Full Idea
If things were distinguished into species according to real essences, it would be impossible to find different properties in two individual substances of the same species, as it is to find different properties in two circles or two equilateral triangles.
Gist of Idea
If we based species on real essences, the individuals would be as indistinguishable as two circles
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 3.06.08)
Book Reference
Locke,John: 'Essay Concerning Human Understanding', ed/tr. Nidditch,P.H. [OUP 1979], p.443
A Reaction
Of course circles or triangles can differ in size. Locke was greatly impressed by individual variation in creatures ('monsters'). My cat isn't just any old cat. Species essentialism must at least acknowledge more than mere essences.