The American Experience series "Dinosaur Wars" brings you this letter from evolutionist Charles Darwin to paleontologist O.C. Marsh. In this letter, Darwin thanks Marsh for his validation of Darwin?s theory of evolution. In science, empirical evidence is necessary to gain acceptance of a hypothesis. Empirical evidence is evidence which justifies a belief. O.C. Marsh identified the Equus parvulus (now Protohippus), an early ancestor to the modern horse, as well as other animal fossils, giving credence to the theory of evolution.