Louis Agassiz was a Swiss naturalist of great prominence in the nineteenth century. He traveled to important intellectual centers in Europe, such as Germany, France and England, before he established himself at Harvard University in the United States. At Harvard he performed his vast work in natural history. The historiography of science has represented Agassiz as the strongest opponent to evolutionary ideas, particularly in relation to human ancestry, that way he was categorized as "anti-evolutionist" and "racist ". I believe the history of science needs to move beyond this representation. Although controversial, Agassiz has made significant contributions with his classification system known as three-fold parallelism. The method was to study the fossil records based on knowledge of geology, embryology and also through his geographical distribution of animal theses, failing to be based only on the system of the surface morphology. Thus, the triple parallelism method is intended to compare the bodies on the anatomy, embryology and geology, to discover the relationship between both the living and the extinct. The purpose of this paper is to review the controversies and the contributions from Agassiz to natural history in the nineteenth century. To this end, I work with sources of symbolic universe of scientific work: the letters written and read by Agassiz and by his interlocutors of private and professional life. The naturalist scientific practice also was the intellectual and creative activity of writing and reading letters. The textual content presents itself rich in cognitive science. Additionally, the correspondences may have the function of bringing the naturalist to reflect on the process of his work and about himself. Therefore, as different of the publications of scientific texts (books and articles), the letters give to the historian the potential to meet more broadly the development by which a scientist arrived at his discoveries and conclusions through reflection of his research practice in reading and writing of correspondences.