The chief question posed by this extensive interpolated passage and the associated changes made to accommodate it in the text is the simple one of why. Why did Tolkien feel the need to add this complication to the story, the only major addition between the manuscript and typescript phases of the book? It’s possible that he wanted to insert some folklore motifs (enchanted sleep, the white deer, the dire consequences of breaking a fairy-tale prohibition no matter what the temptation, echoes of the wild hunt) to make the dwarves’ passage through Mirkwood even more eerie and unsettling than it already was, and quite distinct from the earlier journey in the dark through the Misty Mountains. Certainly the additions help enhance Bilbo’s standing by providing another example of his usefulness with appropriate tasks in the episode of securing the boat, and they develop Bombur’s character by making him a complainer and a drag upon the party, even more of a weak link than little Bilbo.1
The most likely answer, I think, is that the passage was created to pay a narrative debt once Tolkien realized he had incurred one. While he was dealing with that, he seems to have decided to settle several other points that had occurred to him after finishing the story, explaining why the dwarves didn’t hunt food when they were starving (especially since the original draft already mentioned the black squirrels), or use the bows to defend themselves against the spiders or the elves.
Back in what ultimately became Chapter VII, Medwed warned his guests of the dangers they would face in Mirkwood:
[Y]our road through the forest is difficult and dangerous . . . Water is not easy to find there, nor food. For the time is not come for nuts which is all there is growing there that can be eaten . . . I will provide you with skins for carrying water, which you had better fill before you enter the forest. You will see one stream, a strong black one, if you hold to the path, but I doubt if it is good to drink. I have heard that it carries enchantment, and brings a frightful drowziness. I will give you four bows and arrows, but I doubt if you will shoot anything in the dim shadows of that place, without straying from the path – which you MUST NOT DO. And I doubt if it would be good to eat if you shot it.
—pp. 241–2 (emphasis mine).
There is no mention of the enchanted stream in the first rough outline (p. 229) nor in the Plot Notes describing their adventures in Mirkwood (Plot Notes A, pp. 294–6) which Tolkien used as his pre-draft guide when actually composing the chapter, but Medwed’s warning suggests that this was an episode Tolkien intended to include and then simply forgot about. Re-reading the book as he was creating the First Typescript, he seems to have noticed the dropped motif and decided to recast the chapter to include it; hence the interpolation. While he was at it, he accounted for the dwarves not shooting food when they were starving by letting them try with the squirrels and discover them inedible, just as Medwed had predicted (‘I doubt [anything] would be good to eat if you shot it’).
The scene with the dark hart and white hind rather neatly first precipitates the disaster of Bombur’s dunking and subsequent enchantment and then renders the bows useless when the desperate dwarves expend their last arrows.2 And, although this may be unintentional on Tolkien’s part, the revised text fits into the ‘rule of three’ that he consciously applies elsewhere (the three attempts to approach the feasting elves, Bilbo’s three descents into Smaug’s lair)3 with the three warnings Gandalf’s party receive. On Bladorthin’s advice, the dwarves keep their first promise, to send Medwed’s horse back. They try to keep their second promise not to drink from the enchanted stream, and only the thirteenth dwarf does so and then only by mischance, itself the result of an uncanny, unexpected intrusion; while inconvenienced the group is able to continue on with their quest afterwards. The third promise, to stay on the path, they finally break in extremis, and it brings disaster on their heads just as Bladorthin predicted:
‘Don’t stray off the track – if you do it is a thousand to one you’ll never find the path again, or ever get out of Mirkwood; and then I don’t suppose I (or anyone else) will hear of you again!’
— p. 244.
However, had Tolkien kept to the manuscript version of events the wizard would have proved a false prophet, for Bilbo does find the path again, rather easily, and after rescuing his friends leads them back to it as well (see ‘The Theseus Theme’, pp. 337–8). With the revision, Medwed’s and Bladorthin’s dire warnings more closely resemble the results.
Finally, a few miscellaneous points. The ‘little black boat’ itself is interesting, since we are never given any indication whose boat it is nor how it came to be there. If the elves placed it there to replace the fallen bridge, it’s surprising they would prefer to bring an oar with them (a seven days’ journey) each time they wanted to cross the stream, rather than simply leaving one with the boat. Usually the narrator either provides the reader with information where the characters are stumped or admits to his own ignorance on a given point; here there is only silence. All we really know about the boat is that it is small (just large enough for four dwarves at a time), black, and has laid undisturbed for some time (since the painter snaps).4 Thorin’s prudence of standing guard lest ‘any hidden guardian of the boat’ appear not only shows him well-versed in fairy-tale tradition but proves justified by the event, though there’s nothing to indicate that the hart itself, for all the misfortune it brings upon them, is anything but a fugitive from the elven hunt.