In Chapter 12, Darwin continues his exploration of species distributions by turning to two special cases that challenge and refine his broader claims: the global spread of freshwater organisms and the unique biological communities found on oceanic islands. What at first might seem like a minor continuation becomes, by the end of the chapter, a powerful defense of common descent and a striking critique of special creation.
Darwin begins with a puzzle. Isolated land masses like the Galápagos or New Zealand are famous for their endemic species, organisms found nowhere else. But freshwater systems, which are also isolated from one another by land or salt water, show the opposite pattern. Many freshwater organisms (fish excluded) are distributed across vast regions, even appearing on different continents. Why do river and lake species show such global ranges when terrestrial species do not?
To answer this, Darwin proposes that many freshwater organisms have evolved remarkable adaptations for short, frequent migrations from one body of water to another. He recalls his own surprise when collecting freshwater insects and mollusks in Brazil, only to find that they were very similar to forms he had previously found in England. These similarities, he argues, cannot be explained by climate or environment. Instead, they point to effective dispersal mechanisms.
Darwin describes several ways that freshwater species can travel. Some fish tolerate salt water for long enough to cross narrow ocean channels. More importantly, aquatic birds such as ducks and wading birds frequently carry larval stages of aquatic invertebrates on their feet and feathers. Darwin tested this experimentally and found that newly hatched freshwater mollusks could survive up to 24 hours while attached to a duck’s foot: Long enough, he calculated, for the birds to travel 700 miles.
Birds are also important dispersers of seeds. In one of his charmingly detailed experiments, Darwin collected mud from the edge of a pond and planted it. From just three tablespoons, he grew 537 individual plants. Seeds, he showed, can pass through the digestive systems of birds unharmed, and some survive long flights in the mud stuck to feet or feathers. Pond mud, in effect, serves as a vehicle of botanical migration. Nature, Darwin writes:
Like a careful gardener, takes her seeds from a bed of a particular nature and drops them in another equally well fitted for them.
From freshwater systems, Darwin moves to oceanic islands: His real focus. He lays out a series of questions. Why are island biotas often impoverished, with whole groups of organisms absent? Why are some types, like reptiles or flightless birds, overrepresented? Why are so many island species endemic? Why do some herbaceous plants evolve into trees?
Darwin’s answers to these questions begin with colonization. Islands are difficult to reach, and only certain types of organisms are able to make the journey. For example, the Galápagos have many land birds that are endemic, but only two of the eleven marine bird species are. The reason is simple: marine birds can travel between islands much more easily. Likewise, herbaceous plants are more likely to colonize than trees. But once on an island, natural selection may favor increased stature. The result: herbaceous plants evolve into shrubs or trees, as seen in the Galápagos genus “Scalesia", which includes both bush and tree forms, all descended from an ancestral daisy.
Darwin also reflects on the conspicuous absence of certain groups. Frogs, toads, and newts are almost entirely absent from remote oceanic islands. Yet, when introduced by humans, they often thrive and become pests. Since their eggs are killed by saltwater, Darwin argues they would have faced enormous barriers to natural colonization. If they could not get there, they could not evolve there. The same reasoning applies to terrestrial mammals. Remote islands over 300 miles from continental land masses lack native land mammals, with rare exceptions, such as the wolf-like fox of the Falklands, which may have arrived via ice during the last glacial period. In contrast, bats, which can fly over water, are commonly found on oceanic islands.
Land snails present another striking case. They are extraordinarily common on islands and often radiate into numerous species, as in the Galápagos (with 70+ species) and Hawaii (750 species). Yet land snails are easily killed by saltwater. Darwin tested this experimentally and found that snails in hibernation, sealing themselves in with a hardened mucus layer, could survive saltwater immersion for up to 20 days. This is long enough to raft across oceans on vegetation or debris. Modern studies show that small juvenile snails can also be lifted by air currents, dispersing them vast distances.
But perhaps the most striking observation Darwin makes is about the identity of island species. The most important and striking fact, he writes, is that island species are almost always closely related to species on the nearest mainland, even though they are not the same. The Galápagos mockingbirds, finches, and land birds are all uniquely Galápagan, but they bear the clear imprint of their American origin. The same is true for the Cape Verde Islands, whose flora and fauna resemble those of Africa, not South America, even though the environments may be similar.
This pattern, Darwin insists, cannot be explained by independent creation. If species were specially created for their environments, we would expect the Galápagos and Cape Verde islands, so similar in climate, to host similar species. Instead, what matters is geography, not climate. The colonists that reached each archipelago came from different continents, and it is their ancestry, not the local conditions, that defines the character of the local fauna.
Darwin concludes with a discussion of geographic speciation. The Galápagos mockingbirds are again his case study. Each major island has its own endemic mockingbird species, despite the fact that the birds can easily fly between islands. Why haven’t they merged into one population? Darwin believes the answer lies in competition. When a species is introduced into an island already occupied by a closely related species, it will often lose out to the better-adapted resident. This competitive exclusion helps maintain species boundaries, even in the absence of strong physical barriers.
All of this leads Darwin to a final conclusion. There is no need to assume that the same species originated independently in multiple places. A much simpler explanation is that all individuals of a species descend from a common ancestor. Dispersal, adaptation, and local extinction shape the distributions we see today. And while barriers to migration can seem like accidents of geography, Darwin argues they are essential to understanding the evolutionary history of life.
What makes his theory so powerful, he says, is its ability to explain facts that special creation cannot. Whether we study changes over time in the fossil record or changes across space in the modern world, the bond between closely related forms is always the same: inheritance. As Darwin puts it:
Modifications have been accumulated by the same power of natural selection.