One of my favorite novels is Susanna Clarke's Jonathan Strange & Mr. Norrell. When I attempt to explain the book to my friends, I start with the setting: England during the Napoleonic wars. "So it's a historical novel." Well, not exactly. There are droll little commentaries throughout, kind of like in Jane Austen, gently poking fun at the rules of middle-high society. "So it's a comedy of manners." Well, a little, except there's also the fact that, in this version of Georgian England, magic is real. "So it's a fantasy novel." Well, a little, but also not really. Because magic, when the novel opens, is a stuffy academic discipline, embraced by stuffy older gentlemen who find the idea of practicing magic to be vulgar. "Okay, so... umm..." And just as I'm losing them, I usually add in, "And the best part? It's full of FOOTNOTES! Explanations of things and citations to scholarly works written by gentleman-magicians that the author is also making up! It's Brilliant!"
At this point, they usually try to find a way to shift the conversation. But that weird elision of genres turns out to be where Anthony Grafton ends up in his chapter four, "An Enlightenment Interlude." Sliding back a step chronologically, Grafton takes his search for the origins of the footnote from the nineteenth century of Ranke to the eighteenth century of the philosophes. And what he finds is surprising: "Footnotes, in short, spread rapidly in eighteenth-century historiography in part because they were already trendy in fiction." 
Here's the thing: I, perhaps like most people, associate the literature of the enlightenment with the Voltaires and Swifts: scholars more interested in being incisive than being scientific. But Grafton shows how the eighteenth and early nineteenth centuries were actually home to two traditions in scholarly literature: the broadly gesturing and often satirical philosophe, and the erudite antiquarian. That the former is better known -- perhaps "won" the contest for whom the century would be associated with -- is a foregone conclusion. But the fact that the former felt obliged to constantly puncture the pretensions (as they saw it) of the latter points to the widespread existence of the latter.
And in the ultimate irony, the vehicle that the former used to carry their satires and excoriations was, more often than not -- you guessed it -- the footnote itself.
From the prose of Gibbon to the pornographers of Grub Street the eighteenth-century footnote is not one thing. It is a genus with several species: some are citations to sources (of varying degree of precision), some are explanations or clarifications that the erudite address to the ladies, some are snark, and some appear to be entirely made up, used as signifiers to lend the illusion of fact to what was undoubtedly fiction, in a melding of the two as a joke that the reader was expected to be in on.
Had they only existed, Susanna Clarke's scholar-magicians would have been delighted.
 Grafton, 121.
makes a reappearance here as well, as the bastard offspring of the two
traditions, grand narrator above the line, but alternately pedant and
satirist below the line. Sort of the scholarly-literary equivalent of a mullet.