The discipline of biogeography, as has been emphasized repeatedly, was developed because the same species are not found everywhere. Different regions do not have the same complement of species and, moreover, the climate alone is not sufficient explanation for why species are found where they are. As part of the discussion of biogeography and paleobiogeography, several other topics were introduced and discussed, including the role that vicariance and allopatric speciation play in influencing biogeographic patterns: they produce closely related species separated by geographic barriers. The role of geodispersal as a mechanism that alters the distribution of species by moving them from one region to another was also discussed, and it has been argued herein that this is an important biogeographic process that will lead to patterns of different species occurring in different regions or areas of the globe. Thus far, however, in this book the terms regions and areas have been treated in a fairly nebulous fashion, connoting something to do with geography and position on the Earth’s continents or in the Earth’s oceans. However, as biogeography is the study of how life is distributed on Earth, a more rigorous definition of regions and areas is required.