On the Origin of Species | Study Guide

Charles Darwin

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On the Origin of Species | Chapter 12 : Geographical Distribution | Summary

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Summary

Having discussed the geological history of species, Darwin now offers evidence for their geological distribution—a natural extension of his observations regarding the extinct ancestors of many species and genera. He begins with three main points: the distribution of species across the globe cannot be accounted for when considering "climatal and other physical conditions" alone; the distribution of species is related to the presence of physical barriers; and there is a similarity of species across continents at different points. Darwin argues the similarity of species can be accounted for only through "inheritance," while the dissimilarity can also be accounted for through barriers. From these conclusions he suggests it is logical to assume in many cases, species of the same genus—regardless of physical location—are descended from the same ancestor.

The remainder of the chapter is devoted to describing a few methods of dispersal: climate, changes in land and sea level, changing continental positions, and glacial periods. Darwin contends that in many cases, plants and animals would have been dispersed during periodic glacial periods due to advancing and receding glaciers. He argues this would have resulted in changing temperatures that would allow for species to spread across continents. Combined with the change in the position of continental plates over time, this would account for the presence of similar species today and fossil specimens in very different geographical regions. Darwin defends the idea of a glacial period, noting evidence for such events has been supported by previous researchers. He comments on a few more observations regarding the biogeographical distribution of species, noting species moving north to south are more likely to adapt to the region than those moving south to north. Overall, the majority of the chapter is dedicated to Darwin's observations regarding biogeographical patterns and potential reasons for their occurrence.

Analysis

Continuing along the same line as the argument presented in the previous chapter, Darwin uses geological and geographical information to bolster his theory of natural selection. He believes an immense time depth is necessary for physical processes to take place due to natural selection. The crux of his argument in this chapter is in many cases—both in modern times as well as in antiquity—similar species from the same genera appear in very different areas of the world. He notes previous arguments have centered on the idea of independent species formation. However, he argues the specific pattern of where these similar species occur is indicative of a common ancestor. It is often the case fossil specimens physically similar to modern species are found within the same geographical location.

Darwin's idea species originated from a common ancestor was made possible because of the changing views of geological time and processes in the 19th century. His ideas were built upon those stated by his predecessors and contemporaries. However, his insights were also based on numerous observations of similar species in different regions and continents, ancestral fossil species, experiments involving the possibility of seeds being transported over the ocean, and observations of the relative hardiness of seeds in bird scat among others. In sum, while Darwin did not conceive of the idea of a common ancestor in a vacuum, he was able to accurately describe it based on his own observations.

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