In a process called diffusion, people of any one culture may copy from those of another rather than constantly "reinventing" ideas and technologies. They copy things as diverse as pots, plants, plans, books, and automobiles; the techniques for constructing or replicating such artifacts; institutions, whether ecclesiastical, managerial, ceremonial, military, or economic; and ideas or complexes of ideas, including religious beliefs and political ideologies. In the process they transform these things, sometimes beyond recognition. How important, then, are these diffusion processes in determining...