Sigmund Freud is my dear close personal friend, but somewhat obsessed with S-E-X, and not surprisingly, he says that the experience of the uncanny, which is different and more subtle than terrifying or horrifying, has to do with confronting repressed fears, usually sexual. He writes about the recurrent theme of losing one's eyes in the short story, "The Sandman," which I've never read, but I know there is indeed something particularly uncanny in this; I used to feel a little shudder of horror at a lullaby that ran in part, "Birds and the butterflies, pecking at his eyes." In one version of Cinderella, crows peck out an eye from each of the stepmother and stepsisters as they enter the church to watch Cinderella's wedding, and then upon their exit, return to eat the other eye.
R Crumb's strangely bloodless eye sockets in as Eggs stares futiley at his paper creep us out, as does the lump of one eyeball descending the crow's throat. Freud, predictably, says this sort of thing represents castration anxiety, and, well, okay... what the hell, why not. You can't deny there's some sort of sexual revenge going on in Cinderella, and we needn't doubt that Crumb, who digs the toe-jam from the darkest corners of the id - incest, murder, canabalism, cacphagy (look it up) - might have a sexual subtext at work. But with respect to Dr. Freud (Siggy, we used to call him in grade school) I think he might be barking up the wrong complex.
The German word for the uncanny is unheimlich, which means something like un-homely. When Dorothy tells Toto they're not in Kansas anymore, she's expressing the German idea of the uncanny. Freud does some brilliant etymological analysis leading to the conclusion that the root heimlich means both itself, "homely," and its opposite - "secret," and so unheimlich, also is its own antonym. The uncanny, for Freud, is the unfamiliar, which points us back to that which is familiar, but repressed.
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The ending isn't scary in the normal way - it's not as if the new mother shows any ability to harm them, it's just that it's so uncanny. It's illogical, it makes no sense, it isn't possible.
Freud would say this is sexual too, and ok, if that's the way he wants it, but I'll cling to my notion that the uncanny has more to do with the potential for terrible unknowableness than the fear of a wiener-snipping. At the beginning of Night of the Living Dead a brother and sister are arguing as they visit their father's grave (Okay, Freud, shut up, you've had your say!) And in the background, way against the horizon, a shuffling figure meanders slowly, weaving in and out of our field of vision. He's so far off, we can barely see him. And we know, we just know, he's a zombie. It doesn't make sense that he should be, there's no reason for a dead person to be up and walking around - and that's what makes it uncanny. One more example, Gregor Samsa, as rule-bound and reality principled as any man who ever walked, wakes up from a troubled dream to find himself changed into a monstrous vermin. This is the uncanny and our peculiar horror of it. When we were very, very little we struck a bargain with the universe that we would be have reasonably in accordance with its reason, but what if the universe never kept its side of the deal, and after all, why should we expect it to? What if instead of an orderly if unsatisfactory clockwork of cause and effect, and this therefore that, and one-thing-at-a-time, it's a bedlam of eyeball-eating crows and mothers with glass eyes and wooden tails?