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 live (Wille zum Leben) 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 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 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 natural selection. 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, 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. Carl 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,