Full Idea
There are reasons to believe that there are natural kinds that might never be instantiated, such as a transuranic element, capable of existing for some fraction of a second, but which has never actually existed anywhere.
Gist of Idea
There might be uninstantiated natural kinds, such as transuranic elements which have never occurred
Source
Brian Ellis (Scientific Essentialism [2001], 2.05)
Book Reference
Ellis,Brian: 'Scientific Essentialism' [CUP 2007], p.81
A Reaction
He cautiously claims that kinds are ontologically prior to their individual members. I would say that there is no natural kind of the type that he describes. He says you have at least some grounds for predicting what kinds are possible.