 HPC theory provides a more promising account of species as natural kinds than essentialism. HPC kinds need not have a common essential property, so the criticisms of species essentialism are avoided. Furthermore, HPC theory allows that external relations play a significant role in causing similarity among the members of a kind. Traditional essentialism assumes that the essence of a kind is an internal or intrinsic property of a kind’s members, such as the atomic structure of gold or the DNA of tigers. HPC theory is more inclusive because it recognizes that both the internal properties of organisms and the external relations of organisms are important causes of species-wide similarities. For instance, HPC theory but not essentialism cites interbreeding as a fundamental cause of similarity among the organisms of many species.