The Origin of Species · Chapter
Modes of Transition explained
Darwin argues that complex organs can be formed through numerous successive slight modifications, and that transitional grades are often missing due to extinction.
What happens
Darwin argues that complex organs can be formed through numerous successive slight modifications, and that transitional grades are often missing due to extinction. He provides examples of organs changing function, such as the swim-bladder becoming lungs, and ovigerous frena becoming branchiae. He also discusses the possibility of transition through acceleration or retardation of reproduction.
Free preview opens on this chapter — companions, themes, and character notes appear beside the text.
Narrative arc
Story tension across the book — this chapter sits in Rising action.
- Setup
- Escalation
- Breaking point
- Aftermath
- Closing
This passage marks the story's dramatic climax because Darwin shifts from a defensive posture against objectio…
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Follow the arc while readingThemes in this chapter
- Adaptation and the Utilitarian Doctrine
Darwin defends the idea that organs can change function through natural selection, exemplified by the swim-bladder becoming lungs and ovigerous frena becoming branchiae.
- Natural Selection and the Struggle for Existence
The chapter emphasizes that natural selection works through slight modifications and can convert organs to new functions, supporting the theory of descent with modification.
Characters to notice
- Charles Darwin
Author and primary voice, presenting arguments for transitional modes and defending his theory.
- Professor Owen
Cited for his description of the swim-bladder and its relation to vertebrate lungs.
- Fritz Müller
Mentioned for observations on crustaceans changing after maturity.
- Professor Cope
Referenced for his work on acceleration and retardation of reproduction as a mode of transition.
Key passages
“If it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down. But I can find out no such case.”
Darwin asserts that no complex organ has been proven impossible to evolve through gradual steps, so his theory remains intact.
Central claim defending gradualism against potential counterexamples.
“All physiologists admit that the swim-bladder is homologous, or 'ideally similar' in position and structure with the lungs of the higher vertebrate animals: hence there is no reason to doubt that the swim-bladder has actually been converted into lungs, or an organ used exclusively for respiration.”
The swim-bladder and lungs are homologous, supporting the idea that one organ evolved into the other through functional change.
Key example of organ conversion via natural selection.
“Therefore it need not be doubted that the two little folds of skin, which originally served as ovigerous frena, but which, likewise, very slightly aided in the act of respiration, have been gradually converted by natural selection into branchiae, simply through an increase in their size and the obliteration of their adhesive glands.”
The ovigerous frena in one cirripede group gradually evolved into branchiae in another group through natural selection.
Illustrates how organs can change function and structure over time.
