Louis Agassiz (Jean Louis Rodolphe Agassiz) (zhäN lwē rôdôlf´ ăg´əsē), 1807–73, Swiss-American zoologist and geologist, b. Môtiers-en-Vuly, Switzerland. He studied at the universities of Zürich, Erlangen (Ph.D., 1829), Heidelberg, and Munich (M.D., 1830). Agassiz practiced medicine briefly, but his real interest lay in scientific research. In 1831 he went to Paris, where he became a close friend of Alexander von Humboldt and studied fossil fishes under the guidance of Cuvier. In 1832 he became a professor of natural history at the Univ. of Neuchâtel, which he made a noted center for scientific study. Among his publications during this period were Recherches sur les poissons fossiles (5 vol. and atlas, 1833–44), a work of historic importance in the field (although his system of classification by scales has been discarded); studies of fossil echinoderms and mollusks; and Étude sur les glaciers (1840), one of the first expositions of glacial movements and deposits, based on his own observations and measurements.
Agassiz came to the United States in 1846 and two years later accepted the professorship of zoology and geology at Harvard. His first wife died in Germany in 1848, and in 1850 in Cambridge he married Elizabeth Cabot Cary (see Agassiz, Elizabeth Cabot Cary). In the United States he was primarily a teacher and very popular lecturer. His influence extended to the fields of zoology, paleontology, geology, anatomy, and glaciology. Emphasizing advanced and original work, he gave major impetus to the study of science directly from nature and influenced a generation of American scientists. His extensive research expeditions included one along the Atlantic and Pacific coasts of the Americas from Boston to California (1871–72). His Contributions to the Natural History of the United States (4 vol., 1857–62) includes his famous "Essay on Classification," an extension of the theory of recapitulation to geologic time. Despite his own evidences for evolution, Agassiz opposed Darwinism and believed that new species could arise only through the intervention of God.
See biographies by J. Marcou (including letters, 1896), J. D. Teller (1947), E. Lurie (1960, repr. 1967), and C. Irmscher (2013); L. Cooper, Louis Agassiz as a Teacher (rev. ed. 1945).