them, as also to Ricketts, who investigated Mexican typhus and succumbed to it, and to Walter Myers and Everett Dutton, of the Liverpool School, our science owes much in methods and in ideals.
Truly, no field of medicine offers so much of tragedy, of romance and of spectacular discovery as that of the pathogenic protozoa, and few offer such great difficulties. It is, however, one of the most promising fields of present-day effort and one which I would like to present more in detail. It must, however, suffice to end this presentation with mere mention of the successful cultivation of amebæ (Mesnil and Mouton), the cultivation of the trypanosomes (Novy and MacNeal), the discovery by Schaudinn and Hoffman of the spirochete, which we now know to be the cause of syphilis, and the finding of a very similar organism in yaws. Time might also be given to the various trypanosomes, to the spirochetes causing diseases of cattle and poultry and to the Negri bodies of rabies; also the discussion might be extended to include the broader field of tropical medicine, but instead, as it is the direct outcome of the study of protozoa, I must turn to a new phase of research in medicine, that known as chemotherapy.
Chemotherapy
As the study of protozoan diseases progressed it soon became evident that the method of combating such diseases must be different from that used against diseases due to bacteria. The chronicity of amebic dysentery and relapses in malaria indicated that the protozoan diseases are not self-limited and therefore not characterized by the development of immune bodies, similar to those of the acute bacterial diseases; also artificial cultivation failed to demonstrate that protozoa yielded bodies analogous to bacterial toxins, capable of producing, on injection, bodies with efficient antitoxic power. These and other facts precluded, therefore, a therapy based on the principles applied to bacterial vaccines or antitoxins.
The beneficial effect of quinine in the treatment of malaria and the cellucidal action of quinine on the ameba and other protozoan forms indicated that a therapy, to be successful, must be one in which a substance toxic for the protozoa in question is brought into direct contact with it. The establishment of such therapy and incidentally the creation of a new science, that of specific chemical therapeutics, has been the work, in the past seven years of Professor Ehrlich, of the Royal Prussian Institution for Experimental Therapeutics at Frankfurt. This new therapy is based on the principle that "a specific chemical affinity exists between specific living cells and specific chemical substances." This principle has always been the main theme of Ehrlich's work, as is seen in his application of the aniline dyes to the
