The Origin of Species · Chapter
Instinct explained
Darwin addresses objections to natural selection regarding the origin and modification of instincts, arguing that instincts, like physical structures, can be gradually refined through selection of slight variations.
What happens
Darwin addresses objections to natural selection regarding the origin and modification of instincts, arguing that instincts, like physical structures, can be gradually refined through selection of slight variations. He tackles the special difficulty of sterile neuter insects (e.g., worker ants) which cannot reproduce yet exhibit complex instincts and structures; Darwin resolves this by proposing that selection acts on the family level, with fertile individuals passing on tendencies that produce sterile members with advantageous traits. He concludes that even the most remarkable instincts can be explained as consequences of natural selection.
Themes in this chapter
Natural Selection and the Struggle for Existence
Central theme: Darwin applies natural selection to instincts, showing how slight variations can be preserved and accumulated, even in sterile castes via family-level selection.
Variation and Inheritance
Discussed in the context of correlated variations (e.g., beak and feet in titmice) and inheritance of sterile traits through fertile relatives.
Adaptation and the Utilitarian Doctrine
Implied in the argument that instincts and structures are shaped for utility; Darwin defends the gradual, adaptive origin of even the most remarkable instincts.
Characters to notice
- Charles Darwin
Author and narrator; presents the argument that instincts evolve through natural selection and addresses the difficulty of sterile neuter insects.
- Mr. Mivart
Critic whose objection that instinct and structure must change simultaneously is addressed; Darwin argues the changes can be gradual.
- M. Verlot
Cited for the example of double annual stock, where sterile flowers are produced through selection applied to the family, analogous to sterile insect castes.
Key passages
“The force of this objection rests entirely on the assumption that the changes in the instincts and structure are abrupt.”
Mivart's criticism only holds if we assume instinct and structure change suddenly, rather than gradually.
Darwin counters Mivart's objection by emphasizing gradual change.
“Hence, I can see no great difficulty in any character becoming correlated with the sterile condition of certain members of insect communities; the difficulty lies in understanding how such correlated modifications of structure could have been slowly accumulated by natural selection.”
It is not hard to imagine that traits can be linked to sterility in some insects; the real puzzle is how natural selection could gradually build up those traits.
Darwin frames the core difficulty of sterile castes.
“This difficulty, though appearing insuperable, is lessened, or, as I believe, disappears, when it is remembered that selection may be applied to the family, as well as to the individual, and may thus gain the desired end.”
The problem of sterile workers is resolved when we realize that selection can act on whole families, not just individuals.
Key resolution: family-level selection explains sterile castes.