Full Idea
We know from biology that naturally occurring species do not exhibit the constancy required by the concept of natural kind.
Gist of Idea
Species do not have enough constancy to be natural kinds
Source
Harré,R./Madden,E.H. (Causal Powers [1975], 8.IV)
Book Reference
Harré,R/Madden,E.H.: 'Causal Powers: A Theory of Natural Necessity' [Blackwell 1975], p.150
A Reaction
This view has been challenged recently. How much constancy does a natural kind need? Even protons decay eventually, it seems. I think a natural kind just needs a fair degree of stability over a reasonable time-period. Tigers qualify.