Darwin discusses the heritability of instinct and its role in natural selection. Instinct proves difficult for Darwin to define. It is similar to habit, because it consists of actions performed repeatedly by an individual animal. But unlike habit, which Darwin believes animals learn, instinct is inherited. The causes of innate instincts remain unknown, in the same way the causes of physical variations are unknown. Darwin believes that inherited habits—those learned by a parent and subsequently passed on to offspring by hereditary inheritance—may play a role in the construction of these instincts. However instincts come to be, Darwin argues that natural selection acts on them just as it acts on physical variations. If an instinct is advantageous to a species’ survival, natural selection allows organisms with that instinct to survive over others and perpetuate that instinct in their offspring. Thus, natural selection helps create entire species with well-adapted instincts, allowing them to survive in a variety of natural environments.

Darwin provides many examples of species that have innate, advantageous instincts. Certain hens, for example, have exhibited an instinct for laying their eggs in another hen’s nest, allowing them to give birth to numerous offspring without having to take care of all of them. In certain ant populations, some ants are born with “slave” instincts. These ants instinctively care for the rest of the population without any type of training after their birth. Hive bees instinctively construct the honeycomb to hold the greatest amount of honey possible. Darwin provides the results of a number of experiments in which the hive bees shaped whatever wax Darwin gave them, no matter what size or shape, in the same way. The results demonstrate that the construction of the hive holes is instinctual for the species. All of these instincts, Darwin argues, come about as a result of slight modifications over time: Natural selection perpetuates the most advantageous instincts, gradually honing them into their most advanced state in the latest incarnation of a species.

In concluding this chapter, Darwin presents one complication to his theory of the natural selection of instincts, a complication that, in his mind, almost destroys the entire theory. In some ant species, the worker ants, which vary from the nonworker ants in both structure and instinct, are sterile. If these ants are unable to produce offspring, how could their traits be inherited by subsequent generations? Moreover, a number of different structures for these infertile ants exist. How could all of these different structures be passed down through natural selection if the carriers of these structures were always infertile? While Darwin admits how difficult these questions are to answer, he argues that natural selection works on entire species, not just on individual organisms. Therefore, natural selection must have perpetuated the fertile ants in this species, the ones who carried a tendency to produce these sterile members, which proved advantageous to ant society by providing it with worker ants. The parent ants that were able to produce infertile offspring proliferated—not the sterile worker ants themselves. This difficult case illustrates the power of natural selection in selecting characteristics that could have been acquired only through variation and not through habit.


In his continued explanation of natural selection, Darwin discusses the heritability of instinct, shifting focus from the selection of physical characteristics to the selection of mental characteristics. This analysis builds on his previous discussion of natural selection but adds a new dimension: the power of mental characteristics (or instincts) to shape the survival and proliferation of species. Clearly, mere physical strength or structural adaptation is not enough for animals to survive in nature. Animals must know what to do with their physical adaptations and how to use them to live in their environment. They must know how to gather food, build shelter, and hide from their enemies. While the necessity of instincts may seem obvious, in this analysis, Darwin reminds the reader that advantages gained from instincts also become part of natural selection’s shaping of species.

Darwin’s discussion of advantageous mental characteristics in animals implies that organisms have an innate intelligence, and that some use this intelligence to survive better in nature. Darwin argues that the smartest animals are most likely to win out in natural selection, because their intelligence is advantageous to their survival. Indeed, many of the instincts he cites as examples, such as the mathematical precision of hive bees as they make honeycombs, are quite sophisticated. Darwin’s theory of heritability and natural selection of instincts implies that a form of intelligence plays a role in the development of the human species. Social Darwinists would later apply Darwin’s theory of natural selection to questions of human intelligence and how it shapes the human population. They would ask, Is natural intelligence in human beings instinctive and therefore innate, because it is passed on through heredity by innately intelligent parents? Do more intelligent human beings stand a greater chance of survival? Are intelligent humans naturally selected to survive and flourish in the human population?

Although Darwin admits that habit may play some unknown role in the development of variation, he provides an example of a hereditary characteristic—the birthing of sterile worker ants—that would be impossible to shape by habit or choice. Since the characteristic of sterility in worker ants is clearly inherited, some other means of creating variations must exist for these variations to be passed down to subsequent generations. Although Darwin cannot pinpoint the means by which these variations are created—Mendelian genetics and genetic mutation theory would provide those answers in the twentieth century—his example of the sterile worker ants provides a direct refutation of Lamarck’s evolutionary theory. Darwin points this out by asking how an instinct to reproduce sterile ants could be produced through chosen habit. Darwin’s “difficult case” of the sterile worker ants, which he believes could have undermined his own theory, ends up strengthening it instead by refuting the central assumption of Lamarckian evolution—that habits are the cause of evolution.