Iceland has not been known as a contributor to the history of science. This small nation in the North-Atlantic has only in recent decades made its mark on international science. But the Icelandic naturalist Thorvaldur Thoroddsen (1855-1921) is an exception to this generalisation, for he was well known at the turn of the 20th century in Europe and America for his research on the geography and geology of Iceland. Though Thoroddsen's contribution to these sciences is of great interest there is (...) another very interesting side to his career. This is his transformation from "mechanist" to "vitalist," which is the main focus of this paper. Thoroddsen, who began his career as a follower of Darwinism and Haeckelian monism, started to express his worries about materialism in society in 1902 culminating in a full-fledged denial of materialism in politics and science in 1910. Thoroddsen's transformation is in its nature a very complex process, but overall it can be traced to his view that materialism was destroying the Danish and Icelandic societies. In the years before 1902 Thoroddsen was a monistic atheist, relatively liberal in his politics, a firm believer in the power of the natural sciences, and considered the laws of physics the key to human progress. His views were strikingly similar to the ones Haeckel expounded in "The Riddle of the Universe" (1899), whose reception in Germany is explored in this paper in order to contextualise Thoroddsen's transformation. Between 1902 and 1910, however, Thoroddsen rejected this worldview in favour of conservative politics and ontological dualism, becoming a staunch believer in a higher, godly, authority. The theories of Darwin and Haeckel were now obsolete and needed to be replaced by the idea of purpose, direction and the final goal of life (Teleology). In order to understand Thoroddsen's transformation, I focus on the interaction of scientific knowledge, politics and other cultural issues in his writings. This is done by looking at Thoroddsen's views on biology, on the natural sciences in general, and on politics as published in Icelandic articles, manuscripts and letters in the period 1872 to 1910. I show that the transformation was due to influences from the German holistic movement, Thoroddsen's engagement with the Royal elite in Denmark and its political concerns, new developments in physics, and from personal traumas and professional disappointments. I also show that Thoroddsen's transformation seems to be a good example of the social shaping of scientific knowledge. (shrink)