In October 1980, Stanford University biologist Ehrlich and University of Maryland economist Julian Simon drew up a futures contract obligating Simon to sell Ehrlich the same quantities that could be purchased for $1,000 of five metals (copper, chromium, nickel, tin, and tungsten) 10 years later at 1980 prices. If the combined prices rose above $1,000, Simon would pay the difference. If they fell below $1,000, Ehrlich would pay Simon the difference. In October 1990, Ehrlich mailed Simon a check for $576.07. Simon won the bet. Environmental historian Paul Sabin uses this famous wager to frame 40 years of rancorous debate over humanity's impact on the planet and the prospects for human flourishing in his intriguing new book, The Bet: Paul Ehrlich, Julian Simon, and Our Gamble over Earth's Future. In his review, Reason Science Correspondent Ronald Bailey picks a side.