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Chapter 12
Themes
Key
LitCharts assigns a color and icon to each theme in The Origin of Species, which you can use to track the themes throughout the work.
Natural Selection and the Power of Nature
Reason, Argument, and the Scientific Method
Time and Progress
Collaboration and Science
Summary
Analysis
Fresh-water Productions. Though the world’s freshwater habitats (lakes and rivers) seem to be well separated from each other, they have a surprising similarity in species. In one case, a freshwater species from New Zealand was found in mainland South America, yet it’s also possible for the first populations in adjacent streams to differ.
Darwin notes why freshwater habitats may seem difficult to explain under natural selection. It might seem impossible, for example, that fish could migrate from one landlocked lake to another different one. As Darwin shows, however, conditions in the past—and particularly freshwater habitats—were not static.
Themes
Accidental transportation (such as floods and whirlwinds) likely played some role in dispersing freshwater fish, and so too have changes in the flow of rivers, as some diverge over time or run together. Some mollusks have been observed to cling to a duck’s feet and are transported between freshwater habitats that way, and this seems to have been the most common way for many freshwater plants and small animals to be dispersed.
Darwin details some of the ways that seemingly isolated freshwater sources could have allowed for migration between them. On the other hand, his theories also help explain how nearby freshwater environments that seem similar can have very different populations due to a past divergence.
Themes
On the Inhabitants of Oceanic Islands. Fewer species inhabit oceanic islands than larger continental landmasses, yet they are more likely to contain endemic species that are unique to the area. Some islands are unique for their lack of certain species, like how the Galapagos Islands lack reptiles.
Darwin moves on to the topic of oceanic islands. After his extensive studies in the Galapagos Islands, this is a topic that Darwin understands well. Because of their isolation, islands often represent an extreme example of how natural selection develops when there are barriers to migration.
Themes
Absence of Batrachians and Terrestrial mammals on Oceanic Islands. One naturalist noted that “Batrachians” (frogs, toads, and newts) were never found on islands. Darwin found this mostly true, with a handful of exceptions, although most of these exceptions are on islands closer to land and not strictly oceanic islands.
Per the scientific method, Darwin considers the available evidence before putting forward a hypothesis. In this case, Darwin considers another naturalist’s claims about certain amphibians being absent from islands. Darwin finds that the rule is broadly true while also having exceptions—and he considers both of these factors as he builds toward an explanation.
Themes
Literary Devices
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Terrestrial mammals, too, seemed to be conspicuously absent from oceanic islands (though in some cases they were domesticated by indigenous inhabitants). Again, the handful of possible exceptions to the rule are from islands closer to land. Non-terrestrial mammals, like bats, are also found on many oceanic islands.
Darwin explores how there are broad rules about what sort of life is found on islands, but that these rules also tend to have important exceptions. Both rules and exceptions are important when forming a comprehensive scientific theory, as Darwin is attempting to do.
Themes
Some naturalists also noted that the depth of sea between an island and another island or the nearest continent impacts the similarity between species. While Darwin didn’t have time to investigate the matter fully, in his experience, he found that shallower seas between islands resulted in more similarity in species. Darwin admitted it was hard to explain the species composition of some of the most remote islands, but he noted that some small species could have been carried between islands on the feet of flying birds, connecting islands to a greater extent than it would first seem.
As he has in the past, Darwin expresses humility and is careful not to extend his argument too far into matters he doesn’t understand. Nevertheless, these expressions of humility are often followed by a renewed emphasis on what Darwin can conclude from the available evidence. Here, Darwin’s argument about how seemingly remote islands could be connected is a logical extension of his earlier argument about how freshwater environments could be connected.
Themes
On the Relations of the Inhabitants of Islands to those of the nearest Mainland. Darwin found it striking how inhabitants of islands near mainlands often resembled the inhabitants of the mainland while also being quite different. He drew in particular on his experience observing birds in the Galapagos. He concluded that these endemic populations on islands must be related to populations on the nearest continent (or perhaps just a large island).
The birds on the Galapagos are closely associated with Darwin and often credited with helping him reach his epiphany about natural selection in the first place. Islands and other places with endemic populations provide a particularly good environment in which to study natural selection because of the barriers to migration. Darwin learned a lot by comparing endemic species to other species that were dispersed more widely.
Themes
What surprised Darwin in the Galapagos and elsewhere was that new species formed on one island did not spread quickly to others. He did find, however, that “lower” organisms (i.e., ones that were generally smaller and less complex) ranged more widely than “higher” ones. Ultimately, he found it unlikely that species were independently created on each island, believing instead that most islands were at some point colonized by a population from a larger landmass, where the isolated population on the island subsequently developed new adaptations.
Elsewhere in the book, Darwin notes the surprising variety of life he found in the Galapagos. He took this as evidence that there was not much migration happening between the islands and that species had adapted to their own specific environment. It would be tempting to draw the conclusion that unique island species originated on their home island, but Darwin noticed enough similarities between island and mainland life to conclude that most island life probably migrated from a larger landmass at one point.
Themes
Summary of the last and present Chapters. Darwin summarized the effects that climate and geography had on the development of species. He emphasized the idea that in most cases, similar species descended from a single original source and were not independently created at different locations.
The idea that species developed independently in multiple places around the world makes some intuitive sense and may seem to fit with the fossil record. Nevertheless, Darwin’s theories about migration are an even better fit with the evidence and provide a more comprehensive explanation of both the diversity of life as well as similarities in life around the world.
Themes
