Biogeographical data comprise a largely neglected but potentially powerful tool for deciphering the tectonic evolution of the Phanerozoic Earth. This is true because the borders of biogeographical realms, regions, provinces, and subprovinces are natural barriers, some of them tectonic in origin. Yet most major biogeographical realm boundaries, based on floral and faunal distributions, do not coincide with the partly tectonic, partly computer-generated boundaries of plate tectonics.Instead, the paleontologic record shows that (1) a broad intercalary zone separates "northern" from "southern" biogeographical realms, and (2) this broad zone has existed during most, if not all, of Phanerozoic time. Within this intercalary zone, whose width ranges from several hundred to 5,000 km, strata bearing "northern" biotas are intercalated with strata bearing "southern" biotas and, in many areas, admixtures of "northern" and "southern" taxa are present within the same beds.During the Middle and Late Cambrian, the southern realm is the Atlantic Realm. (A globally low Early Cambrian climatic gradient does not permit easy definition of a southern realm.) Following the Cambrian, the two principal southern realms are the Malvinokaffric Realm (Ordovician-early Middle Devonian) and its successor, the Gondwana Realm (Early Permian-Early Cretaceous). The two are separated in time by the later Devonian, an interval of rather cosmopolitan biotas.