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.