This book describes the establishment of the hypothesis that Charles Darwin's "natural selection," reformulated by R. A. Fisher, J. B. S. Haldane, and S. Wright in the light of Mendelian genetics, is the primary or exclusive mechanism for biological evolution. During the 1930s, alternatives such as Lamarchism, macromutations, and orthogenesis were rejected in favor of natural selection acting on small mutations, but there were disagreement about the role of random genetic drift in evolution. By the 1950s, research by T. Dobzhansky, E. B. Ford, and others persuaded leading evolutionists that natural selection was so powerful that drift was generally unimportant. This conclusion was accepted by most; however, a significant minority of biology textbooks and popular articles mentioned drift in the late 1960s.