What does the principle of faunal succession state

The principle of faunal succession states that fossil, animal and plant groups occur in the geological record according to an invariable and determined order, so that, if this order is known, it is possible to determine the relative age of the layers from their fossiliferous content. That is, according to this principle, fossil is equal to time. This principle was initially used as a practical instrument, but years later was explained by Darwin's theory of evolution: Since there is an irreversible biological evolution across geological time, fossils must be ordered in time on an evolutionary scale.