Félix de Azara y Perera, an Aragonese military engineer stationed in the Río de la Plata region between 1781 and 1801, produced during that period one of the most rigorous naturalist works of the 18th century on South American fauna. His Apuntamientos (1802–1805) document 448 bird species and dozens of mammals through direct field observation, describing morphological and behavioral variation among populations with unprecedented precision. What is singular about Azara is not only the exhaustiveness of the data: from his observations, he explicitly abandons the creationist and fixist models dominant in the 18th century — shared by Buffon, his theoretical reference — upon detecting differences between individuals of the same species that did not respond to superficial causes but to internal ones. Azara does not formulate the mechanism of natural selection — that step is Darwin's — but he empirically documents that species can change, introducing the possibility of variation where the Newtonian paradigm demanded closed order. Darwin explicitly cites him in On the Origin of Species (1859) as a source of data on variation, behavior, and interactions between species in Paraguay, and relies on his observations as empirical evidence. The direct citation confirms transmission: Darwin read Azara and relied on him. However, Azara's role in the history of biology is reduced to that of an empirical informant, without recognition as a conceptual precursor who broke with fixism before any naturalist of his generation. His work circulated mainly in French translation (Voyages dans l'Amérique méridionale, 1809), and his Spanish authorship was subsumed under the European naturalist corpus.