Full Idea
Modern essentialists would insist that any two members of the same natural kind must be identical in all essential respects.
Gist of Idea
For essentialists two members of a natural kind must be identical
Source
Brian Ellis (The Philosophy of Nature: new essentialism [2002], Ch.1)
Book Reference
Ellis,Brian: 'The Philosophy of Nature: new essentialism' [Acumen 2002], p.14
A Reaction
For this reason, animals no longer qualify as natural kinds, but electrons, gold atoms, and water molecules do. My sticking point is when anyone asserts that an electron necessarily has (say) its mass. Why no close counterpart of electrons?