Darwin examines the role of geographical isolation and migration in the development of species in particular environments. He begins by observing that the similarity and dissimilarity of organisms in particular locales has little to do with the climate of the region. Similar climates exist all over the world, but these climates support vastly different species. Barriers to species migration affect their development in important ways. Separated land masses (large, continuous areas of land, such as the continents of Australia, Africa, and South America) support extremely different species, as do separated bodies of water. In a location in which migration is impossible, species develop according to the conditions of that particular environment and become highly distinct from species in other locations. Within a continent or sea, species show relationships (or affinities) to one another. In places where migration is possible, the introduction of new species shapes natural selection just as much as the environment does.
The divergent development of species based on geographical separation leads to an important question: Were species created at one point on the earth’s surface, or did they appear simultaneously at many different points? Darwin believes that each individual species formed in one particular location and then spread to other parts of the world through migration. For some species, such as land-living animals, migration is difficult. Those species may not spread as far as a result. However, many ways exist for plants to migrate from location to location, even given geographical barriers such as bodies of water. Some plant seeds can survive in seawater and float from one continent to another. Birds may also carry seeds in their beak, feet, or stomach (to be expelled later) as they fly from continent to continent. Even icebergs may transport seeds to different locations.
Geographical changes have played a role in both limiting and allowing for species migration. Darwin does not believe, as some geologists do, that islands were land masses that broke off of the continents. Even if this were true, the large geographic changes could not have occurred within a short enough time to affect living species. However, as the level of the land changed due to the oscillation of sea levels, water barriers gradually appeared and disappeared. These changes created periods of geographic isolation, during which new species were able to develop, and periods of geographical connection, during which migration allowed the new species to spread throughout other regions.
To illustrate the role of geological change in species development, Darwin points out two large geological changes that explain some patterns of species dispersal. First, the Glacial period (commonly known as the Ice Age) created a uniform climate in which only certain species could survive. At the end of that period, the remaining species had to migrate to mountaintops to find a climate cold enough for them to survive. This migration accounts for the similarity of species on mountain peaks throughout the world. Second, a connection between lands stretching from Western Europe through Siberia to eastern America—a supposed land bridge now covered by the Bering Strait—would have allowed species to migrate by land from Europe and Asia to North and South America, before rising waters cut off the connection between these lands. Darwin hypothesizes that ice melting after the Ice Age increased the ocean’s water level and disconnected these lands from one another, creating a geographical separation of species on these continents that allowed the species to modify separately and diverge. This hypothesis explains how analogous species appear in many different parts of the world, without any apparent means of migration. These geological changes also explain how natural selection occurs in isolated environments, creating the divergent development of species in different locales.
Darwin continues his discussion of the geographical distribution of species by considering how similar species can exist in different freshwater environments that are geographically isolated from one another. Obviously, water-bound species cannot migrate from pond to pond with ease. As a result, a species of fish cannot exist in the freshwater environs of two separate continents. However, oftentimes the same fish species may be found in freshwater ponds of the same continent. Darwin hypothesizes that changes in land levels allowed rivers to flow into one another at certain points, and the migration of fish ceased when the bodies of water were fully separated. Freshwater species also may have been able to migrate when ducks carried plants and shells from one pond to another. Birds, Darwin observes, may also eat seed-eating freshwater fish and expel the seeds in their excrement at a different location.
Darwin addresses the issue of species formation on oceanic islands, which are geographically isolated. Darwin argues that the ability to migrate has shaped which species are found on islands, noting that islands hold fewer species than larger continents. However, islands hold more “endemic species”—species that are found only on that island and nowhere else in the world. Similarly, species that are better able to migrate, such as birds, show less adaptation than species that are unable to migrate. This observation suggests the capacity for migration is just as important as environmental conditions in the formation of adapted species. For example, plants that bear hooked seeds are more widespread across the world than are plants with non-hooked seeds. The hook of the seeds allows them to attach to a bird better, making the seeds more likely to be carried to another area by migration.