Discussing geographical isolation is a helpful way to understand how natural selection produces new species. The voyage of the H.M.S. Beagle took Darwin to the Galápagos Islands, each of which housed its own distinct species of birds, mammals, and plants. This phenomenon alerted Darwin to the possibility that isolation of species from one another results in divergent development.

Continuing his research after the voyage, Darwin noticed the wider distribution of species on continents where migration occurs easily and the much smaller distribution of species found only in geographic isolation. Moreover, the unique species found on islands were most similar to related species found on the nearest mainland continent. All of these factors provided Darwin with evidence that geographical distribution and isolation greatly influence the processes of natural selection and the development of new species.

Darwin argues that all of the existing species in the world have descended from one or a few primary forms that originated in one part of the world. As these original organisms multiplied in subsequent generations, their descendents migrated throughout the world. The surface of the earth changed, due to the Ice Age and the subsequent period of rewarming, which raised water levels and cut off previously connected lands—and populations—from one another.

The geographical isolation of species from one another allowed different variations to be perpetuated in different species, depending on which variations proved advantageous in the environment in question. Moreover, the ease with which birds migrated to and from isolated lands also haphazardly spread species to isolated islands, as birds could carry from place to place seeds, plants, eggs, and food in their beaks, feet, and stomachs. The migration of various species to new, geographically isolated locales allowed for their proliferation in new places, but also their divergence from other species, as whatever traits were apparent in the few species that migrated would be perpetuated in the new population.