child may result from a syphilitic father and an apparently healthy mother, the mother's immunity being shown by the speedy infection of a healthy wet-nurse from the child's lips, while she herself nurses with impunity. Conversely, Profeta's law asserts that an apparently healthy child may sometimes be born of a syphilitic mother (and father), in which case it may be suckled by either the mother or a syphilitic wet-nurse without danger of infection. In either case examination of the blood has demonstrated the existence of the parasites as well as the permanence of the Wassermann reaction, in the mother's blood in the first instance, in the child's in the second. It became evident, from facts of this kind, that immunity from protozoan infection (animal parasites) is not the same thing as immunity from bacterial infection (vegetable parasites). In the latter case the immunity is derived from the antitoxic products of the bacteria themselves. In the case of the animal parasites, we know nothing of the chemical sources of infection and immunity. As a matter of pure speculation, perhaps the immune mother in Colles's law and the immune child in Profeta's law may, like the bacillus carriers of typhoid fever, come under Ehrlich's immunity of the first order (natural immunity), in which the sensitive receptors have either become worn out and insensitive or no longer exist as such. Theorizing aside, the practical outcome of these details was the evident impossibility of curing parasitic diseases by special antitoxins or vaccines, and the practical necessity of finding chemical specifics which would destroy the parasites as quinine does the malarial plasmodium. The next step towards a rational chemotherapy was Ehrlich's discovery that when mice infected with trypanosomes are treated with specific dyes like trypan red in doses that fall short of complete sterilization by some assignable quantity, a race of trypanosomes can be gradually bred which will prove permanently "fast," or resistant to the effects of the drug. The power of parasites to immunize themselves and their descendants against the effect of destructive poisons led Ehrlich to his next move in attempting to checkmate them—the idea of sterilizing the patient's body uno ictu ("mit einem Schlag") by a single dose of medicine. It was "upon this hint that he spake" in formulating his therapia sterilisans magna and we are now in a position to appreciate the value of his discoveries in pharmacodynamics.
In his Harben lectures before the Royal Institute of Public Health in 1907 Ehrlich stated his conviction that pharmacology, toxicology and therapeutics are "the most important branches of medicine." In other words, he believes that the chief end of the physician is to get his patients well. Self-evident as this proposition is, it has a certain paradoxical novelty when we consider the dominance of setiological studies (pathology and bacteriology) through the last half of the nineteenth century and the apparent débâcle of drug therapy which attended the rise of experimental pharmacodynamics. In such natural remedies as