Answer:
Darwin used fossils as pieces of evidence in order to develop the theory of natural selection.
He observed that the fossils of simple organisms are found to be present in much older rocks, whereas fossils of more complex organisms are obtained from the geologically younger rocks. This observation of fossils made him understand how the evolutionary process takes place in due course of time. This evolution takes place due to the natural selection method, where an organism is adapted to a particularly suitable environment.
Thus, the discoveries of various fossils helped in developing the natural selection theory.