It is against the attacks of new diseases that the law of custom is most helpless. The primitive theory of disease and death is so widespread that we may accept it as the belief of mankind before custom gave place to scientific inquiry. The primitive argument was this: the natural state of man is to be healthy, and everything contrary to Nature must be the doing of some hostile agency. When a man feels ill he knows that an evil spirit has entered into him, and since evil spirits do not move unless some person conjures them, his first thought on waking with a headache is "An enemy hath done this." Out of this springs all the complicated ritual of witchcraft, fetish and juju, which by frightening natives into destroying or burying all offal and refuse that might be used against them by a wizard, achieves the right thing for the wrong reason. The "Evil spirit" theory of disease is thus not so very far removed from the bacillus theory: in both the body has been attacked by a malignant visitor which must be expelled before the patient can recover. It is in the methods adopted for making the body an uncomfortable lodging for it that the systems diverge. In all ages the essential part of therapeutics has been faith in the remedy, whether in the verse of the Korân swallowed by the Moslem, in the charm prescribed by the medieval quack, in the "demonstration" of the Christian Scientist, in the prescription of the medical practitioner. Mankind survives its remedies as well as its epidemics. England has a population of nearly forty millions, even though, less than a century ago, as we learn from Creevy's memoirs, blood-letting was regarded as the proper treatment for advanced stages of consumption.