Ernst Haeckel's The Riddle of the Universe is a bold turn-of-the-century attempt to unite science, evolution, and philosophy into one coherent vision of reality. Writing for readers willing to question inherited certainties, Haeckel challenges both narrow specialists and abstract metaphysicians, arguing that only monism can explain the deep "world-riddles" of existence. Drawing on his lifelong research, especially in zoology and human descent, he ranges from embryology and the human body to consciousness, the soul, immortality, and the origins of religion. He insists that consciousness is not a separate spiritual essence but a natural function of the brain, developing gradually in infancy, across the animal kingdom, and through human history alongside language and thought. He also traces the rise of modern science, from geology and physics to biology and anthropology, as it replaces miracle with lawful continuity. At the cosmic scale, he presents the universe as governed by eternal natural law, where worlds are born, evolve, and perish in endless cycles of matter and force. This edition has been fully restored with modern typesetting, custom cover design.