Data from a situation of rapid agricultural change in Costa Rica illustrate the mechanism of agricultural evolution. Figures of the efficiency of labor using intensive and extensive agricultural methods support Boserup's law of least effort, but disagree with Bronson's approach. In a period of population growth, declining soil fertility lowers both yields and the productivity of labor in traditional swidden fields. A new system involving dry terracing and a quadrupling of labor investment is shown to be a more efficient use of labor for some segments of the community. This reversal of labor efficiencies leads farmers to choose the intensive methods and is the mechanism by which agricultural evolution takes place.