Everyone went to the window; we banged heads trying to look out in all directions. As far as I could see, the horizon was clear. I saw nothing at all. "Where are they?" I said. "Coming from the south. We have them on the monitors."

"How many?" Charley said.

"Four."

"Four!"

"Yeah, four."

The main building was south of us. There were no windows in the south wall of the shed.

David said, "We don't see anything. How fast are they coming?"

"Fast."

"Do we have time to run for it?"

"I don't think so."

David frowned. "He doesn't think so. Jesus."

And before I could say anything, David had bolted for the far door, opened it, and stepped out into the sunlight. Through the rectangle of the open door we saw him look to the south, shading his eyes with his hand. We all spoke at once:

"David!"

"David, what the fuck are you doing?"

"David, you asshole!"

"I'm trying to see…"

"Get back here!"

"You stupid bastard!"

But Brooks remained where he was, hands over his eyes. "I don't see anything yet," he said. "And I don't hear anything. Listen, I think maybe we can make a run for-uh, no we can't." He sprinted back inside, stumbled on the door frame, fell, scrambled to his feet, and slammed the door shut, pulled it tight behind him, tugging on the doorknob. "Where are they?"

"Coming," he said. "They're coming." His voice shook with tension. "Oh Jesus, they're coming." He pulled back on the doorknob with both hands, using his whole body weight. He muttered over and over, "Coming… they're coming…"

"Oh great," Charley said. "The fucking guy's cracked."

I went over to David, and put my hand on his shoulder. He was pulling on the doorknob, breathing in ragged gasps. "David," I said quietly. "Let's take it easy now. Let's take a deep breath."

"I just-I have to keep-have to keep them-" He was sweating, his whole body tense, his shoulder shaking under my hand. It was pure panic.

"David," I said. "Let's take a deep breath, okay?"

"I have to-have to-have-have-have-"

"Big breath, David…" I took one, demonstrating. "That feels better. Come on now. Big breath…"

David was nodding, trying to hear me. He took a short breath. Then resumed his quick gasps.

"That's good, David, now another one…"

Another breath. His breathing slowed slightly. He stopped shaking.

"Okay, David, that's good…"

Behind me, Charley said, "I always knew that guy was fucked up. Look at him, talking to him like a fucking baby."

I glanced back, and shot Charley a look. He just shrugged. "Hey, I'm fucking right."

Mae said, "It's not helping, Charley."

"Fuck helping."

Rosie said, "Charley, just shut up for a while, okay?"

I turned back to David. I kept my voice even. "All right, David… That's good, breathe… okay now, let go of the doorknob."

David shook his head, refusing, but he seemed confused now, uncertain of what he was doing. He blinked his eyes rapidly. It was as if he was coming out of a trance.

I said softly, "Let go of the doorknob. It's not doing any good."

Finally, he let go, and sat back on the ground. He began to cry, head in hands.

"Oh Jesus," Charley said. "That's all we need."

"Shut up, Charley."

Rosie went to the refrigerator and came back with a bottle of water. She gave it to David, who drank as he cried. She helped him to his feet, nodded to me that she'd take it from here. I went back to the center of the room, where the others were standing by the workstation screen. On the screen, the lines of code had been replaced by a monitor view of the north face of the main building. Four swarms were there, glinting silver as they moved up and down the length of the building.

"What're they doing?" I said.

"Trying to get in."

I said, "Why do they do that?"

"We're not sure," Mae said.

We watched for a moment in silence. Once again I was struck by the purposefulness of their behavior. They reminded me of bears trying to break into a trailer to get food. They paused at every doorway and closed window, hovering there, moving up and down along the seals, until finally moving on to the next opening.

I said, "And do they always try the doors like that?"

"Yes. Why?"

"Because it looks like they don't remember that the doors are sealed."

"No," Charley said. "They don't remember."

"Because they don't have enough memory?"

"Either that," he said, "or this is another generation."

"You mean these are new swarms since noon?"

"Yes."

I looked at my watch. "There's a new generation every three hours?"

Charley shrugged. "I couldn't say. We never found where they reproduce. I'm just guessing." The possibility that new generations were coming that fast meant that whatever evolutionary mechanism was built into the code was progressing fast, too. Ordinarily, genetic algorithms-which modeled reproduction to arrive at solutions-ordinarily, they ran between 500 and 5,000 generations to arrive at an optimization. If these swarms were reproducing every three hours, it meant they had turned over something like 100 generations in the last two weeks. And with 100 generations, the behavior would be much sharper.

Mae watched them on the monitor and said, "At least they're staying by the main building. It seems like they don't know we're here."

"How would they know?" I said.

"They wouldn't," Charley said. "Their main sensory modality is vision. They may have picked up a little auditory over the generations, but it's still primarily vision. If they don't see it, it doesn't exist for them."

Rosie came over with David. He said, "I'm really sorry, guys."

"No problem."

"It's okay, David."

"I don't know what happened. I just couldn't stand it."

Charley said, "Don't worry, David. We understand. You're a psycho and you cracked. We get the picture. No problem."

Rosie put her arm around David, who blew his nose loudly. She stared at the monitor. "What's happening with them now?" Rosie said.

"They don't seem to know we're here."

"Okay…"

"We're hoping it stays that way."

"Uh-huh. And if it doesn't?" Rosie said.

I had been thinking about that. "If it doesn't, we rely on the holes in the PREDPREY assumptions. We exploit the weaknesses in the programming."

"Which means?"

"We flock," I said.

Charley gave a horse laugh. "Yeah, right, we flock-and pray like hell!"

"I'm serious," I said.

Over the last thirty years, scientists had studied predator-prey interactions in everything from the lion to the hyena to the warrior ant. There was now a much better understanding of how prey defended themselves. Animals like zebras and caribou didn't live in herds because they were sociable; herding was a defense against predation. Large numbers of animals provided increased vigilance. And attacking predators were often confused when the herd fled in all directions. Sometimes they literally stopped cold. Show a predator too many moving targets and it often chased none.

The same thing was true of flocking birds and schooling fish-those coordinated group movements made it harder for predators to pick out a single individual. Predators were drawn to attack an animal that was distinctive in some way. That was one reason why they attacked infants so often-not only because they were easier prey, but because they looked different. In the same way, predators killed more males than females because nondominant males tended to hang on the outskirts of the herd, where they were more noticeable. In fact, thirty years ago when Hans Kruuk studied hyenas in the Serengeti, he found that putting paint on an animal guaranteed it would be killed in the next attack. That was the power of difference.