 Darwin returned to these issues in 1856, following a twelve-year period in which he published his Geological Observations on the Volcanic Islands (1844b), the second edition of his Journal of Researches (1845), Geological Observations on South America (1846), the four volumes on fossil and living barnacles (1851, 54, 55), and Geological Observations on Coral Reefs (1851). In addition, he had published several smaller papers on invertebrate zoology, on geology, and had reported on his experiments on the resistance of seeds to salt water, a topic that would be of importance in his explanation of the population of remote islands.