When developing phylogenetic systematics, the entomologist Willi Hennig adopted elements from Nicolai Hartmann’s ontology. In this historical essay I take on the task of documenting this adoption. I argue that in order to build a metaphysical foundation for phylogenetic systematics, Hennig adopted from Hartmann four main metaphysical theses. These are (1) that what is real is what is temporal; (2) that the criterion of individuality is to have duration; (3) that species are supra-individuals; and (4) that there are levels of (...) reality, each of which may be subject to different kinds of law. Reliance on Hartmann’s metaphysics allowed Hennig to ground some of the main theoretical principles of phylogenetic systematics, namely that the biological categories—from the semaphoront to the highest rank—have reality and individuality despite not being universals, and that they form a hierarchy of levels, each of which may require different kinds of explanation. Hartmann’s metaphysics thereby provided a philosophical justification for Hennig’s phylogenetic systematics, both as a theory and as a method of classification. (shrink)