I would then beg him to explain particularly difficult points in anatomy, and once I ventured to put the question: "Must not all these vertebrates, with their identity in internal skeleton, in spite of all their external differences, have come originally from a common form?" The great master nodded his head thoughtfully, and said: "Ah, if we only knew that! If ever you solve that riddle, you will have accomplished a supreme work." Two months afterwards, in September, 1854, I had to accompany Müller to Heligoland, and learned under his direction the beautiful and wonderful inhabitants of the sea. As we fished together in the sea, and caught the lovely medusæ, I asked him how it was possible to explain their remarkable alternation of generations; if the medusæ, from the ova of which polyps develop to-day, must not have come originally from the more simply organised polyps? To this precocious question, I received the same resigned answer: "Ah, that is a very obscure problem! We know nothing whatever about the origin of species." Johannes Müller was certainly one of the greatest scientists of the nineteenth century. He takes rank with Cuvier, Baer, Lamarck, and Darwin. His insight was profound and penetrating, his philosophic judgment comprehensive, and his mastery of the vast province of biology was enormous. Emil du Bois-Reymond happily compared him, in his fine commemorative address, to Alexander the Great, whose kingdom was divided into several independent realms at his death. In his lectures and works Müller treated no less than four different subjects, for which four separate chairs were founded after his death in 1858--human anatomy, physiology, pathological anatomy, and comparative anatomy. In fact, we ought really to add two more subjects--zoology and embryology. Of these, also, we learned more from Müller's classic lectures than from the official lectures of the professors of those subjects. The great master died in 1858, a few months before Charles Darwin and Alfred R. Wallace made their first communications on their new theory of selection in the Journal of the Linnæan Society. I do not doubt in the least that this surprising answer of the riddle of creation would have profoundly moved Müller, and have been fully admitted by him on mature reflection.