A first-person record of a scientific corps' riverine expedition to collect and classify microscopic parasitic organisms associated with regional vegetation. The narrative combines detailed fieldwork—equipment, methods, daily life aboard a steam vessel—and character sketches of the investigators with encounters in remote environments. Alongside methodical natural history observations, the party encounters a mysterious native people whose forms of communication, including claimed thought dictation and unusual revelations, challenge the explorers' empirical assumptions and prompt broader reflections on the relationship between humans and other organic forms.