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
Darwin suggested that the way organisms are spread across the planet couldn’t be fully explained simply by looking at climate or other physical conditions. He believed that the so-called Old World and New World were both vast enough to encompass just about every imaginable climate, but that in spite of this, they had unique organisms living in them. One of the key factors behind these differences was obstacles to migration.
The topic of migration was important for previous chapters, but in this one, Darwin explores it in even greater detail. Darwin’s theory rests on the idea that species migrate widely (especially over long periods of time), but that there could also be serious obstacles to migration that limited it.
Themes
Single Centres of supposed Creation. The naturalists of Darwin’s time often wondered whether species originated at one or many points on the earth’s surface. Though Darwin admitted it was astounding to consider how a species could originate in one area, then migrate so widely, he ultimately believed that this was the case.
Darwin’s theory rests on the idea that species migrated widely in the past and that that is why there are similarities between creatures in very different parts of the world (as shown in the fossil record and in surviving current species). The alternative (that the same species could have independently arisen in multiple locations) puts less emphasis on migration but involves such an improbable coincidence (Darwin calls it a “miracle”) that it would not happen through natural selection, even over millions of years, let alone frequently.
Themes
Furthermore, Darwin argued that the differences in land mammals in Europe versus the United States suggested clearly that the same species doesn’t originate in two or more points, since these differences seem to be due to the fact that land mammals can’t migrate across oceans. If species could originate in more than one place, there would be more uniformity around the world.
While Darwin believes that animals migrated widely in the past, it is also crucial to this theory that there were barriers to migration. It is the contrast between free migration and barriers to migration that led to the current diversity of life on earth.
Themes
Means of Dispersal. Darwin summarized the findings of Lyell and other writers. Changes in climate were a major force in causing organisms to migrate. Raising and lowering of the sea level (which sometimes either connected or severed connections between land masses) also influenced migration.
Perhaps the most important barrier to migration is water. In the past, sea levels were different, and this means that migration could occur in some areas where it can’t now, and vice versa.
Themes
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Darwin also described what he called “accidental means” of migration, specifically for plants. Experiments showed that some seeds could survive long periods in saltwater, perhaps even long enough to cross an ocean. Birds, insects, driftwood, or even icebergs could have also carried seeds across landmasses. Nevertheless, the barriers were still significant enough to ensure that plants from vastly distant continents, like North America and Europe, would mostly remain distinct.
Plants can’t migrate in quite the same way as animals, but in this section, Darwin shows that in fact there are some unexpected ways that plants could have spread widely. Though each of the dispersal methods Darwin describes here may have been individually unlikely, together, over long periods of time, they help explain similarities between plants in very different parts of the world.
Themes
Literary Devices
Dispersal during the Glacial Period. Many naturalists in Darwin’s time and before marveled at how species on mountain-tops, separated by hundreds of miles, could have so many similarities. Such similarities could be explained by the movements of glaciers, which used to provide a bridge between the climates of different mountaintops before they moved and the climate got warmer.
Even in Darwin’s time, geologists understood how glaciers moved, based on the geological features they left behind in the land. Darwin suggests that the movement of these massive bodies of ice would have affected migration, even providing a way for species to easily travel from one mountaintop to another (until the glacier moved).
Themes
Alternate Glacial Periods in the North and South. Europe in Darwin’s time showed distinct evidence of a glacial period in the distant past. But while the phenomenon was most studied in Europe, it appeared to affect the whole planet. Animal and particularly plant distributions in South America show clear signs of there having been a glacial period at some point in the past.
Darwin explains that because the climate is different in the Northern and Southern Hemispheres, glacial periods and other climate change would have occurred at different times. Though it is not always possible to fully reconstruct the past, the species the survive today often provide indirect evidence of how migration likely occurred in the past.
Themes
It seems, however, that the Southern Hemisphere and Northern Hemisphere went through glacial periods at different times. One of the strangest signs of this that the naturalists in Darwin’s day observed was how many more species seem to have migrated from the north to the south instead of vice versa, which seems to provide evidence of alternating glacial periods. While not all the mysteries of organism dispersion have been resolved, it is still possible to deduce a lot from the evidence that does exist.
A hallmark of Darwin’s thinking (and of the scientific method in general) is that he tries to adapt his theories to fit the evidence instead of vice versa. In this case, he and other naturalists were presented with the surprising finding that species seem to have migrated more from north to south instead of vice versa. Rather than dismissing this finding, Darwin attempts to account for it, relying in part on available geological evidence.
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