It is impossible to estimate the effect of Darwin's book on _The Origin of Species_, published in English in 1858, in German in 1859 unless we fully realize how completely the biologists of that time had turned away from general problems. I can only say that we, who were then the younger men, studying in the fifties, had no idea that a theory of evolution had ever been put forward, for no one spoke of it to us, and it was never mentioned in a lecture. It seemed as if all the teachers in our universities had drunk of the waters of Lethe, and had utterly forgotten that such a theory had ever been discussed, or as if they were ashamed of these philosophical flights on the part of natural science, and wished to guard their students from similar deviations. The over-speculation of the 'Naturphilosophie' had left in their minds a deep antipathy to all far-reaching deductions, and, in their legitimate striving after purely inductive investigation, they forgot that the mere gathering of facts is not enough, that the drawing of conclusions is an essential part of the induction, and that a mass of bare facts, however enormous, does not constitute a science. One of my most stimulating teachers at that time, the gifted anatomist, Jacob Henle, had written as a motto under his picture, 'There is a virtue of renunciation, not in the province of morality alone, but in that of intellect as well,' a sentence which expressly indicated the desirability of refraining from all attempts to probe into the more general problems of life. Thus the young students of that time were nourished only on the results of detailed research, in part indeed interesting enough, but in part dry and, because uncorrelated, unintelligible in the higher sense, and only here and there awakening a deeper interest, when, as in physiology and in embryology, they formed a connected system in themselves. Without being fully clear as to what was lacking, we certainly missed the deeper correlation of the many separate disciplines. It is therefore not to be wondered that Darwin's book fell like a bolt from the blue; it was eagerly devoured, and while it excited in the minds of the younger students delight and enthusiasm, it aroused among the older naturalists anything from cool aversion to violent opposition. The world was as though thunderstruck, as we can readily see from the preface with which the excellent zoologist of Heidelberg, Bronn, introduced his translation of Darwin's book, where he asks this question among others, 'How will it be with you, dear reader, after you have read this book?' and so forth. But before I enter on a detailed examination of the contents of this epoch-making book, I should like to say a few words about the man himself, who thus revolutionized our thinking.