 HPC kinds are more than groups of entities that share stable clusters of similarities. HPC kinds also contain “homeostatic causal mechanisms” that are responsible for the similarities found among the members of a kind. The members of a biological species interbreed, share common developmental programs, and are exposed to common selection regimes. These “homeostatic mechanisms” cause the members of a species to have similar features. Dogs, for instance, tend to have four legs and two eyes because they share genetic material and are exposed to common environmental pressures. An HPC kind consists of entities that share similarities induced by that kind’s homeostatic mechanisms. According to Boyd, species are HPC kinds and thus natural kinds because “species are defined … by … shared properties and by the mechanisms (including both ”external“ mechanisms and genetic transmission) which sustain their homeostasis” (1999b, 81).