In Chapter 11 of the Origin, Darwin turns to one of the most compelling lines of evidence for his theory: the global distribution of organisms. This is the only subject in the book to which he devotes two chapters, a sign of the importance he placed on biogeography. Darwin saw geography not just as a backdrop for life, but as a key to understanding how species evolve and spread.
Naturalists of Darwin’s time were puzzled by the stark differences in faunas across the continents. Africa, South America, and Australia each possessed their own distinct groups of animals, unlike those in Europe and Asia, even when climates and habitats were similar. In a biblical framework, this was difficult to explain. German naturalist Johann Friedrich Blumenbach famously mocked the idea that sloths, which take an hour to crawl six feet, could have migrated all the way from Mount Ararat to South America after Noah’s flood.
Darwin offered a different lens. Instead of taking species distributions as given, he asked why and how these patterns arose. Why do similar climates on different continents support such different organisms? Why do island species resemble those on nearby mainlands, even when separated by hundreds of miles of ocean?
Darwin’s answer was rooted in evolution. He emphasized that barriers to migration, such as oceans, mountains, and deserts, shape the patterns we observe. Organisms evolve in isolation, and over time, even closely related species diverge. This explains why species within a continent are often more similar to each other than they are to organisms in other continents, regardless of climate. On his theory, the key is inheritance: shared ancestry explains biological similarity, and geographic separation explains difference.
Even in the ocean, where physical barriers are less obvious, Darwin found the same principle at work. The marine faunas of the east and west coasts of South America, for example, are strikingly distinct, with hardly a shell, fish, or crab in common. The Andes mountain range, which prevents faunal mixing across the land, appears to have a similar effect under the sea.
Darwin was especially interested in island species. High-altitude species in the Andes differ from those in adjacent lowlands, just as island organisms often differ subtly from their closest mainland relatives. These differences could not be explained by environmental conditions alone. For Darwin, they reflected a deeper bond: descent with modification from common ancestors, shaped by time and isolation.
He denied that evolutionary change results simply from isolation. There is no law of necessary development, he emphasized. Geographic separation alone does not cause change unless organisms also encounter new ecological pressures: Competition, predation, changes in climate. Evolution depends not just on space but on interaction.
A major theme of the chapter is Darwin’s rejection of the idea that similar species in distant regions were independently created. Instead, he argues that such distributions must result from dispersal. When the same species is found in widely separated places, this can often be explained by migration followed by survival in suitable habitats. Those who reject this explanation, he insists, are forced to invoke miracles.
Terrestrial mammals, Darwin notes, rarely have inexplicable distributions. Unlike birds or plants, they cannot easily cross oceans. Thus, we find no mammals naturally shared between Europe and Australia. If species were independently created in different places, why would mammals be absent from some continents altogether?
His conclusion is that each species originates in one area and spreads from there as far as its powers of movement and survival allow. Discontinuous distributions (where the same species appears in widely separated locations) can usually be explained by some form of dispersal, often aided by birds, ocean currents, or changing climate.
Darwin also engages with one of the biggest biogeographic debates of the 19th century: land bridges. Many naturalists believed that distant continents must once have been connected by now-submerged land masses, allowing species to spread before these bridges sank. Darwin remained skeptical, especially for island groups like Hawaii and the Galápagos, which were volcanic in origin and never part of continental landmasses. Until the mid-20th century, land bridges were widely accepted, but today their role has been superseded by the theory of plate tectonics.
Interestingly, Darwin anticipated this shift in thinking. He carried out pioneering experiments to test how seeds might survive dispersal across oceans. He found that out of 87 plant species, 64 germinated after being submerged in salt water for 28 days, and a few even after 137 days. From this, he calculated that seeds could survive transport by ocean currents for at least 900 miles.
Birds also play a role. Darwin conducted feeding experiments in which he forced seeds into the stomachs of fish, fed the fish to birds, and then examined whether seeds remained viable after digestion. He succeeded in growing plants from the excreted seeds, showing that birds could potentially spread plant species over distances of at least 500 miles.
He also emphasized how birds, especially migratory species, might carry seeds on their feathers or feet. On remote volcanic islands, Darwin suggested, nearly any seed that arrives and is suited to the climate could establish itself, especially in the absence of predators or competitors.
He extended these ideas to animal dispersal as well. Ocean currents, Darwin believed, had carried iguanas from mainland South America to the Galápagos, where they evolved into separate marine and land species. A more dramatic example came after Darwin’s time: the recolonization of Krakatoa following its cataclysmic eruption in 1883. Within 40 years, 60% of the original fauna had returned: Strong support for Darwin’s ideas about dispersal and colonization.
The final part of the chapter focuses on how glaciation shaped the distributions of plants and animals. Darwin discusses how species currently found in the Alps, Pyrenees, Siberia, and even North America were once part of a continuous flora that shifted south during the last Ice Age. As glaciers advanced, plants and animals migrated with the changing climate. When the ice retreated, some returned north, while others remained on isolated mountain peaks. This explains the presence of related species on distant but ecologically similar mountain ranges.
He closes with more examples of long-distance dispersal. Nearly fifty flowering plants in Tierra del Fuego are also found in Europe, despite the enormous distance separating the two regions. Many of these species also occur in North America, suggesting a clear migratory pathway. The same logic applies to plants in Africa, Australia, and New Zealand, where related forms occur thousands of miles apart. Darwin believed these patterns were best explained by a combination of long-range dispersal and global climate change during the Glacial Period.
The overall message is clear: geography does not merely describe where organisms are found. It provides evidence for how they got there. Darwin believed that distributions of plants and animals were not static or arbitrary, but the result of historical processes: Dispersal, isolation, environmental change, and descent with modification. Even without the benefit of modern genetics or plate tectonics, Darwin grasped that life’s patterns were dynamic, shaped by deep time and migration.