Page:Popular Science Monthly Volume 48.djvu/396

364 through forces which we may ourselves administer as we go about our usual tasks.

When we group diseases according to causation, we find that many are due to excesses—excesses in food, in drink, in drugs, in exercise, in indolence, in work, in rest, in play. Sometimes the body labors under adverse influences which we do not understand. Sometimes the organism is handicapped from the start by inherited defects. Sometimes subtle poisons which the body itself forms are not properly disposed of, and the organism suffers.

In spite of the vast stores of experience with the human body in disease, clear down to the early part of the last decade, the nature and cause of some of the most common and fatal human maladies were practically unknown. I speak of the diseases which we call infectious. These were so widespread and so mysterious as to foster superstition, and lead to the perpetuation of the earlier notions about angry gods, offended Providence, judgments, warnings, witchcraft, and so forth, long after the legitimate presence of such developmental phases in human thought had passed away. Man had gone on making up inventories of his visible life neighbors in the world, had traced their pedigrees and relationships, had found how closely their evolution was bound to his own, and had got the systems pretty well completed, when his surprised attention was called to an inconceivably populous world of beings lying far beyond the reach of the unaided vision and closely woven into the chain of life. And shortly the so-called "germ theory" of disease had ripened into a well-grounded body of positive knowledge, so far-reaching and significant that more than anything else it has seemed to dominate medical science for more than a decade, and has led to most beneficent practical results in the prevention and the cure of disease.

For a good while after attention had been called to the existence of microbes, most people thought of them—many still do—almost exclusively as inciters of disease. But this is a most narrow conception. The amount of material on the earth available for the temporary uses of living things is limited, and it is largely through the intervention of these lowly but active organisms that when once the material, which for a little while has come under the sway of the life forces, has fallen into death, it is rescued and worked over into available life stuff again. And so, chained over to the material, the cycle of life on the earth is maintained only through these swarming hordes which serve all animate things in serving their own necessities.

We can to-day trace their presence through geologic ages, sharing in the life of earlier times. We are but just beginning to realize their vast economic importance in agriculture, in the