Geographical distributions of terrestrial or aquatic taxa that are broken up by oceans can be explained by either oceanic dispersal or vicariance in the form of fragmentation of a previously contiguous landmass. Oceanic dispersal may be the most important factor in the distributions of animals. Present distribution patterns of animals are largely a result of shifting landmasses. Distribution of animals is limited by various environmental conditions which the most restrictive barriers are the separation of the land by water, high mountain ranges and deserts. Extinction is a gradual process...