 The greatest popularizer of evolutionary theory in the nineteenth century was of course Ernst Haeckel. Although he is now best known for his infamous maxim, “ontogeny recapitulates phylogeny”, his larger project was to put forward an evolutionary metaphysics; in particular to offer a monistic unified account of our reality. His project, overlapping significantly with that of Mach and Herring, sought to embed the developments of psychology within a theory of evolutionary development; evolution was the evolution of not just the species but also of the psyche.