"Because fossils are just bones," Arby said.
"Right. And bones are not behavior. When you think about it, the fossil record is like a series of photographs: frozen moments from what is really a moving, ongoing reality. Looking at the fossil record is like thumbing through a family photo album. You know that the album isn't complete. You know life happens between the pictures. But you don't have any record of what happens in between, you only have the pictures. So you study them, and study them. And pretty soon, you begin to think of the album not as a series of moments, but as reality itself. And you begin to explain everything in terms of the album, and you forget the underlying reality.
"And the tendency," Malcolm said, "has been to think in terms of physical events. To assume that some external physical event caused the extinctions. A meteor hits the earth, and changes the weather. Or volcanoes erupt, and change the weather. Or a meteor causes the volcanoes to erupt and change the weather. Or vegetation changes, and species starve and become extinct. Or a new disease arises, and species become extinct. Or a new plant arises, and poisons all the dinosaurs. In every case, what is imagined is some external event. But what nobody imagines is that the animals themselves might have changed-not in their bones, but their behavior. Yet when you look at animals like these, and see how intricately their behavior is interrelated, you realize that a change in group behavior could easily lead to extinction."
"But why would group behavior change?" Thorne said. "If there wasn't some external catastrophe to force it, why should the behaviour change?"
"Actually," Malcolm said, "behavior is always changing, all the time. Our planet is a dynamic, active environment. Weather is changing. The land is changing. Continents drift. Oceans rise and fall. Mountains thrust up and erode away. All the organisms on the planet are constantly adapting to those changes. The best organisms are the ones that can adapt most rapidly. That's why it's hard to see how a catastrophe that produces a large change could cause extinction, since so much change is occurring all the time, anyway."
"In that case," Thorne said, "what causes extinction?"
"Certainly not rapid change alone," Malcolm said. "The facts tell us that clearly."
"What facts?"
"After every major environmental change, a wave of extinctions as usually followed-but not right away. Extinctions only occur thousands, or millions of years later. Take the last glaciation in North America. The glaciers descended, the climate changed severely, but animals didn't die. Only after the glaciers receded, when you'd think things would go back to normal, did lots of species become extinct. That's when giraffes and tigers and mammoths vanished on this continent. And that's the usual pattern. It's almost as if species are weakened by the major change, but die off later. It's a well-recognized phenomenon."
"It's called Softening Up the Beachhead," Levine said. "And what's the explanation for it?"
Levine was silent.
"There is none" Malcolm said. "It's a paleontological mystery. But I believe that complexity theory has a lot to tell us about it. Because if the notion of life at the edge of chaos is true, then major change pushes animals closer to the edge. It destabilizes all sorts of behavior. And when the environment goes back to normal, it's not really a return to normal. In evolutionary terms, it's another big change, and it's just too much to keep up with. I believe that new behavior in populations can emerge in unexpected ways, and I think I know why the dinosaurs - "
"What's that?" Thorne said.
Thorne was looking at the trees, and saw a single dinosaur hop out into view. It was rather slender, agile on its hind legs, balancing with a stiff tail. It was six feet tall, green-brown with dark-red stripes, like a tiger.
"That," Malcolm said, "is a velociraptor."
Thorne turned to Levine. "That's what chased you up in the tree? It looks ugly."
"Efficient," Levine said. "Those animals are brilliantly constructed killing machines. Arguably the most efficient predators in the history of the planet. The one that just stepped out will be the alpha animal. It leads the pack."
Thorne saw other movement beneath the trees. "There's more."
"Oh yes," Levine said. "This particular pack is very large." He picked up binoculars, and peered through them. "I'd like to locate their nest, he said. "I haven't been able to find it anywhere on the island. Of course they're secretive, but even so…"
The parasaurs were all crying loudly, moving closer to the apatosaur herd as they did so. But the big apatosaurs seemed relatively indifferent; the adults nearest the water actually turned their backs to the approaching raptor.