When I completed my seventieth year at Rapallo, on February 16th, I was overwhelmed with a mass of congratulations, letters, telegrams, flowers, and other gifts, most of which came from unknown readers of _The Riddle of the Universe_ in all parts of the world. If my thanks have not yet reached any of them, I beg to tender them in these lines. But I should be especially gratified if they would regard this work on the wonders of life as an expression of my thanks, and as a literary gift in return. May my readers be moved by it to penetrate deeper and deeper into the glorious work of Nature, and to reach the insight of our greatest German natural philosopher, Goethe: "What greater thing in life can man achieve Than that God-Nature be revealed to him?" ERNST HAECKEL. JENA, _June 17, 1904_. THE WONDERS OF LIFE THE WONDERS OF LIFE I TRUTH Truth and the riddle of the universe--Experience and thought--Empiricism and speculation--Natural philosophy--Science--Empirical science--Descriptive science--Observation and experiment--History and tradition--Philosophic science--Theory of knowledge--Knowledge and the brain--Æstheta and phroneta--Seat of the soul, or organ of thought: phronema--Anatomy, physiology, ontogeny, and phylogeny of the phronema--Psychological metamorphoses--Evolution of consciousness--Monistic and dualistic theories of knowledge--Divergence of the two ways of attaining the truth. What is truth? This great question has occupied the more thoughtful of men for thousands of years, and elicited myriads of attempts to answer it, myriads of truths and untruths. Every history of philosophy gives a longer or shorter account of these countless efforts of the advancing mind of man to attain a clear knowledge of the world and of itself. Nay, even "world-wisdom" itself, or philosophy in the proper sense of the word, is nothing but a connected effort to unite the general results of man's investigation, observation, reflection, and thought, and bring them to a common focus. Without prejudice and without fear, philosophy would tear the mantle from "the veiled statue of Sais," and attain a full vision of the truth. True philosophy, taken in this sense, may proudly and justly style itself "the queen of the sciences."