BEFORE Sir Ronald Ross's epoch-making discovery, there was no more puzzling problem in medicine than the cause of malaria; no secret in Nature more cunningly hid than the malaria secret. Malaria was known to be connected with swamps, and to be reduced by drainage and cultivation. Yet, as if merely to confuse, men found that to flood some swamps actually improved health; and elsewhere that drainage and the cultivation of the soil produced the most serious and devastating outbursts of the disease. Yet again, malaria was found not only in swamps, but also often on hills and dry sandy deserts. Some jungle-covered land was singularly free from malaria: other jungle land was intensely malarial. In fact, malaria existed on soils of every conceivable variety, of every age in geological time. It was-impossible to point to any mineral, chemical, or vegetable condition essential to its presence. It was, and had been for hundreds of years, a dark, inscrutable mystery.