This book is the first detailed biography of Ernst Mayr. He was an ‘architect’ of the Synthetic Theory of Evolution, and the greatest evolutionary biologist since Charles Darwin, influential historian and philosopher of biology, outstanding taxonomist and ornithologist, and naturalist. He is one of the most widely known biologists of the 20th century. Mayr used the theories of natural selection and population thinking as theoretical models within the framework of historical biological studies. He suggested that various competing paradigms may exist side by side and more or less pronounced ’revolutions’ may occur in different fields from time to time. Changes of concepts have a much stronger effect on the development of biological sciences than the discovery of new facts. Mayr was the first to emphasize the role of biopopulations, thereby pointing out the basic difference between ’population thinking’ and typological essentialism. Population thinking takes into consideration the uniqueness of each individual and unlimited variation of populations which may lead to the development of new species. On the other hand, typologists assume that the unchanging essence of each species determines variation and fixed limits of variation preclude speciation from occurring except through saltation. Jürgen Haffer majored in geology and paleontology obtaining a PhD degree at the University of Göttingen. He became an exploration geologist and lived in South and North America, Iran, Egypt, and Norway. During these assignments he studied the bird faunas of Amazonia and Iran and has been in close communication with Ernst Mayr. He also co-published a biography of Erwin Stresemann, Mayr’s teacher and friend in Berlin, Germany.