Keith Thomson is professor emeritus of natural history@the University of Oxford and senior research fellow of the American Philosophical Society. His recent books include The Young Charles Darwin (Yale University Press, 2009) and A Passion for Nature: Thomas Jefferson and Natural History (Thomas Jefferson Memorial Foundation and University of North Carolina Press, 2008). Address: American Philosophical Society, 105 South Fifth Street, Philadelphia, PA 19106. E-mail: kthomson@amphilsoc.org