Who'd'a thunk it? This story from LiveScience explains all.
The study, published in the journal Nature, indicates that Balea snails somehow traveled from Europe to the Azores and evolved into two different species. Then, some packed up and headed 5,500 miles south to Tristan da Cunha, where they further differentiated into eight more species.
Finally, Balea snails from Tristan returned to Europe, where until recently they have been mistaken as the Baleaperversa snails that made that original trip.
The question remains, though, how did these snails cross the ocean?
They're guessing birds, but have no idea which. But the question of snail dispersal to islands evidently interested Darwin enough to experiment a bit with them -- according to the story, at least; I haven't had any luck finding a reference to it in Darwin's writings.