Natural Kinds
Links
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'.
Natural Kinds and Projectible Predicates
http://sowi.iwp.uni-linz.ac.at/Sorites/Natural_Kinds.html
In this article Axel Mueller applies some reflections upon Goodman's paradox to the problem of natural kinds.
Properties
http://plato.stanford.edu/entries/properties/
Entry in the Stanford Encyclopaedia of Philosophy by Chris Swoyer. Principally concerned with existence and identity conditions.