 While HPC theory is better at capturing the features of species than essentialism, does HPC theory provide an adequate account of species as natural kinds? Here are two potential problems with HPC theory. HPC theory’s objective is to explain the existence of stable similarities within groups of entities. However, species are also characterized by persistent differences. Polymorphism (stable variation within a species) is an important feature of nearly every species. Species polymorphisms are easy to find. Consider sexual dimorphism: within any mammalian species there are pronounced differences between males and females. Or consider polymorphism in the life cycles of organisms. The lives of organisms consist of dramatically different life stages, such as the difference between the caterpillar and butterfly stages of a single organism. HPC theorists recognize the existence of polymorphism, but they do not recognize polymorphism as a central feature of species in need of explanation. HPC theorists privilege and attempt to explain similarities. In addition to Boyd’s ‘homeostatic’ mechanisms we need to recognize ‘heterostatic’ mechanisms that maintain species variation. (For further discussion of polymorphism in species see Ereshefsky and Matthen 2005 and Magnus 2008.)