Natural Kinds
Links
Properties
http://plato.stanford.edu/entries/properties/
Entry in the Stanford Encyclopaedia of Philosophy by Chris Swoyer. Principally concerned with existence and identity conditions.
Category Induction and Representation
http://cogsci.soton.ac.uk/~harnad/Papers/Harnad/harnad87.categorization.html
Online text of a chapter by Stevan Harnad in `Categorical Perception: The Groundwork of Cognition'.