But Raven is equally himself with the bigger picture, which accounts for his affection for Darwin's last paragraph. In his office in the late afternoon, he returns to the world outside his garden. "Individuals express themselves through others," he says. "They influence other people, they share values, and that way an individual's life goes on, generation after generation.

"The earth is the only place we know in the universe with living organisms; thus it is special. With 6 billion people on the planet, it becomes harder and harder to measure the effects of individuals, yet the outcome is formed precisely by the interaction of individuals. All that adds up to the ongoing stream of humanity and human accomplishment. We have relatively short lives, and yet by preserving the world in a condition that is worthy of us, we win a kind of immortality. We become stewards of what the world is."

"Gardeners?" I ask him.

"Gardeners," he says