Sentence:  When in a meadow, or, as now, in a quiet, already <WSD>duskening</WSD> wood, he would involuntarily begin to wonder if, through this silence, he might perhaps hear the entire, enormous world traversing space with a melodious whistle, the bustle of distant cities, the pounding of sea waves, the singing of telegraph wires above the deserts. 
Meaning of the word marked with <WSD> tags:  To grow or become (dusky).