In this chapter, Darwin tackles one of the most fascinating and controversial aspects of animal behavior: Instinct. His goal is not to explain the neurological basis of instinctive behavior, nor to speculate on the physiology of animal minds. Instead, he asks a more evolutionary question: how could natural selection have shaped complex, seemingly purposeful behaviors that occur without learning or reasoning? Darwin approaches instinct with the same framework he applies to morphology: as a product of gradual, heritable variation accumulated by natural selection.
He defines instinct broadly and practically. An action is instinctive, he writes, when performed by many individuals in the same way, without their knowing for what purpose it is performed. These behaviors are not the result of conscious intention but are innate and often arise without prior experience. Today, we might describe these as behaviors with a genetic basis expressed automatically, behaviors that develop and function independently of extensive learning or cognition.
Darwin insists that instincts vary, just as physical traits do. If instincts can vary, even slightly, then natural selection can preserve and accumulate those variations. Over time, this process can give rise to remarkably complex behaviors. A complex instinct, Darwin argues, can only arise through the slow and gradual accumulation of numerous, slight, yet profitable variations. Even if intermediate stages are not visible in current species, they may have existed in extinct ancestors, or might still be found in related lineages. The absence of intermediates, as always in Darwin’s framework, is not a disproof of gradualism but a limitation of our observations.
To illustrate this principle, Darwin offers an especially puzzling example: aphids that yield sweet secretions to ants. At first glance, this appears to violate Darwin’s broader claim that no animal performs an action for the exclusive good of another species. However, Darwin does not leave this case unanswered. He conducts an experiment: removing ants from aphids and attempting to stimulate them to secrete using a hair. The aphids do not respond. But when a single ant is reintroduced, it successfully “milks” several aphids using its antennae. Even very young aphids exhibit this behavior, suggesting it is innate, not learned.
Darwin concludes that this is not an act for the good of the ants. Instead, it is likely that removal of the viscid fluid benefits the aphids, perhaps by preventing harmful buildup. What appears at first to be altruism across species lines is, in fact, mutualism: a coevolved relationship that benefits both participants. Modern science confirms this view. Certain aphid species rely on ants for protection, and some have lost the ability to excrete without ant stimulation. The ants, in turn, benefit from a stable food source.
Having established that instincts can vary, Darwin now must show that they are heritable. He turns to domestic animals, where instinctive behaviors are easier to observe and compare across breeds. From these, he selects three examples to support his argument: the brood parasitism of cuckoos, the slave-making behavior of certain ant species, and the cell-building behavior of hive bees. The last two, he writes, have been considered by naturalists the most wonderful of all known instincts.
The brood parasitism of the European cuckoo, Darwin proposes, likely evolved from the behavior seen in American cuckoos, which only occasionally lay eggs in other birds’ nests. He suggests that this occasional behavior conferred advantages, perhaps by allowing earlier migration, or by freeing the parent from the burden of caring for young of different ages. If the offspring raised by other birds were stronger as a result, or if the mother was less encumbered, then the behavior would gradually become fixed. This is a classic Darwinian scenario: a rare but beneficial behavior becomes more frequent through selection.
The slave-making instincts of ants are even more dramatic. Darwin provides detailed observations of "Formica sanguinea" raiding nests of "F. fusca" to capture and enslave their brood. These enslaved ants, once matured, work as if in their own colony. Darwin notes that geographic variation exists in the degree of dependence on slaves, even within the same species, providing further evidence that this instinct is variable and thus evolvable. Modern studies have confirmed the evolutionary origin of this behavior, showing it is shaped by ecological conditions and genetic predispositions.
Perhaps the most striking case Darwin discusses is the construction of honeycomb cells by hive bees. These hexagonal cells, built by scores of worker bees simultaneously, are marvels of geometry and efficiency. Darwin calculates that one pound of wax requires bees to consume fifteen pounds of dry sugar. Selection, he argues, should therefore favor economy in wax usage. The hexagonal structure minimizes material while maximizing storage space. Darwin notes that stingless tropical bees “Melipona” build less precise cells, which he interprets as an intermediate stage in the evolution of comb-building. This supports his central claim: even such a complex instinct could evolve through many small, beneficial modifications.
But then Darwin turns to what he calls the most serious difficulty of all: The case of neuter insects. In ants, bees, and termites, sterile workers and soldiers exist in distinct castes. These individuals never reproduce, so how could natural selection favor the traits that define their role? If they leave no offspring, how can their instincts evolve?
At first, Darwin admits, this problem seemed insuperable and even fatal to his whole theory. But he finds an answer by shifting the level at which selection acts. Natural selection does not have to act only on individuals. It can act on families or extended kin groups. If a trait in a sterile worker improves the success of the colony, and if the genes responsible are also carried by the reproductive individuals, then those genes can still be passed on. Darwin gives the analogy of plant and animal breeding. A vegetable may be killed for its flavor, or a cow for its fat, yet the breeder continues the lineage using relatives that share those favorable traits. Selection works on groups, not just individuals. This is the principle we now call inclusive fitness or kin selection.
Darwin anticipates much of modern evolutionary thinking about social behavior. He recognizes that natural selection can explain not only physical traits and solitary behaviors but also the most complex cooperative systems in the animal kingdom. In The Descent of Man, he expands on this to explain altruism in humans, suggesting that behaviors like loyalty, courage, and sympathy can be favored through group selection. A tribe made up of such individuals, he writes, would be victorious over other tribes. This vision foreshadows theories of group selection and reciprocal altruism that would emerge more than a century later.
Darwin concludes the chapter by affirming that instincts are just as variable, and just as subject to selection, as physical traits. His shift from typological thinking to population thinking allowed him to see that behavior evolves not as fixed patterns, but as diverse and heritable traits subject to environmental pressures. With this chapter, Darwin extends the reach of natural selection beyond bodies and into behavior, providing yet another pillar for his theory of evolution.