Disease was but a form of this eternal struggle for existence; it was the attempt of invisible lower forms to master the higher human tissues and make them their feeding-ground. The original enemies of man, the wild beasts, were subdued or tamed or driven forth into the deserts as soon as savage life was passed. Then began the fiercer contest for the possession of his own cells and tissues and organs. Enemies that he could not see migrated out of the surrounding elements into his system as soon as it became delicate enough to stir their appetite, and for ages there were no weapons against them; chance now and again offered one; but generally he groped about in his frantic ignorance for anything that would ease the pain from these gnawing foes within him. Out of this rose by slow steps a kind of quackery they called the science of medicine; but the conflict still remained unequal; the invisible enemies had the best of it, and they were ever being recruited by new enemies out of space, which bred new and more appalling plagues. Not till it was found that the newer these settlers were the more virulent were their ravages was there any chance of a real science of medicine arising from this everlasting agonism. The first beginnings of a true science appeared in the attempts to deplete the soil by setting tamed and exhausted specimens of their foes to feed on it. A soil once reft of the elements that invited and fitted any disease germs seldom suffered in any serious degree from them again. Soon by their new electro-microscopes or clirolans they were able to classify the infinitely minute foods of these infinitely minute pasturers on the human tissue. Their microscopes, enormously though they had added to the power of human vision into the atomic world, had been unable to advance beyond the discovery and complete classification of the invisible organisms. Their clirolans combined photography with electro-microscopy in such a way that every change in the systems of their minute foes was recorded; they were able to see the elements taken from the human system absorbed and sifted of their nutritive powers, and the débris or manure ejected and left to poison the human tissues; it was not the presence of the organisms themselves, or even their destruction of essential elements that generally produced the disease, but the accumulation of the exhausted excreta, clogging the various functions. At first medical science satisfied itself with cultivating feeble and underbred germs, and turning them loose on the human body in order to make them exhaust the elements which attracted their kin. Next they discovered the chemical combination that, introduced into the body, would neutralise the poisonous qualities of the bacterial débris. Last of all by their vimolans or photo-electric analysers they found the exact food which attracted each form of microbe to the tissue and nourished them there; and they experimented