Two themes ran through the previous chapters. One was that subsistence farmers are intelligent, well-trained, inventive and practical-minded, and have numerous tricks for increasing net crop production; the other that their farms provide very low yields and are subject to severe physical constraints, reflecting environments that are finite, risky and indifferent to farmers’ needs. Some readers may find these two themes contradictory: if subsistence farmers are so skillful, why can’t they overcome environmental limits as well as modern farmers do? The answer, explored in detail in this chapter, is that the limits are built into the basic biology of food production. In fact, they can only be overcome, even partially and temporarily, with the massive, costly and energetically inefficient inputs provided to modern farmers by the industrial sector.