364 POPULAR SCIENCE MONTHLY. 



neficence 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 inher- 

 ited 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 na- 

 ture 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 mys- 

 terious as to foster superstition, and lead to the perpetuation of 

 the earlier notions about angry gods, offended Providence, judg- 

 ments, warnings, witchcraft, and so forth, long after the legiti- 

 mate 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 popu- 

 lous 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 signifi- 

 cant 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 exist- 

 ence of microbes, most people thought of them many still do 

 almost exclusively as inciters of disease. But this is a most nar- 

 row 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 vine- 



