Appearance of the Concept in Nietzsche's Work

As the 1880s began, Nietzsche began to speak of the "Desire for Power" (Machtgelüst); this appeared in The Wanderer and his Shadow (1880) and Daybreak (1881). Machtgelüst, in these works, is the pleasure of the feeling of power and the hunger to overpower.
Wilhelm Roux published his The Struggle of Parts in the Organism (Der Kampf der Teile im Organismus) in 1881, and Nietzsche first read it that year. The book was a response to Darwinian theory, proposing an alternative mode of evolution. Roux was a disciple of and influenced by Ernst Haeckel who believed the struggle for existence occurred at the cellular level. The various cells and tissue struggle for finite resources, so that only the strongest survive. Through this mechanism, the body grows stronger and better adapted. Lacking modern genetic theory and assuming a lamarckian or pangenetic model of inheritance, the theory had plausibility at the time.
Nietzsche began to expand on the concept of Machtgelüst in The Gay Science (1882), where in a section titled “On the doctrine of the feeling of power,” he connects the desire for cruelty with the pleasure in the feeling of power. Elsewhere in The Gay Science he notes that it is only “in intellectual beings that pleasure, displeasure, and will are to be found,” excluding the vast majority of organisms from the desire for power.
Léon Dumont (1837–77), whose 1875 book Théorie Scientifique de La Sensibilité, le Plaisir et la Peine Nietzsche read in 1883, seems to have exerted some influence on this concept. Dumont believed that pleasure is related to increases in force. In The Wanderer and Daybreak, Nietzsche had speculated that pleasures such as cruelty are pleasurable because of exercise of power. But Dumont provided a physiological basis for Nietzsche’s speculation. Dumont’s theory also would have seemed to confirm Nietzsche’s claim that pleasure and pain are reserved for intellectual beings, since, according to Dumont, pain and pleasure require a coming to consciousness and not just a sensing.
In 1883 Nietzsche coined the phrase “Wille zur Macht” in Thus Spoke Zarathustra. The concept, at this point, was no longer limited to only those intellectual beings that can actually experience the feeling of power; it now applied to all life. The phrase Wille zur Macht first appears in part 1, "1001 Goals" (1883), then in part 2, in two sections, “Self-Overcoming” and “Redemption” (later in 1883). “Self-Overcoming” describes it in most detail, saying it is an “unexhausted procreative will of life.” There is will to power where there is life and even the strongest living things will risk their lives for more power. This suggests that the will to power is stronger than the will to survive.
Schopenhauer's "Will to life" thus became a subsidiary to the will to power, which is the stronger will. Nietzsche thinks his notion of the will to power is far more useful than Schopenhauer's will to live for explaining various events, especially human behavior—for example, Nietzsche uses the will to power to explain both ascetic, life-denying impulses and strong, life-affirming impulses in the European[clarify? link?] tradition, as well as both master and slave morality. He also finds the will to power to offer much richer explanations than utilitarianism's notion that all people really want to be happy, or the Platonist's notion that people want to be unified with the Good.
Nietzsche read William Rolph’s Biologische Probleme around mid-1884, and it clearly interested him, for his copy is heavily annotated. He made many notes concerning Rolph. Rolph was another evolutionary anti-Darwinist like Roux, who wished to argue for evolution by a different mechanism than the struggle for existence. Rolph argued that all life seeks primarily to expand itself. Organisms fulfill this need through assimilation, trying to make as much of what is found around them into part of themselves, for example by seeking to increase intake and nutriment. Life forms are naturally insatiable in this way.
Nietzsche's next published work was Beyond Good and Evil (1886), where the influence of Rolph seems apparent. Nietzsche writes, "Even the body within which individuals treat each other as equals ... will have to be an incarnate will to power, it will strive to grow, spread, seize, become predominant — not from any morality or immorality but because it is living and because life simply is will to power." Beyond Good and Evil has the most references to “will to power” in his published works, appearing in 11 aphorisms; The influence of Rolph and its connection to “will to power,” also continues in book 5 of Gay Science (1887) where Nietzsche describes will to power as the instinct for “expansion of power,” fundamental to all life.
Karl Wilhelm von Nägeli's 1884 book Mechanisch-physiologische Theorie der Abstammungslehre, which Nietzsche acquired around 1886 and subsequently read closely, also had considerable influence on his theory of will to power. Nietzsche wrote a letter to Franz Overbeck about it, noting that it has “been sheepishly put aside by Darwinists”. Nägeli believed in a “perfection principle,” which led to greater complexity. He called the seat of heritability the idioplasma, and argued, with a military metaphor, that a more complex, complicatedly ordered idioplasma would usually defeat a simpler rival. In other words, he is also arguing for internal evolution, similar to Roux, except emphasizing complexity as the main factor instead of strength.
Thus, Dumont’s pleasure in the expansion of power, Roux’s internal struggle, Nägeli’s drive towards complexity, and Rolph’s principle of insatiability and assimilation are fused together into the biological side of Nietzsche’s theory of will to power, which is developed in a number of places in his published writings. Having derived the “will to power” from three anti-Darwin evolutionists, as well as Dumont, it seems appropriate that he should use his “will to power” as an anti-Darwinian explanation of evolution. He expresses a number of times the idea that adaptation and the struggle to survive is a secondary drive in the evolution of animals, behind the desire to expand one’s power—the will to power.
Nonetheless, in his notebooks he continues to expand the theory of the will to power. Influenced by his earlier readings of Boscovich, he began to develop a physics of the Will to Power. The idea of matter as centers of force is translated into matter as centers of will to power. Nietzsche wanted to slough off the theory of matter, which he viewed as a relic of the metaphysics of substance.
These ideas of an all inclusive physics or metaphysics built upon the will to power do not appear to arise anywhere in his published works or in any of the final books published posthumously, except in the above mentioned aphorism from Beyond Good & Evil, where he references Boscovich (section 12). It does recur in his notebooks, but not all scholars treat these ideas as part of his thought.