The Formation of Vegetable Mould Through the Action of Worms (Barnes & Noble Digital Library)

Darwin had been intrigued by the earthworm for forty years, but it wasn’t until 1881 that he produced the volume that would illuminate this “unsung creature which, in its untold millions, transformed the land as the coral polyps did the tropical sea.” The volume, which focused on the fascinating behavior and ecology of the earthworm, sold thousands of copies in its first weeks.