Although not falling within our sphere, it would be wrong, too, not to recognize also the growth of medicine, especially through the knowledge of bacteria and their functions, and of disease germs and the methods of combating them. The world can never forget the debt it owes to Pasteur and Lister and many later investigators in this field. To follow out this subject further would be to encroach upon the field of the chapters following, but, more important and fundamental still than all the facts discovered and the phenomena investigated has been the establishment of certain broad scientific principles which have revolutionized modern thought and shown the relation between sciences seemingly independent. The law of conservation of energy in the physical world and the principle of material and organic evolution may well be said to be the greatest generalizations of the human mind. Although suggestions in regard to them, particularly the latter, are to be found in the writings of early authors, the establishment and general acceptance of these principles belong properly to the middle of the nineteenth century. They stand as the crowning achievement of the scientific thought of the period in which we are interested. Any mere enumeration of the vast fund of knowledge accumulated by the efforts of man through observation and experiment in the period in which we are interested would be a dry summary, and yet would give some measure of what this marvelous period has accomplished. As in geography, man’s energy has in recent years removed the reproach of a “Dark Continent,” of “unexplored” central Asia and the once “inaccessible polar regions,” so in the different departments of science, he has opened up many unknown fields and accumulated vast stores of knowledge. It might even seem as if the limit of the unknown were being approached. There remains, however, this difference in the analogy, that in science the fundamental relations—as, for example, the nature of gravitation, of matter, of energy, of electricity; the actual nature and source of life—the solution of these and other similar problems still lies in the future. What the result of continued research may be no one can predict, but even with these possibilities before us, it is hardly rash to say that so great a combined progress of pure and applied science as that of the past hundred years is not likely to be again realized. _Scientific Periodical Literature in 1818._