On the Origin of Species for Existence" Darwin includes "slow-breeding man" among other examples of Malthusian population growth. In his discussions on morphology, Darwin compares and comments on bone structures that are homologous between humans and other mammals. Darwin's early notebooks discussed how non-adaptive characteristics could be selected when animals or humans chose mates, with races of humans differing over ideas of beauty. In his 1856 notes responding to Robert Knox's "The Races of Man: A Fragment", he called this effect sexual selection. He added notes on sexual selection to his "big book on species", and in mid-1857 he added a section heading; "Theory applied
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