In this chapter (and the one that follows), Darwin discusses specific facts and causes of geographical distribution and attempts to explain this distribution via a speculative history in accordance with the principles of natural selection. First, he notes the importance of physical barriers in blocking the migration of species. The isthmus of Panama is an example: Although only a narrow strip of land separates it, the marine life to the east and the west of the Panama isthmus differs radically. Some plants, unlike mammals, he writes, have migrated across the vast distances of the sea, which explains their often remarkable geographical distribution. Proponents of independent creation theory would attribute this to separate acts of spontaneous creation. Countering this, Darwin writes, “[T]he view of each species having been produced in one area alone and having subsequently migrated from that area as far as its powers of migration and subsistence under past and present conditions permitted, is the most probable” (809). This view can account for the close relationships between species in different parts of the world, something that the competing theory can’t explain.

In a subsection dedicated to the means of wide dispersal of plant life, Darwin describes an experiment in which he puts seeds out to sea and mentions the migratory pattern of birds (that could carry the seeds), and the possibility of drift timber bringing life ashore islands.