Origin of "In which ..."

Solution 1:

In the 19th century, chapter titles in novels frequently started with "In which". Consider Uncle Tom's Cabin.

-1. In which the Reader is introduced to a Man of Humanity
-2. The Mother
...
-9. In which it appears that a Senator is but a Man
-10. In which Propriety gets into an Improper State of Mind

So three of the 18 chapters here begin with "in which". Other authors used this as well, including Henry Fielding, Charles Dickens, Robert Louis Stevenson, and William Makepeace Thackeray. I have no idea who originated it.

Solution 2:

This looks like a title of a narration. The full form would be something like:

The story in which I form the suspicion that I am not Nature's intended audience.

Solution 3:

Ellipsis, "(grammar, rhetoric) omission of a grammatically required word or phrase that can be inferred", is the most likely explanation of how such titles became common. For example, if you do an ngrams lookup for in which and click on the 1500-1681 books link, and at that page click the second link, you'll see a title page dated 1673, with a 40-word title that is, in broad strokes, "TWO LETTERS: [etc.] In which most things of Note [are] handled." This illustrates the convention of mentioning a form (e.g. book, missive, chapter) and then briefly summarizing its contents with an "In which" clause. Chapter titles of form "In which x" are shorter versions of "This is a chapter in which x", formed from the latter by ellipsis.