Charles Darwin is famous for the theory of evolution by means of natural selection. The theory hinges on the mutability of species, whereby the propagation of certain favorable traits within members of a species may gradually result in the evolution of that species. The question of when Darwin first came to believe in the mutability of species – when he became a “convinced transmutationist” – has long been a point of contention among historians of science. There are two prevailing theories on the topic. The early conversion hypothesis states that Darwin developed a belief in the transmutation of species while on the Beagle voyage based on observed similarities between the fossils he was collecting and extant species in the area.