THE RAPID DEVELOPMENT OF MEDICINE IN THE LAST FIFTY YEARS.--THE INFLUENCE OF DARWIN.--PREVENTIVE MEDICINE.--THE DISSEMINATION OF MEDICAL KNOWLEDGE.--THE DEVELOPMENT OF CONDITIONS IN RECENT YEARS WHICH ACT AS FACTORS OF DISEASE.--FACTORY LIFE.--URBAN LIFE.--THE INCREASE OF COMMUNICATION BETWEEN PEOPLES.--THE INTRODUCTION OF PLANT PARASITES.--THE INCREASE IN ASYLUM LIFE.--INFANT MORTALITY.--WEALTH AND POVERTY AS FACTORS IN DISEASE. Certain conditions have arisen in the past fifty years which have profoundly affected the thoughts, the beliefs and the activities of man. Within this period what is generally known as Darwinism, including under this evolution, has developed. Unlike theories which came from philosophical speculation only, the theory of evolution was one which could be subjected to observation and experiment. It freed man's mind from dogmas, it stimulated the imagination, it enlarged the territory in which it seemed possible to extend knowledge by the methods of science, and has resulted in an enormous increase of knowledge. This has been more striking in medical science than elsewhere, and in this of more far-reaching influence. Evolution coincided with another important development. History shows that all great periods of civilization have at their back sources of energy. In the civilizations of the past such sources of energy have come from the enslavement of conquered peoples or from commerce, or more direct forms of robbery, which have enabled a favored class to appropriate for its purposes the results of the work of others. While these sources have not been absent in the development of our civilization, the great source of energy has come from the rapid, and usually wasteful and reckless, utilization of the stored energy of the earth. The almost incredible advance in medical and other forms of scientific knowledge and the utilization of this knowledge is largely due to the greater forces which we have become possessed of. Disease plays such a large part in the life of man and is so closely related to all of his activities that the changes in this period must have exerted an influence on disease. We have already seen that within the period we have obtained knowledge of the causes of disease and the conditions under which these causes became operative. The mystery which formerly enveloped disease is gone; disease is recognized as due to conditions which for the most part are within the control of man, and like gravity and chemical attraction it follows the operation of definite laws. There has been developed within the period what is known as preventive medicine, which aims rather at prevention than cure, and the resources of prevention are capable of much greater extension.