The degree of similarity, at the generic and familial levels, among Holocene terrestrial mammal faunas is strongly and negatively correlated with the geographic distance between them. Oceanic barriers and climatic differences also contribute significantly to the variation in faunal similarity, but distance alone explains as much as 56/ of the variation. This analysis justifies the use of the degree of similarity among fossil mammal faunas as an estimator of paleogeographic distance.
The distance between the centers of regions can be predicted by the biogeographic transfer function CENTER D = 18570 − 156 (SIMFAM), where distance is measured in kilometres and SIMFAM is the faunal similarity at the familial level, as measured by the Simpson Index. Maps of the Holocene world on which this equation is used to estimate distance display relatively minor deviations from actual geography and suggest that this method is a potentially powerful paleogeographic technique.