Observation that taxonomic groups of animals follow each other in time in a predictable manner.
Sequences of successive strata and their corresponding fauna have been matched to form a composite picture detailing the history of the Earth, especially from the beginning of the Cambrian Period . Faunal succession is the fundamental tool of stratigraphy and is the basis for the geologic time scale. Floral (plant) succession is also an important tool. Climate and conditions throughout the Earth's history can be studied using the successive groups because living organisms reflect their environment.