Full Idea
For Plantinga, essences are entities in their own right and will have properties different from what instantiates them. Hence he will need individual essences of individual essences, distinct from the essences. I see no way to avoid a hierarchy of them.
Gist of Idea
Plantinga's essences have their own properties - so will have essences, giving a hierarchy
Source
comment on Alvin Plantinga (Actualism and Possible Worlds [1976]) by Robert C. Stalnaker - Mere Possibilities 4.4
Book Reference
Stalnaker,Robert C.: 'Mere Possibilities' [Princeton 2012], p.118
A Reaction
This sounds devastating for Plantinga, but it is a challenge for traditional Aristotelians. Only a logician suffers from a hierarchy, but a scientist might have to live with an essence, which contains a super-essence.
Related Ideas
Idea 16469 Plantinga has domains of sets of essences, variables denoting essences, and predicates as functions [Plantinga, by Stalnaker]
Idea 16471 I accept a hierarchy of properties of properties of properties [Stalnaker]