 Another response to the species are individuals thesis is offered by proponents of an alternative approach to natural kinds. According to Boyd (1999a, 1999b), Griffiths (1999), Wilson (1999), Millikan (1999), and Wilson et al. (2007), species are natural kinds on a proper conception of natural kinds. These authors adopt Boyd’s Homeostatic Property Cluster (HPC) theory of natural kinds. HPC theory assumes that natural kinds are groups of entities that share stable similarities. HPC theory does not require that species are defined by traditional essential properties. The members of Canis familiaris, for example, tend to share a number of common properties (having four legs, two eyes, and so on), but given the forces of evolution, no biological property is essential for membership in that species. For HPC theory, the similarities among the members of a kind must be stable enough to allow better than chance predictions about the properties found among the members of a kind. For instance, given that we know that Lizzy is a dog, we can predict with greater than chance probability that Lizzy will have four legs.