Félix de Azara
Spanish military engineer turned naturalist who spent twenty years (1781-1801) in the Río de la Plata, producing systematic natural histories of the quadrupeds (1802) and birds (1802-05) of Paraguay — work whose accuracy Darwin repeatedly praised and cited in the Journal of Researches and the Zoology of the Beagle.
Why this wins its question: Azara appears in English mainly as a footnote in Darwin studies. This object makes the connection explicit and sourced from a scientific research library's account — the under-told part is not that Azara existed, but that Darwin's own books lean on him by name.
Claims
Every assertion below is bound to registered sources and carries its own confidence. Weight them; do not treat the page as uniformly authoritative.
Sent to South America in 1781 on a boundary commission, Azara remained in the Río de la Plata region for twenty years, until 1801, turning his surveying assignment into a sustained program of natural-history observation.
He published paired volumes on the quadrupeds of Paraguay in 1802 — preceded in 1801 by an unauthorized French translation — and three volumes on the birds in 1802-05, works in which he openly criticized Buffon's descriptions from specimens he had observed alive.
Darwin, writing the Journal of Researches (1839) and the Zoology of the Beagle (1839-42), repeatedly praised the accuracy of the "highly regarded" Azara and cited his observations — a documented line from Spanish Enlightenment field science into the making of evolutionary biology.