The notion that the world around us is continuously evolving is a platitude; we rarely grasp its full implications. We do not ordinarily think, for example, of an epidemic disease changing its character as the epidemic spreads. Nor do we think of evolution in plants and animals as occurring in a matter of days or weeks, though it does. And we do not ordinarily imagine the green world around us as a scene of constant, sophisticated chemical warfare, with plants producing pesticides in response to attack, and insects developing resistance. But that is what happens, too.

If we were to grasp the true nature of nature-if we could comprehend the real meaning of evolution-then we would envision a world in which every living plant, insect, and animal species is changing at every instant, in response to every other living plant, insect, and animal. Whole populations of organisms are rising and falling, shifting and changing. This restless and perpetual change, as inexorable and unstoppable as the waves and tides, implies a world in which all human actions necessarily have uncertain effects. The total system we call the biosphere is so complicated that we cannot know in advance the consequences of anything that we do.[1] That is why even our most enlightened past efforts have had undesirable outcomes-either because we did not understand enough, or because the ever-changing world responded to our actions in unexpected ways. From this standpoint, the history of environmental protection is as discouraging as the history of environmental pollution. Anyone who is willing to argue, for example, that the industrial policy of clear-cutting forests is more damaging than the ecological policy of fire suppression ignores the fact that both policies have been carried out with utter conviction, and both have altered the virgin forest irrevocably. Both provide ample evidence of the obstinate egotism that is a hallmark of human interaction with the environment. The fact that the biosphere responds unpredictably to our actions is not an argument for inaction. It is, however, a powerful argument for caution, and for adopting a tentative attitude toward all we believe, and all we do. Unfortunately, our species has demonstrated a striking lack of caution in the past. It is hard to imagine that we will behave differently in the future. We think we know what we are doing. We have always thought so. We never seem to acknowledge that we have been wrong in the past, and so might be wrong in the future. Instead, each generation writes off earlier errors as the result of bad thinking by less able minds-and then confidently embarks on fresh errors of its own.