 A second concern with HPC theory involves the identity conditions of species. The members of a species vary in their traits. Moreover, they vary in their homeostatic mechanisms. Over time and across geographic regions, the members of a single species are often exposed to different homeostatic mechanisms. Given such variation, what causes organisms with different traits and exposed to different homeostatic mechanisms to be members of the same species? The common answer is genealogy: the members of a species form a continuous genealogical entity on the tree of life. A species’ homeostatic mechanisms are mechanisms of one species because they affect organisms that form a unique lineage. Boyd and promoters of HPC theory recognize the importance of genealogy and see historical relations as one type of homeostatic mechanism. However, Boyd does not see genealogy as the defining aspect of species, and this goes against a fundamental assumption of biological systematics: species are first and foremost continuous genealogical entities. Boyd is quite clear that similarity and not genealogical connectedness is the final arbitrator of species sameness (1999b, 80). This assumption makes sense given that Boyd believes that species are kinds and kinds are ultimately similarity-based classes that play a role in induction. But this view of the identity conditions of species conflicts with the standard view in biological systematics that species are continuous genealogical lineages (Ereshefsky 2007).