" Darwin’s point about variation often goes unappreciated today in philosophical discussions, even though it has been uncontroversial for well over a century. Recent discussions of natural kinds, prompted by the seminal ideas of Saul Kripke and Hilary Putnam, often assume that one can revive essentialism. Yet if species are natural kinds no such revival is in prospect. Kripke and Putnam largely restricted their discussions to the cases of elements and compounds, and with good reason. For, given the insights of neo-Darwinism, it’s clear that the search for some analogue of the microstructural essences can’t be found. No genetic or karyotypic property will play for species the role that atomic number does for the elements. "

― Philip Kitcher