I came across this in The Voyage of the Beagle (1839):

The expression of this [Trigonocephalus] snake’s face was hideous and fierce; the pupil consisted of a vertical slit in a mottled and coppery iris; the jaws were broad at the base, and the nose terminated in a triangular projection. I do not think I ever saw anything more ugly, excepting, perhaps, some of the vampire bats. I imagine this repulsive aspect originates from the features being placed in positions, with respect to each other, somewhat proportional to the human face; and thus we obtain a scale of hideousness.

I added it to a new section of the Wikipedia article about the uncanny valley, but I thought someone around here might also find it interesting.

I wonder if there were other similar descriptions before Darwin.