Full Idea
It seems to me that Plato in the later dialogues, beginning with the second half of 'Parmenides', wants to substitute a theory of genera and theory of principles that constitute these genera for the earlier theory of forms.
Gist of Idea
Plato moves from Forms to a theory of genera and principles in his later work
Source
report of Plato (Parmenides [c.366 BCE]) by Michael Frede - Title, Unity, Authenticity of the 'Categories' V
Book Reference
Frede,Michael: 'Essays in Ancient Philosophy' [University of Minnesota 1987], p.27
A Reaction
My theory is that the later Plato came under the influence of the brilliant young Aristotle, and this idea is a symptom of it. The theory of 'principles' sounds like hylomorphism to me.