At various times, Thoreau earned a living by lecturing or working at his family's pencil factory. He was not a talented speaker, but he had a natural gift for mechanics. According to Henry Petroski, Thoreau discovered how to make a good pencil out of inferior graphite by using clay as the binder; this invention improved upon graphite found in New Hampshire in 1821 by Charles Dunbar. Later Thoreau converted the factory to producing plumbago, used to ink typesetting machines. Frequent contact with minute particles of graphite may have weakened his lungs.
After 1850 he became a land surveyor, "traveling a good deal in Concord," and writing natural history observations about the 26 mile² (67 km²) township in his Journal, a two million word document that he kept for 24 years. He also traveled to Quebec once, Cape Cod twice, and Maine three times, landscapes that inspired his "excursion" essays, A Yankee in Canada, Cape Cod, and The Maine Woods, in which travel itineraries frame his thoughts about geography, history, and philosophy. He began writing the Journal at Emerson's suggestion. His first entry on October 22, 1837 reads, "'What are you doing now?' he [Emerson] asked. 'Do you keep a journal?' So I make my first entry today."
Hailed as an early American environmentalist, Thoreau wrote essays on autumnal foliage, the succession of forest trees, and the dispersal of seeds, collected in Excursions. Scientists regard these works as anticipating ecology, the study of interactions between species, places, and seasons. He was an early advocate of recreational hiking and canoeing, of conserving natural resources on private land, and of preserving wilderness as public land. Thoreau was also one of the first American supporters of Darwin's theory of evolution. Although he was not a vegetarian, he ate relatively little meat and advocated vegetarianism as a means of self-improvement.
Thoreau was not without his critics. Scottish author Robert Louis Stevenson judged Thoreau's endorsement of natural simplicity over the tangles of modern society to be a mark of effeminacy: "...Thoreau's content and ecstasy in living was, we may say, like a plant that he had watered and tended with womanish solicitude; for there is apt to be something unmanly, something almost dastardly, in a life that does not move with dash and freedom, and that fears the bracing contact of the world. In one word, Thoreau was a skulker. He did not wish virtue to go out of him among his fellow-men, but slunk into a corner to hoard it for himself. He left all for the sake of certain virtuous self-indulgences." However, English novelist George Eliot, writing in the Westminster Review, characterized such critics as uninspired and narrow-minded: "People—very wise in their own eyes—who would have every man's life ordered according to a particular pattern, and who are intolerant of every existence the utility of which is not palpable to them, may pooh-pooh Mr. Thoreau and this episode in his history, as unpractical and dreamy." Throughout the 19th century, Thoreau was dismissed as a cranky provincial, hostile to material progress. In a later era, his devotion to the causes of abolition, Native Americans, and wilderness preservation have marked him as a visionary.