The Origin of Species · Chapter
A Part Developed in Any Species in an Extraordinary Degree or Manner, in Comparison with the Same Part in Allied Species, Tends to Be Highly Variable explained
Darwin argues that when a part or organ is unusually developed in one species compared to its close relatives, it tends to be highly variable.
What happens
Darwin argues that when a part or organ is unusually developed in one species compared to its close relatives, it tends to be highly variable. He draws parallels with domestic breeds under rapid selection, where the selected traits show great variability. In nature, such extraordinary development implies recent and extensive modification, leaving the part still variable until natural selection stabilizes it. He cites examples from pigeons, cirripedes, and secondary sexual characters, and notes that the rule does not apply to uniformly developed structures like bat wings.
Themes in this chapter
Variation and Inheritance
The chapter focuses on the high variability of parts that have undergone recent and extraordinary modification, linking it to the generative variability still present.
Natural Selection and the Struggle for Existence
Darwin describes a constant struggle between natural selection, reversion, and variability, with selection eventually stabilizing the modified part.
Adaptation and the Utilitarian Doctrine
The extraordinary development of a part implies it is of high importance to the species, yet it remains variable until natural selection fixes it.
Characters to notice
- Mr. Waterhouse
Darwin credits Mr. Waterhouse with first remarking on the rule that extraordinarily developed parts are highly variable.
- Professor Owen
Professor Owen independently arrived at a similar conclusion regarding the variability of unusually developed parts.
Key passages
“When a part has been developed in an extraordinary manner in any one species, compared with the other species of the same genus, we may conclude that this part has undergone an extraordinary amount of modification since the period when the several species branched off from the common progenitor of the genus.”
If a part is unusually developed in one species relative to its close relatives, it likely underwent extensive change since diverging from a common ancestor.
Darwin establishes the core premise linking extraordinary development to recent and extensive modification.
“It is only in those cases in which the modification has been comparatively recent and extraordinarily great that we ought to find the generative variability, as it may be called, still present in a high degree.”
Only when modification is both recent and extreme should we expect to see high variability, because selection has not yet fixed the trait.
Darwin explains why variability persists in recently modified parts.
“The rule applies very strongly in the case of secondary sexual characters, when displayed in any unusual manner.”
The rule is especially evident in secondary sexual characteristics that are unusually developed.
Darwin highlights secondary sexual characters as a key example of the rule.