The Origin of Species · Chapter
Objections to the Theory of Natural Selection as Applied to Instincts: Neuter and Sterile Insects explained
Darwin addresses objections that instinct and structure must vary simultaneously, arguing that gradual changes in either can be coordinated by natural selection.
What happens
Darwin addresses objections that instinct and structure must vary simultaneously, arguing that gradual changes in either can be coordinated by natural selection. He then confronts the major difficulty of sterile neuter insects (e.g., worker ants) that differ greatly from fertile members yet cannot reproduce. He resolves this by proposing that selection acts on the family (the fertile parents), which transmit a tendency to produce sterile offspring with advantageous modifications. He further explains the origin of distinct castes through the gradual production of extreme forms, citing examples from ants, butterflies, and crustaceans, and emphasizes that this case demonstrates the power of natural selection without reliance on inherited habit.
Themes in this chapter
Natural Selection and the Struggle for Existence
Central theme: Darwin defends natural selection against objections by showing how it can account for the evolution of instincts and sterile castes through family-level selection.
Variation and Inheritance
Key to the argument: slight spontaneous variations are accumulated over generations, and the tendency to produce sterile forms is inherited through fertile parents.
Adaptation and the Utilitarian Doctrine
The utility of sterile castes (division of labor) is emphasized as a driving force for their evolution via natural selection.
Characters to notice
- Charles Darwin
Author and narrator; presents and defends the theory of natural selection against objections regarding instincts and neuter insects.
- Sir John Lubbock
Provided drawings of the jaws of worker ants of different sizes, aiding Darwin's analysis of caste gradation.
- Mr. Bates
Cited for analogous cases of caste differences in ants from his work 'Naturalist on the Amazons'.
- Mr. Wallace
Mentioned for his explanation of multiple female forms in Malayan butterflies, analogous to ant castes.
- Fritz Müller
Referenced for his observation of two distinct male forms in Brazilian crustaceans, supporting the concept of multiple castes.
- Lamarck
Mentioned in the context of the doctrine of inherited habit, which Darwin contrasts with his own explanation.
Key passages
“It has been objected to the foregoing view of the origin of instincts that 'the variations of structure and of instinct must have been simultaneous and accurately adjusted to each other, as a modification in the one without an immediate corresponding change in the other would have been fatal.'”
Critics argue that for natural selection to work, changes in body form and behavior must happen at the same time and be perfectly matched, otherwise the organism would not survive.
Darwin introduces and then refutes this objection by showing that gradual, sequential changes can be coordinated.
“Hence, we may conclude that slight modifications of structure or of instinct, correlated with the sterile condition of certain members of the community, have proved advantageous; consequently the fertile males and females have flourished, and transmitted to their fertile offspring a tendency to produce sterile members with the same modifications.”
Small beneficial changes that appear in sterile individuals can be preserved because the fertile parents that produce them thrive and pass on the tendency to generate such sterile forms.
This is Darwin's key resolution to the difficulty of sterile castes: selection acts on the family line, not the sterile individual.
“I am surprised that no one has advanced this demonstrative case of neuter insects, against the well-known doctrine of inherited habit, as advanced by Lamarck.”
Darwin notes that the case of sterile insects provides strong evidence against Lamarck's idea that habits acquired during life can be inherited, since sterile workers cannot pass on their habits.
Darwin contrasts his theory with Lamarck's, highlighting the power of natural selection over inherited habit.
