Geographical barriers can also hinder species migration. For example, some seeds die in salt water, making their migration across oceans difficult. Darwin notes, however, that perhaps these geographical barriers did not always exist. Islands separated from large continents by narrow channels support species similar to those that exist on the nearby continent. It is possible, then, that at some point those narrow channels did not exist, and species were able to move freely between those lands until rising water levels created a geographical barrier.
Most important, Darwin notes that the species of any island are always most similar to the species of the nearest mainland—even if their climates are dissimilar, such as in the case of the Galapagos Islands and South America. Migration is more likely when bodies of land are closer to one another. Seeds and animals are likely to be transported from, for example, South America to the nearby Galapagos Islands. Islands with similar climates, such as the Galapagos and Cape de Verde archipelagos, can support extremely different species. If species cannot easily migrate between two islands, even those with similar climates, the islands may not exhibit the same species. It is not climate that dictates the existence of species, but rather the species’ ability to migrate from one location to another.
Geographical isolation is crucial to Darwin’s theory of natural selection. It provides a mechanism for species to diverge and produce different variations that allow survival in various environments. Not surprisingly, geographical isolation holds a specific importance for Darwin, as it was his research in the Galapagos Islands aboard the H.M.S. Beagle that sparked his theorizing on the origin and development of species. Darwin theorized that the geographical isolation of the Galapagos led to the development of its unique, yet perfectly adapted species—some of which were similar to those species found on other continents, and others of which had never been seen before.
Darwin again draws on Lyell’s theory of gradual change in geological formations on the earth’s surface to explain both the migration possibilities of the past and the geographical barriers that exist today, accounting for both the affinities and divergences in existing species. Lyell’s theory explains how existing geographical barriers seem to make migration impossible. If seas did not exist in the past where they do now, for example, it’s possible that now-disconnected lands were once connected, allowing animals to pass freely between them. In addition, Lyell’s claim that climatic changes play a role in geological change, such as those that occurred during the Ice Age, led Darwin to conclude that climatic change also plays a role in the evolution of species. Climatic changes help reveal times during which species throughout the world were the same. Only those species that could adapt to the brutal cold of the Ice Age, for example, were able to survive. As the climate warmed and the ice melted, surviving species dispersed, which explains the affinity of species in different parts of the world.
Darwin suggests that even though they have led to the wondrous existence and perfect adaptation of the species we see today, both migration and survival are matters of chance. Although he offers many hypotheses to explain how species migrate and disperse, many of the migrations and dispersals seem to be random occurrences. For example, Darwin suggests that a seed might stick to a bird’s feet and remain there while the bird migrates to a different continent. Also, the species that managed to cross between lands before water barriers formed were those that were able to establish themselves in their new environment. It remains uncertain, however, whether a migratory species will always survive in its new environment. Seeds may be transported to places that lack the soil that will allow them to grow. Animals may migrate to a new area, only to become the prey of other animals. Climates may be too cold or too warm for migrated species to survive.