The Swiss-American naturalist Jean Louis Rodolphe Agassiz (1807-1873) giving a lecture. Agassiz studied with enthusiasm and detail fossil fishes and between 1833-1844 he published all his observations in five volumes. Agassiz first proposed and then verified that glaciers slowly move and also that in the past they occupied vast areas during the existence of an Ice Age. In 1846 Agassiz moved to the United States and in 1861 he became an American citizen. He always opposed the evolutionary theory proposed by Darwin, although his work on fossils helped to estabilish it.