In the third decade of the nineteenth century, biology reached out again. Schleiden and Schwann demonstrated that cells are the structural units of every organism, whether animal or plant. This discovery, denounced in no uncertain terms as an impossible theory for several years, revised our whole concept of living things. It was, perhaps, more important as a philosophical innovation than as the opening investigation of the field of research now termed cytology; nevertheless the study of the cell has yielded concrete results of extraordinary value, considering the limited period during which it has been carried on. Our leading investigator in this department of knowledge, Edmund B. Wilson, has happily had the individual experience of seeing the establishment of the theory that all cells come from pre-existing cells; of seeing the proper description of cell division and of fertilization; the identification of the cell nucleus as the most important cell organ; the discovery that the chromosomes, with their peculiarly exact methods of division, are the material basis of heredity; and finally, of seeing the determination of the architecture of the germ plasm and of the mechanism by which the units of heredity, the genes, are transmitted to the succeeding generation.

Physiology, as I have said, had its birth in the researches of Harvey; but for two centuries and a half it led a rather moribund existence. It was restored to vigor by the genius of Johannes Miiller (1801-1858), of whom Verworn said:"He is one of those monumental figures that the history of each science brings forth but once."Miiller was not only a great investigator, he was a most stimulating teacher. Du Bois-Reymond and Helmholz are two of the distinguished scholars whom he led into scientific work. Miiller's contributions to physiology were prodigious, yet he is revered to-day less for the specific results of his researches than for his innovations in physiological methodology. He made controlled experiments, he studied animal behavior, he introduced quantitative measurements, he interpreted results both in the terms of physics, chemistry, and mathematics, and in the terms of psychology. In fact, it was from Muller's laboratory that there came the first recognition of the close association between the operations of the mind and the functioning of the central nervous system.

Physiology, in the seventy-two years since Muller's death, has probably made the greatest progress of any of the biological subdivisions. The introduction of quantitative methods permitted the statistical study of sensation, of tropistic response, and even of more complex types of behavior. It stimulated the more precise investigation of all sorts of bodily functions. It gave us a more accurate conception of the body as machine, and showed us how to accelerate or retard the wheels of the various parts. It showed us, moreover, the interdependence of these different parts and demonstrated the unity of the organism as a whole. It made possible such apparent miracles as the development of the egg without fertilization. It budded off colloidal chemistry and biophysics. It was the starting point of immunology and serology, those Siamese twins of science that have revolutionized medicine. Most important of all, because of the effect such demonstrations have on belief in the universal applicability of the scientific method, it proved the influence of the secretions of the body upon behavior.

One might dwell upon the rise of comparative anatomy, and histology, and embryology, and paleontology, and anthropology, upon the development of more natural systems of classification, upon life histories, upon geographical distribution. These subjects certainly are important. The knowledge such departments give us of the world in which we live is sound. It delivers a history whose accuracy is more to be depended upon than those social chronicles which come to us under that name. One could justify such studies even on economic grounds. But these sub-sciences - sciences of the classical type, as Ostwald calls them - have had little influence, by themselves, in changing man's intellectual habits. But this is not to their discredit. Collectively they furnished the evidence by which Charles Darwin proved the fact that Organic Evolution had occurred, a generalization which has done more to overthrow bigotry and install rationalism than any other. And when I use the words proved and fact, I use them intentionally. Only those unfamiliar with the evidence have the impudence to claim that the Evolution Concept is an undemonstrated hypothesis.

Darwin showed how the chaotic series of facts relating to the geological succession of organisms, to geographical distribution of forms, to morphological similarities, to persistence of vestigial organs, to the recurrence of embryo-logical phases in such distinct groups as mammals, birds, and reptiles, to change of form under domestication,were each and all, individually and collectively, brought into intelligible order only through the idea of evolution. Since these biological facts number hundreds of thousands, since they include the entire compendium of biological knowledge, in truth, I venture to call Darwin's generalization the greatest effort of the human mind. It is true that we do not know all that we should like to know about the way in which evolution took place, but that is another story; of the phenomenon itself we are assured.

The evolution concept stimulated every line of biological work. Through its guidance thousands of problems - upon which numerous observations had been made, but where the meaning of these observations was obscure - were problems no longer. Their solution was at hand. But these results, despite their value, were not the most significant effects. The impact of the idea upon the human mind is what has really counted. When the notion of the genetic unity of living things was once completely grasped, it became apparent that the temptation to set apart certain human problems as definitely beyond the reach of the inquiring mind was simply a manifestation of the primitive interdiction of taboo. Measured thus, the value of Darwin's work became incalculable. Single-handed he had rent the veil of mysticism which had impeded human progress from the beginning of time. No longer was it possible to maintain a holy of holies into which the scientist was not permitted to enter. It must be admitted that after sixty years there is still a battle line defending the hallowed ground; but it is sadly harassed, and its morale is low. In places an entry has been made, and the invaders have consolidated their gains. The results are seen in sociology, genetics, and psychology. The historical researches of the sociologists - showing, as they have, that moral ideas are matters of custom - have opened up the way to a more acceptable framework of society based on reason and knowledge. The geneticists have solved the mechanical problem of heredity, and have gained a clear idea of the factors involved in somatic development. Thus a whole category of problems has been removed from the realm of the mysterious, taken to mundane precincts, and arranged ready for systematic attack. The psychologists have made sufficient progress to demonstrate that the activities of the mind have a physical basis. They have shown how easily sensation, perception, and even intelligence may be measured. They are fast coming to the point where they can tell us not only how we learn and when we learn, but also what we do with our learning and what our learning does with us. It is not too much to say that, within another generation or so, the most serious problem the human race can ever face, mental adjustment to reality, will have had at least a formal solution.