On the Origin of Species the second edition, Darwin added an epigraph from Joseph Butler affirming that God could work through scientific laws as much as through miracles, in a nod to the religious concerns of his oldest friends. The "Introduction" establishes Darwin's credentials as a naturalist and author, then refers to John Herschel's letter suggesting that the origin of species "would be found to be a natural in contradistinction to a miraculous process": WHEN on board HMS "Beagle", as naturalist, I was much struck with certain facts in the distribution of the inhabitants of South America, and in the geological relations of the present
On the Origin of Species this was a common pattern. Darwin discussed ways that species could be dispersed across oceans to colonise islands, many of which he had investigated experimentally. Chapter XII continues the discussion of biogeography. After a brief discussion of freshwater species, it returns to oceanic islands and their peculiarities; for example on some islands roles played by mammals on continents were played by other animals such as flightless birds or reptiles. The summary of both chapters says: ... I think all the grand leading facts of geographical distribution are explicable on the theory of migration (generally of the more dominant forms of