Here, the speakers thoughts have been interrupted, "Yes: but," by a disturbing presence or has had a sudden realization of the delicate state of his mortality. And this idea is "striding" closer and closer to the speaker as though it were an impending doom. The deep green of the hemlock tree is a contrast to the leaves and the feathers of the peacocks. The hemlock, in this poem seems to mock the speakers mortality because hemlock trees are big and can outlive any human being.