"After every major environmental change, a wave of extinctions has 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."