Some people think even greater surprises may await us. "A leading British ethno-biologist," wrote the Economist in 1995, "thinks a megatherium, a sort of giant ground sloth which may stand as high as a giraffe ... may lurk in the fastnesses of the Amazon basin." Perhaps significantly, the ethnobiologist wasn't named; perhaps even more significantly, nothing more has been heard of him or his giant sloth. No one, however, can categorically say that no such thing is there until every jungly glade has been investigated, and we are a long way from achieving that.

But even if we groomed thousands of fieldworkers and dispatched them to the farthest corners of the world, it would not be effort enough, for wherever life can be, it is. Life's extraordinary fecundity is amazing, even gratifying, but also problematic. To survey it all, you would have to turn over every rock, sift through the litter on every forest floor, sieve unimaginable quantities of sand and dirt, climb into every forest canopy, and devise much more efficient ways to examine the seas. Even then you would overlook whole ecosystems. In the 1980s, spelunkers entered a deep cave in Romania that had been sealed off from the outside world for a long but unknown period and found thirty-three species of insects and other small creatures — spiders, centipedes, lice — all blind, colorless, and new to science.