(alphabetical by author)

Burroughs, John. “The Grist of the Gods” from Leaf and Tendril (1898): from the original book or as reprinted in Bill McKibben, ed., American Earth, pp. 159-168.

Darwin, Charles. The Decent of Man, Chapter 5 on "Moral Sense" (1874 edition). Available online here.

Darwin, Charles. from Voyage of H.M.S. Beagle (1836): “Retrospect on Our Voyage,” selection reprinted in Elder & Finch, eds., Nature Writing: The Tradition in English (pp. 155-159)

Descartes, Rene. Sixth Meditation. Available online here.

Emerson, Ralph Waldo. Nature (1836): the two opening sections, entitled “Introduction” and “Nature” (about 5 pp. total).

Hume, David. An Enquiry Concerning the Principles of Morals. Available here.

Malthus, Thomas, An Essay on the Principle of Population, Chapters 1-2. Available online here.

Marsh, George. From Man and Nature, “Principle Causes of the Destruction of the Forest” and “American Forest Trees,” pp. 270-278. Available online here.

Muir, John. “A Wind-Storm in the Forests” (from The Mountains of California, 1894).
 
Seton, Ernest Thompson. From Wild Animals I Have Known (1898): selections from “Note to the Reader” and “Lobo” as reprinted in Elder & Finch, eds., Nature Writing: The Tradition in English (pp. 305-312).

Thoreau, Henry David. Walden (1854): “Where I Lived, and What I Lived For” and “Conclusion”; Journals (any brief selection – we’ve provide a Google link to a full volume, read any page or two to get a sense of the style).
 
Twain, Mark. Life on the Mississippi (1883): chapter XI (about 6 pages); Adventures of Huckleberry Finn (1884): chapter XIX, first two pages.

Whitman, Walt. From Leaves of Grass (1891-1892 edition): “Song of Myself,” sections 1, 14, 31, 32, 52.