Full Idea
One could be an essentialist about natural kinds (of tigers, or water) while holding that every actual instance or sample of a natural kind is only accidentally an instance or a sample of that kind.
Gist of Idea
Maybe the identity of kinds is necessary, but instances being of that kind is not
Source
Penelope Mackie (How Things Might Have Been [2006], 10.2)
Book Reference
Mackie,Penelope: 'How Things Might Have Been' [OUP 2006], p.173
A Reaction
You wonder, then, in what the necessity of the kind consists, if it is not rooted in the instances, and presumably it could only result from a stipulative definition, and hence be conventional.