[In "The Map’] [t]his disjunction or questionable relation exists within language itself: does Bishop start by questioning the color differences on the map, or do alliteration and rhyme call "shallows" forth from "shadows" to generate the questions? In "Filling Station," Bishop exploits this process whereby words of similar sounds but different meanings trigger metaphysical speculation. The "dirty" family filling station, run by the father in a "dirty, / oil-soaked monkey suit" and ‘several quick and saucy / and greasy sons" – "all quite thoroughly dirty" – hums to a repetition of "oily" and "dirty" and insistent rhymes to them. When Bishop proceeds to the metaphysical question – "Why, oh why, the doily?" – the very question seems generate by the literal pattern of the poem: "doily" includes "oily." "Somebody embroidered the doily"; "Somebody / arranges the rows of cans so that they softly say: / ESSO – SO – SO – SO"; somebody set the poem humming to a rhyme of -y, as in dirty, oily and doily; "somebody loves us all." The questions and answers repeat and revise the dominant pattern of sounds found in the poem. Although they may rise from observation, they may also spring from the partly fortuitous and partly planned literal pattern of the poem; there is no way of telling which came first."
From Mutlu Konu Blasing, "The Re-Verses of Elizabeth Bishop," Chapter 6 in American Poetry: the Rhetoric of Its Forms (New Haven: Yale University Press, 1987), 107-108.