"We will end this century as a wholly urban species," writes Doug Saunders. For his book Arrival City, Saunders canvassed the slums, favelas and shantytowns of cities around the world. Instead of poverty and desperation, Saunders found an engine of upward social mobility. He takes us on a virtual tour of a village in China, and our own backyard, South LA. Is urban life good for people, and for the planet?