Other than that, I have nothing to add except that last week I was discussing this very issue over lunch with a colleague. He's heard me say before the H.P. Lovecraft is a post-Darwinian author, and he added that Howard is "post-Nietzschean." Yes, that's what you'd hear if you overheard two English professors talking at Ruby Tuesdays.
So, without any other context,
- H.P. Lovecraft = post-Darwinian
- Robert E. Howard = post-Nietzschean