I'm afraid there's a blatant answer to your question and that is that the prof had no idea of those kind of things modern biology tells us - or didn't bother about thinking of them in relation to his mythology; they are not something to concern a person writing a mythology anyway.
But the world "after the fall" looks like familiar post-books of Moses-world where the humans have fallen to the primordial sin and everything's different from the paradise that once was... Maybe decaying had no part in the natural renewal of the nature in the West?
