 Biological species are often given as one of the classic examples of a “natural kind”. The philosophy of systematics has had a major influence on recent work on classification and natural kinds (Dupré 1993, 2002; Wilson et. al. 1997; Boyd 1999; Griffiths 1999; Wilson 1999; Okasha 2001; Walsh 2006). For example, can there be a notion of natural kind or essentialism consistent with the “population thinking” found in biology (Mayr 1975, Sober 1980; Ariew 2008)? 