David L. Hull is Professor of Philosophy at Northwestern University. His publications include Darwin and His Critics (1983), The Metaphysics of Evolution (1989), and Science as a Process: An Evolutionary Account of the Social and Conceptual Development of Science (1991).Michael Ruse is Professor of Philosophy and Zoology at the University of Guelph. He is founder and editor of the journal Biology and Philosophy and on the editorial board of a number of scientific journals. His publications include The Philosophy of Biology (1989), The Darwinian Paradigm (1989), Evolution Naturalism (1994), and Monad to Man: The Concept of Progress in Evolutionary Biology (1996).