The Origin of Species · Chapter
On the Relations of the Inhabitants of Islands to Those of the Nearest Mainland explained
Darwin examines the striking affinity between species inhabiting islands and those of the nearest mainland, using the Galapagos Archipelago as a key example.
What happens
Darwin examines the striking affinity between species inhabiting islands and those of the nearest mainland, using the Galapagos Archipelago as a key example. He argues that this relationship, inexplicable by independent creation, is explained by colonization from the nearest source followed by modification through natural selection. He also discusses how barriers, competition, and the nature of immigrants shape the distinct species on different islands within the same archipelago.
Themes in this chapter
Geographical Distribution and Barriers
Central theme: islands' inhabitants are related to the nearest mainland due to colonization, with barriers like deep sea channels limiting dispersal and leading to distinct species.
Natural Selection and the Struggle for Existence
Natural selection favors different varieties on different islands due to varying competitors and conditions, as seen in the Galapagos mocking-thrushes.
Common Descent and the Tree of Life
The affinity of island species to mainland species supports descent from a common progenitor, with modification after colonization.
Characters to notice
- Charles Darwin
Author and narrator; presents observations and arguments on island biogeography.
- Dr. Hooker
Cited for his Flora of the Galapagos Archipelago and for insights on plant affinities in New Zealand and Kerguelen Land.
- De Candolle
Referenced regarding the wide range of lower organisms and the distribution of naturalized species.
Key passages
“The most striking and important fact for us is the affinity of the species which inhabit islands to those of the nearest mainland, without being actually the same.”
The key observation is that island species are closely related to, but distinct from, species on the nearest continent.
Introduces the central pattern Darwin seeks to explain.
“Facts, such as these, admit of no sort of explanation on the ordinary view of independent creation; whereas, on the view here maintained, it is obvious that the Galapagos Islands would be likely to receive colonists from America...”
These facts cannot be explained by independent creation, but are easily understood if colonists arrived from America and then modified.
Contrasts creationist view with Darwin's natural explanation.
“The really surprising fact in this case of the Galapagos Archipelago... is that each new species after being formed in any one island, did not spread quickly to the other islands.”
What is truly surprising is that new species on one Galapagos island did not rapidly colonize neighboring islands.
Highlights the role of barriers and competition in limiting dispersal.