The Origin of Species · Chapter
Absence of Batrachians and Terrestrial Mammals on Oceanic Islands explained
Darwin examines the striking absence of frogs, toads, newts, and terrestrial mammals on oceanic islands, contrasting them with the presence of bats and land-shells.
What happens
Darwin examines the striking absence of frogs, toads, newts, and terrestrial mammals on oceanic islands, contrasting them with the presence of bats and land-shells. He argues that this pattern is best explained by the difficulty of transport across seawater—batrachians and their spawn are killed by saltwater, and terrestrial mammals cannot cross wide seas, whereas bats can fly and land-shells can survive immersion. He also notes a correlation between the depth of sea separating islands and the degree of affinity of their mammalian faunas, supporting his theory of occasional means of transport over former land connections.
Themes in this chapter
Geographical Distribution and Barriers
The chapter focuses on the absence of batrachians and terrestrial mammals on oceanic islands, highlighting the role of oceanic barriers and the efficacy of occasional transport.
Adaptation and the Utilitarian Doctrine
Darwin uses the presence of endemic bats and land-shells to illustrate how species adapt to new homes after transport, consistent with natural selection.
Characters to notice
- Charles Darwin
Author and narrator; conducts experiments on land-shell immersion and synthesizes observations on island biogeography.
- Mr. Wallace
Cited for his admirable researches on the Malay Archipelago, showing the relation between sea depth and mammalian faunal affinity.
- Mr. Gould
Mentioned as Dr. A. A. Gould, who gave striking instances of land-shell distribution in the Pacific.
- Mr. Tomes
Cited for his special study of bats, noting their enormous ranges and presence on far distant islands.
Key passages
“With respect to the absence of whole orders of animals on oceanic islands, Bory St. Vincent long ago remarked that Batrachians (frogs, toads, newts) are never found on any of the many islands with which the great oceans are studded.”
Bory St. Vincent noted that frogs, toads, and newts are absent from nearly all oceanic islands.
Darwin verifies this observation and uses it to argue against independent creation.
“Although terrestrial mammals do not occur on oceanic islands, aerial mammals do occur on almost every island.”
While land mammals are missing, bats are found on nearly every oceanic island.
This contrast supports Darwin's explanation that bats can fly across seas, while terrestrial mammals cannot.
“I have carefully searched the oldest voyages, and have not found a single instance, free from doubt, of a terrestrial mammal (excluding domesticated animals kept by the natives) inhabiting an island situated above 300 miles from a continent or great continental island.”
After reviewing early exploration records, Darwin found no clear case of a native land mammal on an island more than 300 miles from a continent.
This empirical generalization underpins his argument for the difficulty of oceanic dispersal for terrestrial mammals.