Peter H. Raven, Ph.D., is director of the Missouri Botanical Garden and Engelmann professor of botany at Washington University at St. Louis. He oversees the garden's internationally recognized research program in tropical botany--one of the world's most active in the study and conservation of imperiled tropical habitats. Raven's botanical research and work in the area of tropical conservation have earned him numerous honors and awards, including a MacArthur Fellowship. He has written 17 textbooks and more than 400 articles, and he is a member of th National Academy of Science and the National Research Council. George B. Johnson, Ph.D., is a professor of biology at Washington University in St. Louis and a professor of genetics at the university's School of Medicine. He is a prolific author of life science texts and curriculum products in a variety of media. New to his list of works are the Explorations of Human Biology CD-ROM and the textbook Human Biology, both offered by Wm. C. Brown Publishers. Johnson is acknowledged as an authority on population genetics and evolution variability, and he has published more than 50 research papers dealing with these and related topics. Visitors to the St. Louis Zoo can appreciate Johnson's work in the Living World, the educational center of which he is the founding director.