Biogeography may be defined simply as the study of the geographical distribution of organisms, but this simple defmition hides the great complexity of the subject. Biogeography transcends classical subject areas and involves a range of scientific disciplines that includes geogra phy, geology and biology. Not surprisingly, therefore, it means rather different things to different people. Historically, the study of biogeogra phy has been concentrated into compartments at separate points along a spatio-temporal gradient. At one end of the gradient, ecological biogeography is concerned with...