In her comprehensive Henry David Thoreau: A Life, Walls—who has previously written about Thoreau’s “turn to science”—calls attention to the pivotal moment when he began to use his journal as he never had before. On November 8, 1850, a year or so after his naturalist’s regimen had begun, Thoreau “wrote up everything he noticed and thought during his daily walk as one long entry.” He did the same the next day, and two days later, Walls notes, and then again a couple of days after that, and the next day,

filling pages with a stream-of-consciousness flow of words as if he were writing while walking: “I pluck,” “I heard,” “I saw yesterday,” “I notice.”

“And this is what truly staggers the mind,” Walls goes on. “From this point, Thoreau did not stop doing this, ever—not until, dying and almost too weak to hold a pen, he crafted one final entry.”

University of Chicago

A week after that first extended entry, he wrote, “I feel ripe for something; it is seed time with me—I have lain fallow long enough.” Thoreau went on, “My Journal should be the record of my love.” At the same time, his journal was a repository of constant measurements, minute and expansive: of the depth of streams, the wingspan of a moth, the number of bubbles trapped beneath the frozen surface of the pond. “What are these pines & these birds about? What is this pond a-doing? I must know a little more,” Thoreau had written back in 1846, when his journal had still been a source to plunder for other writing projects, not yet a compendium of exhaustive field notes. Now his quest for unifying order became more focused, and he set out to pursue it by counting the petals on a blossom or the rings in the stump of a fallen tree—hoping not to lose a sense of beauty and mystery in the process.

The tension between the particular and the whole wasn’t new. Transcendentalists like Emerson were searching for unity in nature, but resisted what seemed to them the blinkered reliance on deductive reasoning and empirical research enforced by encroaching science. Such methods tended to “cloud the sight,” Emerson said, and he endorsed instead a conception of nature as “the symbol of spirit.” That Emersonian notion of natural phenomena as the embodiment of what his mentor called “ideas in the mind of God” had once thrilled Thoreau, as Walls writes. But by the time Thoreau reoriented his life, he needed more direct contact with the “marrow of nature.” Thoreau had already framed the poet-scientist dilemma in 1842, when he reviewed a series of natural-history reports published by the State of Massachusetts: How could such dry summaries hold any interest for the general reader? Where, Thoreau asked in his review in the Transcendentalist literary magazine The Dial, was the joy of nature?

Reading Humboldt’s most popular books, Cosmos, Views of Nature, and Personal Narrative, during his evenings of study, Thoreau learned a way of weaving together the scientific and the imaginative, the individual and the whole, the factual and the wonderful. A vast array of observations, Humboldt insisted, revealed “unity in diversity”—each fact and detail of nature threading together into an interconnected whole. Even before he adopted his systematic regimen, Thoreau’s journal—packed with observations about the songs of birds, the chirping of crickets, the careless pace of the fox, the scent of musk, the “dreamy motions” of fish’s fins—was proof of his visceral relationship to nature. In Thoreau and the Language of Trees, the writer Richard Higgins describes Thoreau sniffing the bark of twigs, listening to the creaking of hardwoods in winter, sampling the taste of lichens (he liked rock tripe and Iceland moss best), delighting in the play of light and shadow in the canopy of trees.

University of California

“We must look a long time before we can see,” Thoreau had concluded in his Dial essay on the “Natural History of Massachusetts,” pronouncing that “the true man of science … will smell, taste, see, hear, feel, better than other men.” Moving beyond Emerson’s grand and spiritual ideas of nature, Thoreau became part of a lively scientific discourse, aware of the latest discoveries, and he used the libraries at Harvard and the Boston Society of Natural History extensively. He collected fish specimens for the zoologist and geologist Louis Agassiz at Harvard. And though he was a little squeamish about gathering birds’ eggs for another scientist there, he agreed to commit “deliberate murder” if the advancement of science demanded it.