Full Idea
Leibniz points out that it is a concealed premise of Locke's argument that if a natural kind exists it must have many instances, but there seems no a priori objection to the idea of a species with just one member.
Gist of Idea
Locke needs many instances to show a natural kind, but why not a single instance?
Source
report of Gottfried Leibniz (New Essays on Human Understanding [1704], 6.6.311) by Nicholas Jolley - Leibniz and Locke on Essences p.200
Book Reference
'Leibniz: Critical and Interpretive Essays', ed/tr. Hooker,Michael [Manchester 1982], p.200
A Reaction
I can't see this bothering Locke. Generally we formulate nominal essences by induction from bundles of ideas, but we can formulate a cautious first stab at it from one instance. If you see a new creature, is it a normal one, or a 'monster'?