That Machen should exert should a powerful influence over Lovecraft (who praises him profusely in his Supernatural Horror in Literature) is no surprise. The two men shared a great deal, perhaps chief among them being the "scientific" grounding of their weird tales. That is, both Lovecraft and Machen shared the belief that, far from liberating mankind from fear, science would in fact make him ever more aware of just how terrifying the cosmos actually was. It is this theme (among others) that The Great God Pan uses as its foundation and develops to remarkable effect.
The story begins as two men, Raymond and Clarke, discuss upcoming brain surgery to be performed on a young woman named Mary. Raymond wishes to perform the surgery in order to vindicate his theories about the universe:
"Look about you, Clarke. You see the mountain, and hill following after hill, as wave on wave, you see the woods and orchard, the fields of ripe corn, and the meadows reaching to the reed-beds by the river. You see me standing here beside you, and hear my voice; but I tell you that all these things—yes, from that star that has just shone out in the sky to the solid ground beneath our feet—I say that all these are but dreams and shadows; the shadows that hide the real world from our eyes. There is a real world, but it is beyond this glamour and this vision, beyond these 'chases in Arras, dreams in a career,' beyond them all as beyond a veil. I do not know whether any human being has ever lifted that veil; but I do know, Clarke, that you and I shall see it lifted this very night from before another's eyes. You may think this all strange nonsense; it may be strange, but it is true, and the ancients knew what lifting the veil means. They called it seeing the god Pan."The surgery goes off as Raymond intends and the consequences, though tragic for Mary, are exactly as he expected:
"She will awake in five minutes." Raymond was still perfectly cool. "There is nothing more to be done; we can only wait."The story then picks up years later, as several young men are dying under mysterious circumstances. The only connection between these deaths is that all of the men is a young woman named Helen Vaugh, who is described as "at once the most beautiful woman and the most repulsive they had ever set eyes on." One need not read the story to know that the prelude to The Great God Pan, involving the surgery done to poor Mary, and the later events involving Helen Vaughn are also connected, but I won't say here the nature of that connection, for fear of lessening the impact of the tale.
The minutes passed slowly; they could hear a slow, heavy, ticking. There was an old clock in the passage. Clarke felt sick and faint; his knees shook beneath him, he could hardly stand.
Suddenly, as they watched, they heard a long-drawn sigh, and suddenly did the colour that had vanished return to the girl's cheeks, and suddenly her eyes opened. Clarke quailed before them. They shone with an awful light, looking far away, and a great wonder fell upon her face, and her hands stretched out as if to touch what was invisible; but in an instant the wonder faded, and gave place to the most awful terror. The muscles of her face were hideously convulsed, she shook from head to foot; the soul seemed struggling and shuddering within the house of flesh. It was a horrible sight, and Clarke rushed forward, as she fell shrieking to the floor.
Three days later Raymond took Clarke to Mary's bedside. She was lying wide-awake, rolling her head from side to side, and grinning vacantly.
What's impresses me still is that, even though the central mystery of the story is one we have likely seen before, given how often Machen is pastiched (mainly through the influence of Lovecraft, who himself took more than a few cues from his older contemporary), it still possesses great power. Like Lovecraft, Machen has a tendency toward unnecessarily complex constructions, purple prose, and somewhat flat characters, but his ideas are so vibrant that they overcome all these flaws. Reading The Great God Pan is an unsettling experience and I wish I could say precisely why. As I said, the story, though original, even scandalous, in its time, has been copied so often in the last century or more that one would think it would no longer pack much punch -- and yet it does. If you've never read it before, I recommend doing so; I'll be curious to hear if anyone else finds it as disturbing as I do.