You must fight the last boss with 3 special souls equipped: Flame Demon, Giant Bat, & Succubus.
Then when you beat him, the game will not end...instead you will get instructions to go back to the black mist door--and it will open for you now. --- There is no theory of evolution, just a list of creatures that Chuck Norris allows to live.
^ The three ancient books. I got the souls wrong though, due to the wording on the third book. It says beautiful "Nightmare", which I thought was referring to Nightmare (since horses are often described as beautiful and since Succubi employ pleasant dreams instead of nightmares).
I couldn't figure out what to do with them because having two bullet souls was throwing me off. In any event, it's not very clear what you're supposed to do.