Walt Whitman - One of the Human Kinks lyrics

How is it that in all the serenity and lonesomeness of solitude, away off here amid the hush of the forest, alone, or as I have found in prairie wilds, or mountain stillness, one is never entirely without the instinct of looking around, (I never am, and others tell me the same of themselves, confidentially,) for somebody to appear, or start up out of the earth, or from behind some tree or rock? Is it a lingering, inherited remains of man's primitive wariness, from the wild animals? or from his savage ancestry far back? It is not at all nervousness or fear. Seems as if something unknown were possibly lurking in those bushes, or solitary places. Nay, it is quite certain there is—some vital unseen presence.