, had mutated from potato spindle tuber. They were still identifying it in the labs at the university, but the sick grass made him sure they were going to find something different, something new.

Nirgal felt sick. The names of the diseases alone were enough to do it. He stared at his hands, which had been plunged thick in infected plants. Through the skin, into the brain, some kind of spongiform encephalopathy, mushroom growths of brain blooming everywhere.

“Is there anything we can do to fight it?” he said.

Sax and Tariki looked at him.

“First,” Sax said, “we have to find out what it is.”

That turned out to be no simple matter. After a few days, Nirgal returned to his basin. There he could at least do something; Sax had suggested removing all the potatoes from the potato fields. This was a long dirty task, a kind of negative treasure hunt, as he turned up diseased tuber after tuber. Presumably the soil itself would still hold the viroid. It was possible he would have to abandon the field, or even the basin. At best, plant something else. No one yet understood how viroids reproduced; and the word from Sabishii was that this might not even be a viroid as previously understood.

“It’s a shorter strand than usual,” Sax said. “Either a new viroid, or something like a viroid but smaller still.” In the Sabishii labs they were calling it “the virid.”

A long week later, Sax came back up to the basin. “We can try to remove it physically,” he said over dinner. “Then plant different species, ones that are resistant to viroids. That’s the best we can do.”

“But will that work?”

“The plants susceptible to infection are fairly specific. You got hit by a new one, but if you change grasses, and types of potatoes — perhaps cycle out some of your potato-patch soil…” Sax shrugged.

Nirgal ate with more appetite than he *had had for the previous week. Even the suggestion of a possible solution was a great relief. He drank some wine, felt better and better. “These things are strange, eh?” he said over an after-dinner brandy. “What life will come up with!”

“If you call it life.”

“Well, of course.”

Sax didn’t reply.

“I’ve been looking at the news on the net,” Nirgal said. “There are a lot of infestations. I had never noticed before. Parasites, viruses…”

“Yes. Sometimes I worry about a global plague. Something we can’t stop.”

“Ka! Could that happen?”

“There’s all kinds of invasions going on. Population surges, sudden die-offs. All over. Things in disequilibrium. Upsetting balances we didn’t even know existed. Things we don’t understand.” As always this thought made Sax unhappy.

“Biomes will eventually come into equilibrium,” Nirgal suggested.

“I’m not sure there is such a thing.”

“As equilibrium?”

“Yes. It may be a matter of…” He waved his hands about like gulls. “Punctuated equilibrium, without the equilibrium.”

“Punctuated change?”

“Perpetual change. Braided change — sometimes surging change — ”

“Like cascading recombinance?”

“Perhaps.”

“I’ve heard that’s a mathematics only a dozen people can really understand.”

Sax looked surprised. “That’s never true. Or else, true of every math. Depends on what you mean by understand. But I know a bit of that one. You can use it to model some of this stuff. But not predict. And I don’t know how to use it to suggest any — reactions on our part. I’m not sure it can be used that way.” He talked for a while about Vlad’s notion of holons, which were organic units that had subunits and also were subunits of greater holons, each level combining to create the next one up in emergent fashion, all the way up and down the great chain of being. Vlad had worked out mathematical descriptions of these emergences, which turned out to come in more than one kind, with different families of properties for each kind; so if they could get enough information about the behavior of a level of holons and the next level up, they could try to fit them into these mathematical formulae, and see what kind of emergence they had; then perhaps find ways to disrupt it. “That’s the best approach we can take for things this little.” “ The next day they called up greenhouses in Xanthe, to ask for shipments of new starts, and flats of a new strain of Himalayan-based grass. By the time they arrived, Nirgal had pulled out all the grass in the basin, and much of the moss. The work made him sick, he couldn’t help it; once, seeing a concerned marmot patriarch chattering at him, he sat down and burst into tears. Sax had retreated into his customary silence, which only made things worse, as it always reminded Nirgal of Simon, and of death generally. He needed Maya or some other courageous expressive speaker of the inner life, of anguish and fortitude; but here was Sax, lost in thoughts that seemed to happen in some kind of foreign language, in a private idiolect he was now unwilling to translate.

They went to work planting new starts of Himalayan grasses throughout the basin, concentrating on the stream banks and their veinlike tracery under the trickles and ice. A hard freeze actually helped, as it killed the infected plants faster than the ones free of infection. They incinerated the infected plants in a kiln down the massif. People came from the surrounding basins to help, bringing replacement starts for planting later.

Two months passed, and the invasion surge weakened.

The plants that remained seemed to be more resistant. Newly planted plants did not get infected or die. The basin looked like it was autumn, though it was midsummer; but the dying had stopped. The marmots looked thin, and more concerned than ever; they were a worrying species. And Nir-gal could see their point. The basin looked ravaged. But it seemed the biome would survive. The yiroid was subsiding, eventually they could hardiy even find it, no matter how hard and long they centrifuged samples. It seemed to have left the basin, as mysterious in departure as in arrival.

Sax shook his head. “If the viroids that infect animals ever get more robust…” He sighed. “I wish I could talk to Hi-roko about it.”

“I’ve heard them say she’s at the north pole,” Nirgal said sourly.

“Yes.”