The stratigraphic record is punctuated not by orogenic rhythms but by numerous brief episodes of mass extinction of organisms followed by invasions of new forms into vacated ecological zones. These events vary in scale from local to worldwide. They form a natural basis for classifying time-stratigraphic units and for correlating fossiliferous strata between distant points. Geological series and systems are comparable to chapters and books of printed history. They represent collections of related biological and physical episodes made more intelligible by grouping. The most conspicuous paleontological breaks lie at the top of the Devonian, Permian, Triassic, and Cretaceous systems. These and many lesser paleontological boundaries coincide with obscure paraconformities. This relationship suggests a universal physical control, such as eustatic changes in sea level. The boundaries of many paleontological zones are recognizable on 2 or more continents and, for all practical purposes, are the same age throughout their extent.