Differences and similarities in the way marine and continental organisms occupy space are briefly reviewed. Among them, the `peninsula effect' (the decline of species richness with distance from the source) is compared with the `bay effect'. Two cases, corals in Mochima Bay, Venezuela and fishes in the Baltic Sea, are presented as examples. The facts that the world's oceans are larger, continuous and three-dimensional, with fewer evident geographical barriers than there are on land, explain why marine biogeographical regions are less well-defined and geographical ranges of marine taxa more wide-spread. This generalization has, however, been questioned following recent findings of extremely rich and highly endemic benthic faunas. This problem is discussed using an index of cosmopolitanism to compare terrestrial and marine biotas.