The Origin of Species · Chapter
Dispersal During the Glacial Period explained
Darwin explains how the Glacial period accounts for the distribution of identical alpine and arctic species on distant mountain summits and polar regions.
What happens
Darwin explains how the Glacial period accounts for the distribution of identical alpine and arctic species on distant mountain summits and polar regions. He describes how cold climates allowed northern species to migrate southward, and as the climate warmed, they retreated northward or to high elevations, leaving isolated populations on mountains. This process explains the close relationship between species in Europe, North America, and other regions, and supports his theory of modification over independent creation.
Themes in this chapter
Geographical Distribution and Barriers
Darwin uses the Glacial period to explain how species came to inhabit distant mountain summits and polar regions, and how barriers like oceans and climate change affect distribution.
Adaptation and the Utilitarian Doctrine
The chapter discusses how species adapted to cold climates migrated and later modified when isolated, illustrating the utilitarian principle of adaptation to changing conditions.
Characters to notice
- Mr. H. C. Watson
Noted that alpine plants of Scotland are allied to those of northern Scandinavia.
- Asa Gray
Showed that more plants are identical between Europe and America than formerly supposed; also noted plants on White Mountains are same as those of Labrador and Europe.
- Agassiz
Called vivid attention to the Glacial period, providing a simple explanation for the distribution of identical species.
- Edward Forbes
Explained the former influence of the glacial climate on the distribution of European inhabitants.
- James Dwight Dana
Described closely allied crustaceans in the Mediterranean and seas of Japan in his admirable work.
- Linnaeus
Concluded in 1747 that the same species must have been independently created at many distinct points.
Key passages
“The identity of many plants and animals, on mountain-summits, separated from each other by hundreds of miles of lowlands, where Alpine species could not possibly exist, is one of the most striking cases known of the same species living at distant points, without the apparent possibility of their having migrated from one point to the other.”
It is remarkable that identical plants and animals are found on mountain peaks far apart, with lowlands between them where such species cannot survive, making it seem impossible for them to have migrated directly.
Darwin introduces the puzzle of disjunct alpine distributions.
“As the cold came on, and as each more southern zone became fitted for the inhabitants of the north, these would take the places of the former inhabitants of the temperate regions.”
As the climate cooled, northern species moved into southern areas that became suitable for them, displacing the original temperate species.
Darwin describes the southward migration during glaciation.
“Thus we can understand the identity of many plants at points so immensely remote as the mountains of the United States and those of Europe.”
This explains why the same plant species are found on mountains in the United States and Europe, despite the great distance between them.
Darwin concludes that glacial migration explains transatlantic plant similarities.