I'm thinking of de Quincey and his swerve into the subject of crocodiles during The English Mail Coach. He achieves this swerve by, one, talking about coaches, then, two, talking about coachmen, then, three, comparing one specific coachmen to a crocodile (they both have "a monstrous inaptitude for turning round"), then, four, saying that "all things change or perish," then, five, connecting that idea to crocodiles, "Mr. Waterton tells me that the crocodile does not change -- that a cayman, in fact, or an alligator, is just as good for riding upon as he was in the time of the Pharaohs," then, six, continuing with Egypt, crocodiles, Mr. Waterton, and the idea of crocodiles being ridden. "The use of the crocodile has now been cleared up -- it is to be ridden; and the use of man is, that he may improve the health of the crocodile by riding him a fox-hunting before breakfast."
By now the author has moved a long way from English mail coaches but he gets back by picking up change again: "Perhaps, therefore, the crocodile does not change, but all things else do: even the shadow of the pyramids grows less. And often the restoration in vision of Fanny and the Bath road, makes me too pathetically sensible of that truth." So he has an idea, change, not-change, and attaches unusual things to that idea: here, crocodiles: the thought stays steady, the basic drive endures, but the decoration is unexpected.
The mind, I'm guessing, high, loose, curious, ready to take the unusual thing when it finds it and continue with that thought, not stopping at a simple statement like, "The coachman was like a crocodile" (which he could have done, or not mentioned crocodiles, never compared the coachman to a crocodile, compared him to an elephant instead, compared him to a rock or an oven, compared him to nothing, never mentioned him, never written the essay, never written anything, died at birth, been illiterate, been literate but lost both hands in an accident at the age of five, been literate but had wet cow hide stitched over his hands to dry and cripple him as happened to one of the two captains in charge of the Mongol ships that tried to invade Japan and were wrecked in the thirteenth century -- the great Khan not forgiving of this escapade --), but willing to find out what else crocodiles can do. What comes into his head when he thinks about crocodiles? He thinks, he wonders, he recalls Charles Waterton, an Englishman who rode a cayman and wrote about it in his book Wanderings in South America, the North-West of the United States, and the Antilles, in the years 1812, 1816, 1820, and 1824; with original instructions for the perfect preservation of birds, &c., for cabinets of natural history (1825) -- Waterton says --
By the time the cayman was within two yards of me, I saw he was in a state of fear and perturbation: I instantly dropped the mast, sprang up, and jumped on his back, turning half round as I vaulted, so that I gained my seat with my face in a right position. I immediately seized his fore-legs, and by main force twisted them on his back; thus they served me for a bridle.
He now seemed to have recovered from his surprise, and probably fancying himself in hostile company, be began to plunge furiously, and lashed the sand with his long and powerful tail. I was out of reach of the strokes of it, by being near his head. He continued to plunge and strike, and made my seat very uncomfortable. It must have been a fine sight for an unoccupied spectator.
And it's possible that the idea of crocodiles being hard to turn round came to him from Waterton's story in the first place, in other words it might have come to him before he mentioned Waterton in the essay. Thinking of the coachman struggling to turn around he might have had a memory of the paragraph about this cayman trying to dislodge the Englishman from its back: that beast plunging and lashing the sand. The animal in Waterton's story does not sound like a nimble turner, not mobile and lissom, more like an angry plank, and so de Quincey, remembering or seeing that the coachman had "a monstrous inaptitude for turning round," might have considered Waterton's cayman.
Therefore he compares the coachman to a crocodile. But he doesn't bring Waterton's name into the writing straight away; he spends the rest of the coachman paragraph describing himself and the coachman's daughter Fanny. "In fact, the utter shadowyness of our relations to each other, even after our meetings through seven or eight years had been very numerous, but of necessity had been very brief, being entirely on mail-coach allowance -- timid, in reality, by the General Post-Office -- and watched by a crocodile belonging to the antepenultimate generation." Waterton is in abeyance, then Waterton is there: "The Fannies of our island -- though this I say with reluctance -- are not improving; and the Bath road is notoriously superannuated. Mr. Waterton tells me that the crocodile does not change ..." but that name, Waterton, might have been lurking in the atmosphere throughout the story of Fanny in the previous paragraph and we didn't know, we couldn't know, it's impossible to know.
Charles Waterton fits into de Quincey's essay and helps him onwards but the essay could have been different, in another life de Quincey might not have read Waterton or heard of Waterton, Waterton might have died on his cayman and never written the book about his Wanderings (instead he waited for several decades before he tripped on a bramble, fell on a log, and perished on a chair, an ending not more strange than the death of someone we knew who lived through rural starvation in the American Depression, then endured World War II, then died this year by falling off the bottom rung of a ladder) which means that de Quincey would never have written him into the essay about the Mail Coach and how arbitrary is Waterton (now that I think about it, I who had taken him for granted before, inevitable feature, like the mountains, which are only there because the earth kicks up, and they move, says Francis Ponge, they move, but so slowly it's inhuman, alien regression from the pitch of creation, wearing away in the wind --): what an arbitrary thing he is, but nailed into de Quincey's essay. In fact the Wikipedia page about the essay says that "Perhaps the most memorable and frequently-cited portion of Part I is De Quincey's comparison of one veteran mail-coachman to a crocodile."* I don't know how they know that this is true.
* So it says as of today, August 25th, 2012. Someone might edit that away in the future. Today, though, it says it.