Carl Linnaeus, a pious Swedish doctor with a huckster's flair, believed that life belonged in tidy, static categories. Georges-Louis de Buffon, an aristocratic polymath and keeper of France's royal garden, viewed life as a dynamic, ever-changing swirl of complexities. Both began believing their work to be difficult, but not impossible--how could the planet possibly hold more than a few thousand species? Stunned by life's diversity, both fell far short of their goal. But in the process they articulated starkly divergent views on nature, on humanity's role in shaping the fate of our planet, and on humanity itself.