The late twentieth century has witnessed a remarkable growth in scientific interest in the subject of extinction. It is hardly a new subject--Baron Georges Cuvier had first demonstrated that species became extinct back in 1786, not long after the American Revolution. Thus the fact of extinction had been accepted by scientists for nearly three-quarters of a century before Darwin put forth his theory of evolution. And after Darwin, the many controversies that swirled around his theory did not often concern issues of extinction. On the contrary, extinction was generally considered as unremarkable as a car running out of gas. Extinction was simply proof of failure to adapt. How species adapted was intensely studied and fiercely debated. But the fact that some species failed was hardly given a second thought. What was there to say about it? However, beginning in the 1970s, two developments began to focus attention on extinction in a new way. The first was the recognition that human beings were now very numerous, and were altering the planet at a very rapid rate--eliminating traditional habitats, clearing the rain forest, polluting air and water, perhaps even changing global climate. In the process, many animal species were becoming extinct. Some scientists cried out in alarm; others were quietly uneasy. How fragile was the earth's ecosystem? Was the human species engaged in behavior that would eventually lead to its own extinction?