On the Origin of Species | Study Guide

Charles Darwin

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On the Origin of Species | Chapter 13 : Geographical Distribution—continued | Summary

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Summary

In this chapter Darwin discusses the distribution of classes, genera, and species in regards to freshwater systems and islands. He notes freshwater systems in disparate countries will sometimes contain very similar freshwater species despite oceanic barriers one might think would be impassible. He goes on to describe how this might have occurred, positing "accidental means" such as floods or whirlwinds, as well as the distribution of some species by others—such as the transportation of shells by ducks and the movement of plants through the digestive system of other species.

Regarding oceanic islands, Darwin is struck by the distribution of species on islands. There are fewer species inhabiting oceanic islands compared to the mainland. Islands will often have frogs, toads, newts, and other amphibious creatures not found on the mainland. In contrast, terrestrial or aerial mammals are found on almost every island. There is a correlation between the species present on an island and the mainland, and the relative degree of proximity between the two. This relationship also holds true for island archipelagos. Species do not spread rapidly from one island to another.

In all cases Darwin emphasizes these patterns cannot be explained by creationism, but by the theory of natural selection along with the acceptance of uniformitarianism and the antiquity of the earth. Paraphrasing naturalist Edward Forbes, Darwin notes, "There is a striking parallelism in the laws of life throughout time and space." He argues these laws explain the variation and similarities seen in freshwater systems and oceanic islands. The end of the chapter concludes Darwin's thoughts on geographical distribution, but his concluding paragraphs emphasize the overall importance of understanding how natural selection plays out over time and space.

Analysis

The continuation of the discussion of geographical distributions makes clear this is a subject Darwin has greatly considered. There is a profound correlation between geological processes and natural selection. This chapter, together with Chapter 12, is one of the first biogeographical studies of animals and plants across the world. While this is a very important aspect of biological and ecological work today—one that informs conservation practices—it was a relatively new idea at the time of publication.

The observations Darwin makes regarding freshwater systems and oceanic islands are based mainly on reflections he made during his voyage aboard the HMS Beagle. In all cases, while he is talking about the variation in classes, genera, and species he observed in locations such as New Zealand and the Galápagos Islands, Darwin frames these observations in geological time. It is obvious the power of the theory of natural selection in explaining some of these processes lies with Darwin's understanding of the time depth involved in these processes. This is clearly shown in the appearance of species in new environments having traveled from the mainland to a nearby—or relatively nearby—island.

Lyell's influence is especially apparent in this chapter. Using the principle of uniformitarianism, Darwin is able to explain why island species may be so similar, and yet different, from nearby mainland species. He states: "In looking to the long succession of past ages ... we find that species in certain classes differ little from each other." These geological processes, in conjunction with natural properties—such as the relative ability of certain classes of animals to move to different locations—help explain observations Darwin makes, such as the dearth of frogs, toads, and newts in contrast to the presence of mammals on island locations.

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