The extension of the theory of evolution to man was, naturally, one of the most interesting and momentous applications of it. If all other organisms arose, not by a miraculous creation, but by a natural modification of earlier forms of life, the presumption is that the human race also was developed by the transformation of the most man-like mammals, the primates of Linné—the apes and lemurs. This natural inference, which Lamarck had drawn in his simple way, but Darwin had at first explicitly avoided, was first thoroughly established by the gifted zoologist, Thomas Huxley, in his three lectures on Man's Place in Nature (1863). He showed that this "question of questions" is unequivocally answered by three chief witnesses—the natural history of the anthropoid apes, the anatomic and embryological relations of man to the animals immediately below him, and the recently discovered fossil human remains. Darwin entirely accepted these conclusions of his friend eight years afterwards, and, in his two-volume work, The Descent of Man and Sexual Selection (1871), furnished a number of new proofs in support of the dreaded "descent of man from the ape." I myself then (1874) completed the task I had begun in 1866, of determining approximately the whole series of the extinct animal ancestors of the human race, on the ground of comparative anatomy, embryology, and paleontology. This attempt was improved, as our knowledge advanced, in the five editions of my Evolution of Man. In the last twenty years a vast literature on the subject has accumulated. I must assume that you are acquainted with the contents of one or other of these works, and will turn to the question, that especially engages our attention at present, how the inevitable struggle between these momentous achievements of modern science and the dogmas of the Churches has run in recent years.

It was obvious that both the general theory of evolution and its extension to man in particular must meet from the first with the most determined resistance on the part of the Churches. Both were in flagrant contradiction to the Mosaic story of creation, and other Biblical dogmas that were involved in it, and are still taught in our elementary schools. It is creditable to the shrewdness of the theologians and their associates, the metaphysicians, that they at once rejected Darwinism, and made a particularly energetic resistance in their writings to its chief consequence, the descent of man from the ape. This resistance seemed the more justified and hopeful as, for seven or eight years after Darwin's appearance, few biologists accepted his theory, and the general attitude amongst them was one of cold scepticism. I can well testify to this from my own experience. When I first openly advocated Darwin's theory at a scientific congress at Stettin in 1863, I was almost alone, and was blamed by the great majority for taking up seriously so fantastic a theory, "the dream of an after-dinner nap," as the Göttinger zoologist, Keferstein, called it.

The general attitude towards Nature fifty years ago was so different from that we find everywhere to-day, that it is difficult to convey a clear idea of it to a young scientist or philosopher. The great question of creation, the problem how the various species of plants and animals came into the world, and how man came into being, did not exist yet in exact science. There was, in fact, no question of it.

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.

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.

To these leading masters in biology, and to all other anatomists, physiologists, zoologists, and botanists up to 1858, the question of organic creation was an unsolved problem; the great majority regarded it as insoluble. The theologians and their allies, the metaphysicians, built triumphantly on this fact. It afforded a clear proof of the limitations of reason and science. A miracle only could account for the origin of these ingenious and carefully designed organisms; nothing less than the Divine wisdom and omnipotence could have brought man into being. But this general resignation of reason, and the dominance of supernatural ideas which it encouraged, were somewhat paradoxical in the thirty years between Lyell and Darwin, between 1830 and 1859, since the natural evolution of the earth, as conceived by the great geologist, had come to be universally recognised. Since the earlier of these dates the iron necessity of natural law had ruled in inorganic nature, in the formation of the mountains and the movement of the heavenly bodies. In organic nature, on the contrary, in the creation and the life of animals and plants, people saw only the wisdom and power of an intelligent Creator and Controller; in other words, everything was ruled by mechanical causality in the inorganic world, but by teleological finality in the realm of biology.

Philosophy, strictly so called, paid little or no attention to this dilemma. Absorbed almost exclusively in metaphysical and dialectical speculations, it looked with supreme contempt or indifference on the enormous progress that the empirical sciences were making. It affected, in its character of "purely mental science," to build up the world out of its own head, and to have no need of the splendid material that was being laboriously gathered by observation and experiment. This is especially true of Germany, where Hegel's system of "absolute idealism" had secured the highest regard, particularly since it had been made obligatory as "the royal State-philosophy of Prussia"—mainly because, according to Hegel, "in the State the Divine will itself and the monarchical constitution alone represent the development of reason; all other forms of constitution are lower stages of the development of reason." Hegel's abstruse metaphysics has also been greatly appreciated because it has made so thorough and consistent a use of the idea of evolution. But this pretended "evolution of reason" floated far above real nature in the pure ether of the absolute spirit, and was devoid of all the material ballast that the empirical science of the evolution of the world, the earth, and its living population, had meantime accumulated. Moreover, it is well known how Hegel himself declared, with humorous resignation, that only one of his many pupils had understood him, and this one had misunderstood him.

From the higher standpoint of general culture the difficult question forces itself on us: What is the real value of the idea of evolution in the whole realm of science? We are bound to answer that it varies considerably. The facts of the evolution of the individual, or of ontogeny, were easy to observe and grasp: the evolution of the crust of the earth and of the mountains in geology seemed to have an equally sound empirical foundation; the physical evolution of the universe seemed to be established by mathematical speculation. There was no longer any serious question of creation, in the literal sense, of the deliberate action of a personal Creator, in these great provinces. But this made people cling to the idea more than ever in regard to the origin of the countless species of animals and plants, and especially the creation of man. This transcendental problem seemed to be entirely beyond the range of natural development; and the same was thought of the question of the nature and origin of the soul, the mystic entity that was appropriated by metaphysical speculation as its subject. Charles Darwin suddenly brought a clear light into this dark chaos of contradictory notions in 1859. His epoch-making work, The Origin of Species, proved convincingly that this historical process is not a supernatural mystery, but a physiological phenomenon; and that the preservation of improved races in the struggle for life had produced, by a natural evolution, the whole wondrous world of organic life.

To-day, when evolution is almost universally recognised in biology, when thousands of anatomic and physiological works are based on it every year, the new generation can hardly form an idea of the violent resistance that was offered to Darwin's theory and the impassioned struggles it provoked. In the first place, the Churches at once raised a vigorous protest; they rightly regarded their new antagonist as the deadly enemy of the legend of creation, and saw the very foundations of their creed threatened. The Churches found a powerful ally in the dualistic metaphysics that still claims to represent the real "idealist philosophy" at most universities. But most dangerous of all to the young theory was the violent resistance it met almost everywhere in its own province of empirical science. The prevailing belief in the fixity and the independent creation of the various species was much more seriously menaced by Darwin's theory than it had been by Lamarck's transformism. Lamarck had said substantially the same thing fifty years before, but had failed to convince through the lack of effective evidence. Many scientists, some of great distinction, opposed Darwin because either they had not an adequate acquaintance with the whole field of biology, or it seemed to them that his bold speculation advanced too far from the secure base of experience.