CONGRUENCE BETWEEN TAXONOMIC AND BIOGEOGRAPHIC HIERARCHIES. A convincing line of evidence is the correspondence between the hierarchy of taxonomic categories and the hierarchy of biogeographic regions, with the clearest patterns occurring at the largest scales. Thus, higher taxa (orders and families) tend to have fairly broad distributions within a continental landmass, presumably reflecting the ancient origin and long confinement of major lineages, while progressively less differentiated forms (genera and species) tend to be con fined to small areas nested within those regions. As pointed out earlier, nested patterns are required by the hierarchical classifications of both organisms and biogeographic regions, so this apparent pattern could be an artifact if based merely on taxonomic classifications. Nevertheless, the relationships between the two hierarchies suggest that the progressively finer biogeographic subdi visions reflect a history of less ancient and less severe barriers between more recently differentiated lineages (more on this in Chapter 12)