On the Origin of Species | Study Guide

Charles Darwin

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On the Origin of Species | Chapter 8 : Instinct | Summary



In this chapter Darwin discusses the idea natural selection informs not only physical characteristics but instincts as well. He fails to define instinct, but compares it to habits, and argues his readers generally know what he is discussing when he uses the term. At the core of his argument is the fact that "the instinct of each species is good for itself," but not for another species, thus fulfilling one of the tenets of natural selection. Darwin returns to domesticated animals, this case focusing on dogs. He argues different breeds possess different instincts—such as herding in sheep dogs—and in all cases, humans would not have selected for these traits if they did not already exist at the outset.

Darwin describes three cases in which natural selection informs animal/insect instincts: the nest-stealing instinct of cuckoos, the slave-making instinct of ants, and the hive-making instinct of bees. He goes into great detail describing each case and argues that, in all instances, he can see how natural selection could select for these instincts because of the adaptive advantage they provide to each species of interest. After discussing these examples, Darwin addresses the argument against the selection of instincts in the wild—that of sterile male ants. He addresses the processes that might have selected for neutered male ants, arguing the main issue is in the misinterpretation of the speed with which these changes occur. There are only objections if the argument is made these changes occur over very short periods of time. Thus, he concludes this chapter by stating natural selection can be applied not only to "corporeal" structures but to instincts as well.


Having spent approximately half of the book focusing on natural selection pertaining to animals' physical traits, in this chapter Darwin focuses his efforts on instinctual traits.

Despite the fact all of the examples within the book thus far have been qualitative and illustrative—rather than quantitative—Darwin is able to accurately describe the process of natural selection. He relies on descriptions of specific processes to elucidate the concepts he is discussing and uses bottom-up data (specific examples) to answer top-down processes (natural selection).

Once again, Darwin anticipates objections to his argument in his discussion of sterile male ants. Critics of natural selection believe traits can appear almost instantaneously, but Darwin again emphasizes the slow nature of natural selection.

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