Seventy-seven years ago Alexander von Humboldt delivered, in this very spot, the lectures which afterwards made up his famous work, _Cosmos, the Elements of a Physical Description of the World_. As he touched, in passing, the obscure problem of the origin of the organic population of our planet, he could only say resignedly: "The mysterious and unsolved problem of how things came to be does not belong to the empirical province of objective research, the description of what _is_." It is instructive to find Johannes Müller, the greatest of German biologists in the nineteenth century, speaking thus in 1852, in his famous essay, "On the Generation of Snails in Holothurians": "The entrance of various species of animals into creation is certain--it is a fact of paleontology; but it is _supernatural_ as long as this entrance cannot be perceived in the act and become an element of observation." I myself had a number of remarkable conversations with Müller, whom I put at the head of all my distinguished teachers, in the summer of 1854. His lectures on comparative anatomy and physiology--the most illuminating and stimulating I ever heard--had captivated me to such an extent that I asked and obtained his permission to make a closer study of the skeletons and other preparations in his splendid museum of comparative anatomy (then in the right wing of the buildings of the Berlin University), and to draw them. Müller (then in his fifty-fourth year) used to spend the Sunday afternoon alone in the museum. He would walk to and fro for hours in the spacious rooms, his hands behind his back, buried in thought about the mysterious affinities of the vertebrates, the "holy enigma" of which was so forcibly impressed by the row of skeletons. Now and again my great master would turn to a small table at the side, at which I (a student of twenty years) was sitting in the angle of a window, making conscientious drawings of the skulls of mammals, reptiles, amphibians, and fishes.