The Origin of Species · Chapter
Summary of the Last and Present Chapters explained
Darwin summarizes the arguments from the previous chapters on geographical distribution, emphasizing that despite our ignorance of past climatic and geological changes, the evidence supports the theory that all individuals of the same speci
What happens
Darwin summarizes the arguments from the previous chapters on geographical distribution, emphasizing that despite our ignorance of past climatic and geological changes, the evidence supports the theory that all individuals of the same species, and all species of the same genus, descend from common parents. He highlights the role of barriers, the Glacial period, and means of occasional transport in shaping distribution. The principles of migration, modification, and natural selection explain the patterns of life across time and space, including the peculiarities of oceanic islands and the parallelism between succession in time and distribution in space.
Themes in this chapter
Geographical Distribution and Barriers
The chapter extensively discusses how barriers (land or water) separate and form zoological and botanical provinces, and how migration and subsequent modification explain the distribution of species.
Common Descent and the Tree of Life
Darwin argues that all individuals of the same species and all species of the same genus are descended from common parents, a core tenet of common descent.
Natural Selection and the Struggle for Existence
The accumulation of modifications by natural selection is invoked as the mechanism driving the diversification of forms after migration.
Characters to notice
- Edward Forbes
Darwin references Forbes's insistence on the striking parallelism in the laws of life throughout time and space.
Key passages
“if we make due allowance for our ignorance of the full effects of changes of climate and of the level of the land, which have certainly occurred within the recent period, and of other changes which have probably occurred... the difficulty is not insuperable in believing that all the individuals of the same species, wherever found, are descended from common parents.”
If we account for our limited knowledge of past climate and land-level changes, it is not impossible to believe that all individuals of a species share common ancestors.
Darwin acknowledges the challenges posed by incomplete geological and climatic records but argues they do not undermine the theory of common descent.
“As the late Edward Forbes often insisted, there is a striking parallelism in the laws of life throughout time and space; the laws governing the succession of forms in past times being nearly the same with those governing at the present time the differences in different areas.”
Edward Forbes emphasized that the rules determining how species succeed each other over time are similar to those that explain why different species exist in different regions today.
Darwin uses Forbes's observation to link the patterns of extinction and succession in the fossil record with contemporary biogeography.
“We can thus understand the high importance of barriers, whether of land or water, in not only separating but in apparently forming the several zoological and botanical provinces.”
Barriers like oceans or mountain ranges are crucial because they not only separate populations but also help create distinct biological regions.
This highlights the role of geographic isolation in speciation, a key concept in Darwin's theory.