The Origin of Species · Chapter
Effects of Habit and of the Use or Disuse of Parts; Correlated Variation; Inheritance explained
Darwin examines how changed habits and the use or disuse of parts lead to inherited modifications, using examples like the domestic duck's lighter wing bones and heavier leg bones compared to the wild duck.
What happens
Darwin examines how changed habits and the use or disuse of parts lead to inherited modifications, using examples like the domestic duck's lighter wing bones and heavier leg bones compared to the wild duck. He discusses correlated variation, where changes in one part of the body affect others, citing cases such as white cats with blue eyes being deaf and black pigs surviving paint-root poisoning. The chapter also explores the laws of inheritance, including the tendency for peculiarities to reappear at corresponding ages, and addresses the claim that domestic varieties revert to wild forms when feral, arguing there is no strong evidence for this.
Themes in this chapter
Variation and Inheritance
Explores how use and disuse of parts, correlated variation, and inheritance laws shape the transmission of traits across generations.
Domestication and Artificial Selection
Uses domestic animals and plants to demonstrate the effects of habit, use/disuse, and correlated variation, and argues that domestic varieties do not inevitably revert to wild forms.
Characters to notice
- Geoffroy Saint-Hilaire
Cited for his work on monstrosities and correlations between distinct parts.
- Professor Wyman
Communicated to Darwin the observation that Virginia farmers select black pigs because they survive eating paint-root.
- Heusinger
Provided facts that white sheep and pigs are injured by certain plants while dark-colored individuals escape.
- Dr. Prosper Lucas
His treatise on inheritance is described as the fullest and best on the subject.
Key passages
“I find in the domestic duck that the bones of the wing weigh less and the bones of the leg more, in proportion to the whole skeleton, than do the same bones in the wild duck; and this change may be safely attributed to the domestic duck flying much less, and walking more, than its wild parents.”
Domestic ducks have lighter wing bones and heavier leg bones compared to wild ducks, likely because they fly less and walk more.
Example of how disuse (less flying) and use (more walking) lead to inherited anatomical changes.
“Thus cats which are entirely white and have blue eyes are generally deaf; but it has been lately stated by Mr. Tait that this is confined to the males.”
White cats with blue eyes are often deaf, though Mr. Tait reports this only occurs in males.
Illustrates correlated variation, where a color trait is linked to a sensory defect.
“If strange and rare deviations of structure are truly inherited, less strange and commoner deviations may be freely admitted to be inheritable.”
If rare abnormalities can be inherited, then more common variations likely are too.
Darwin argues for the general inheritance of variations, using rare cases as evidence.
“But there is not a shadow of evidence in favour of this view: to assert that we could not breed our cart and race-horses, long and short-horned cattle, and poultry of various breeds, and esculent vegetables, for an unlimited number of generations, would be opposed to all experience.”
There is no evidence that domestic varieties inevitably revert to wild forms; experience shows we can maintain distinct breeds indefinitely.
Darwin refutes the claim that domestic varieties revert when feral, supporting the use of domestication as a model for natural selection.