Randall Fuller in Nature:
One night in 1851, Henry David Thoreau woke from a dream. In it, astride two ungovernable horses — literal nightmares — he galloped through the woods, “but the horses bit each other and occasioned endless trouble and anxiety, and it was my employment to hold their heads apart”. At that time, the 34-year-old naturalist–writer was trying to reconcile two contending forces in his life: the transcendentalism he had long espoused and the rigours of science he had recently discovered. Transcendentalism emerged in the mid-1830s as an intoxicating set of philosophical, literary and spiritual tendencies unified by discontent with American life. Its core tenet, derived from Romantic philosophy, was that God permeated everything. As the movement's best-known spokesperson, philosopher Ralph Waldo Emerson, put it, “behind nature, throughout nature, spirit is present”. Transcendent divinity could be perceived only through intuition and inspiration — not through reason — and was best found in nature. This set of beliefs fuelled the experiment in self-reliance and simple living on Walden Pond in Concord, Massachusetts, in the mid-1840s that led to Thoreau's masterpiece, Walden, published in 1854. But something happened to Thoreau before his best-known book was finished, subtly changing its final form, making it more empirical, and thus scientific. Always fascinated by the natural world, he began to ponder more on its physical processes. Every day, rain or shine and usually for at least four hours, he tramped through the woods and fields surrounding his native Concord, collecting specimens and writing down what he observed in a series of small notebooks. He measured and weighed, pressing the leaves of red currant, poison sumac and many other species for his herbarium. In the evening, he transferred his observations into larger journals. Occasionally he worried about this new tendency. “I fear,” he wrote on 19 August 1851, “that the character of my knowledge is from year to year becoming more distinct and scientific — that, in exchange for views as wide as heaven's cope, I am being narrowed down to the field of the microscope.” He tried to fit the data he gathered into a larger, transcendentalist vision of the cosmos, but his focus was inexorably drawn to the material world.
In 1860, he encountered Charles Darwin's On the Origin of Species, published the previous year. He was already familiar with Darwin, having devoured The Voyage of the Beagle (1839) almost a decade earlier. Thoreau first learnt of Origin at a dinner party on New Year's Day 1860, a gathering that included radical abolitionist Franklin Sanborn, child-welfare reformer Charles Loring Brace and transcendental philosopher Bronson Alcott. This quartet of progressive intellectuals discussed the book at length, and Thoreau was immediately captivated.