The Unified Neutral Theory of Biodiversity and Biogeography . Stephen P. Hubbell. Princeton University Press, Princeton, NJ, 2001. 389 pp. $75, £52. ISBN 0-691-02129-5. Paper, $29.95, £19.95. ISBN 0-691-02128-7. Hubbell presents a formal mathematical theory that attempts to combine two approaches to population biology, the study of species richness and relative abundance, and the study of the geographic distribution of species.