As Todd Weir observes in the introduction to this volume, the monism of Ernst Haeckel (1834–1919) was "a naturalistic worldview based chiefly on the theory of biological evolution."¹ But what, exactly, was "the" theory of biological evolution, according to Haeckel? And to what extent might the reverse have been true and the theory have depended on the worldview? For the foundations of monism demanded more from evolution than the mere transformation of species, and not all of the competing theories of Haeckel's day could satisfy those demands equally well. In fact, Haeckel rejected some of them quite vehemently for their incompatibility with monism.