NOTICE: The project that is the subject of this report was ap proved by the Council of the National Academy of Sciences. The members of the committee responsible for the report were chosen for their special competences and with regard for appropriate balance.

Financial Support: The development of this report was supported by the Department of Energy, the John D. and Catherine T. MacArthur Foundation, and funds from the National Academy of Sciences and the National Research Council. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the organizations or agencies that provided support for the project.