A great many of our sanitary measures, if now witnessed by our ancestors of three thousand years ago, would probably seem perfectly natural and proper. The fumigation of ships and buildings would seem a very proper mode of burning incense to the hostile god who was causing yellow fever. The modern man, his descendant, is burning the pyrethrum to kill the living spirit, the mosquito, which he has found by experiment really causes the disease. Our ancestor would accept as a perfectly natural explanation the use of oil on the stagnant waters if he were told that this was a libation to propitiate the angry god who was inflicting on man malarial fevers. His descendant is using the oil really to kill the living beings, mosquito larvæ, which cause malaria. As man advanced in intelligence and civilization he threw aside his belief in spirits and numberless higher beings as causes of disease among his fellowmen. He now began to attribute disease to abnormal conditions in the blood and tissues of the body, caused by unfavorable environment, such as filth, food, clothing, climatic conditions, etc. His efforts for the prevention of disease during all this period had little or no effect. It is probable that through long ages the human race remained stationary in numbers or increased very slowly, due principally to the fact that men were unable to affect favorably their sanitary condition, or to ward off in any way the fearful epidemic scourges that every now and then swept through the ranks of mankind. Up to the time of the discovery of America, Europeans had been making no attempt whatever to prevent disease and with our present knowledge and point of view, we can see that any sanitary attempt on their part would necessarily have failed. They had an entirely wrong conception of disease and an erroneous theory of its cause. The mortality rates in most parts of Europe at this time were as high as its birth rates. In England, the population had not increased for several centuries, or if it had increased, the increment was so slight that it could not easily be measured. Every now and then virulent epidemics would sweep through Europe and carry off a large portion of the population. The figures as given in some of these epidemics are almost inconceivable. [Illustration: Concreted Ditch. Gatun.] For instance, it is stated by Hecker that the epidemic of plague in the fourteenth century carried off from Europe some twenty-five millions of the population, and from China alone, thirteen millions. That again, in the fourteenth and fifteenth centuries, millions of the population of Germany and other countries were carried off by the “sweating sickness.”