“And if they got tired of being agriculturalists, if they found they didn’t like where it was leading them in their particular adaptation, they were able to give it up. They didn’t say to themselves, ‘Well, we’ve got to keep going at this even if it kills us, because this is the right way to live.’ For example, there was once a people who constructed a vast network of irrigation canals in order to farm the deserts of what is now southeastern Arizona. They maintained these canals for three thousand years and built a fairly advanced civilization, but in the end they were free to say, ‘This is a
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