Full Idea
Let any one examine his own thoughts, and he will find, that as soon as he supposes or speaks of essential, the consideration of some species, or the complex idea, signified by some general name, comes into his mind.
Gist of Idea
In our ideas, the idea of essence is inseparable from the concept of a species
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 3.06.04)
Book Reference
Locke,John: 'Essay Concerning Human Understanding', ed/tr. Nidditch,P.H. [OUP 1979], p.440
A Reaction
This wouldn't stop an individual having a distinct essence, if essences are distinctive combinations of these species qualities. Thus if my dog is particularly ferocious, it combines the species of dog and the species of ferocious in a unique way.