"At the core of Freud’s initial theory of psychoanalysis was his proposal of the instinctual system, which included two fundamental classes of instincts. The first were the life-preservative instincts. These included the needs for air, food, water, and shelter and the fears of snakes, heights, and dangerous humans. These instincts served the function of survival. Freud’s second major class of motivators consisted of the sexual instincts. “Mature sexuality” for Freud culminated in the final stage of adult development—the genital stage, which led directly to reproduction, the essential feature of Freud’s mature sexuality. Astute readers might sense an eerie familiarity. Freud’s two major classes of instincts correspond almost precisely to Darwin’s two major theories of evolution. Freud’s life-preservative instincts correspond to Darwin’s theory of natural selection, which many refer to as “survival selection.” And his theory of the sexual instincts corresponds closely to Darwin’s theory of sexual selection. Freud eventually changed his theory by combining the life and sexual instincts into one group called the “life instincts” and adding a second instinct known as the “death instinct.” He sought to establish psychology as an autonomous discipline, and his thinking moved away from its initial Darwinian anchoring."