 Following Quine, some realists have expanded upon his intuition that a natural kind’s members share a natural property by counting families of such properties that are contingently clustered in nature (Boyd 1991, 1999a; Millikan 1999). These families of properties cluster together over time either because the presence of some properties in the family favours the presence of others or because there are underlying internal mechanisms and/or extrinsic contextual mechanisms which tend to secure the co-occurrence of the properties. A natural kind is any such family of co-occurring properties that may be employed in inductive inferences or for the purposes of scientific explanation. Cluster kind realists will readily concede that, depending on the case, environmental pressures may affect and alter the set of properties associated with a kind over time. Therefore, in such cases, none of the properties themselves need be individually necessary for kind membership.