Full Idea
Kripke's tiger example shows that a nominal essence is not necessary for the existence of a natural kind; examples from Putnam show that a nominal essence is not sufficient either.
Gist of Idea
Nominal essence may well be neither necessary nor sufficient for a natural kind
Source
report of Saul A. Kripke (Naming and Necessity lectures [1970]) by Alexander Bird - Philosophy of Science Ch.3
Book Reference
Bird,Alexander: 'Philosophy of Science' [UCL Press 2000], p.101
A Reaction
None of the characteristics of a tiger is essential to it. The appearance of water doesn't fix its reference. The move is towards an external view, that what matters for natural kinds is the real essence, not human conventions about it. I agree.