Greatly influenced by Charles Darwin, the famed German zoologist and philosopher Ernst Haeckel (1834-1919) boldly defended the fact of organic evolution and seriously considered its far-reaching ramifications for science, philosophy, and theology. Advocating the interplay of empirical evidence and rational speculation, The Riddle of the Universe is his most daring, comprehensive, and successful work. Its monistic and naturalistic worldview offers a cosmic perspective and evolutionary framework that supplants traditional theistic beliefs in God, free will, and the personal immortality of the human soul. This classic volume remains a tour de force of critical thought, free inquiry, and intellectual value.