"Anatomy of Monotony" is a poem from the second edition (1931) of Wallace Stevens's first book of poetry, Harmonium.
Yet the spaciousness and light In which the body walks and is deceived, Falls from that fatal and barer sky, And this the spirit sees and is aggrieved.
The poet conceives us as evolving and increasingly civilized products of an earthly process.
The spirit sees this and is aggrieved, for it would harbor experience in some place that transcends nature, free from the contingencies of earth and universe.
This reading is supported by the naturalistic tenor of the Harmonium collection as a whole, and specifically by the parallel of "Invective Against Swans".