When we simply plow through Aristotle’s text changing every “what is” to “essence” we will be prone to fall into exactly the sort of error that is thoroughly discredited by the insights of evolution. In Aristotle’s system, the “what”, when said of a natural thing, refers to a compositeof matter and form; when said of a being without matter thing, it refers to form alone. The first “what” is inseparable from a change and flux so far as it is defined with matter, and so if we say that “the what” means “essence” than the essence of natural things is changeable. Changeability is not the whole of its essence (for form is essential to its essence too) but it is inseparable from it. In this sense of essence, the essences of the things the natural scientist studies are changeable. The difficulty is that “essence”, because of its absolute character, cannot mix with the idea of being changeable. The mind recoils from the idea of “changeable essence” as a contradiction, or at least an extremely poor choice of words. This is why when we find various good reasons to say that essences (or what we thought were essences) are not fixed, it is better to junk the idea of essence altogether. This is fine, but even after we junk the word “essence” we are still left with Aristotle’s “the what”, which, when applied to natural things, includes matter and is therefore capable of change.
But Aristotle also says that nature is more form than matter, does he not, even though it is made up of both? And matter is proportioned to form -- so how much change can a thing tolerate before its potencies are affected, and it no longer has the same potencies as before?