 Darwin offered a naturalistic alternative to the theological account. Marshalling anecdotes from zookeepers, breeders, and his own observations, Darwin presented evidence that instincts adapt to a changing environment. One process by which they evolve is unguided variation and natural selection. However, like many of his contemporaries, Darwin also accepted an alternative “inherited habits” model of instinct evolution. Habits are behaviors that originate as intelligent acts and become increasingly automatic over repeated performances. Although historians disagree about which mechanism Darwin took to be the more significant, it appears that he took both natural selection and inherited habits to be involved in the evolution of instinct. As he explained in The Origin, “If we suppose any habitual action to become inherited – and I think that this does sometimes happen – then the resemblance between what originally was a habit and an instinct becomes so close as not to be distinguished” (1859, 187). Darwin’s dual-mechanism view inspired other researchers to explore the relationship between intelligence, habit, and instinct. Of particular note is George Romanes, a protege of Darwin who sought to clarify and elaborate his mentor’s theory of instinct. In his books on animal intelligence, Romanes (1882; 1883) distinguished two types of instinct: primary instincts that evolve by a process of natural selection, and secondary instincts that evolve by the inheritance of acquired habits.