and Wassermann have shown the possibility of preparing specific cellular poisons for cancer analogous to those used in curing protozoan diseases. The final clue which will unravel the mystery of this complex disease would not appear to be as yet fully in hand, and yet I think no one of those most conversant with the problem would be surprised to find to-morrow that it has been discovered and that cancer was curable.
Protozoology.—It is of interest that about the year 1890, when bacteriologists ceased to announce discoveries with their accustomed regularity, owing to the fact that all readily recognized pathologenic bacteria had been discovered, the systematic study of protozoa began and some of the single-cell forms of life in the animal kingdom soon took a place as disease-producers alongside the corresponding form of the vegetable kingdom. Until this time, protozoa had been found in only two diseases of man, dysentery and malaria. In the year 1890 appeared the first books on the subject of protozoa as causes of disease, a small volume of one hundred pages by L. Pfeiffer, followed in the next year by Doflein's more extensive discussion of the same subject from the broader biological point of view. The bacteriologists of the preceding decade had by their efforts limited the number of diseases in which a bacterial etiology could be readily shown and it was natural, therefore, that the attention of investigators turned to the study of other microorganisms as factors in the production of disease. The careful technique of the bacteriologist had shown the methods to be used in the study of etiology, and, undoubtedly, the publications of Pfeiffer and Doflein stimulated general interest in the search for pathogenic protozoa. However this may be, it is a matter of record that in 1890 "only two human diseases were suspected of being caused by protozoa. . . . To-day more than fifteen are known or suspected to be of protozoan origin" (Calkins).
In the discussion of bacteriology I have referred to Leeuwenhoek as the first to see bacteria; he was likewise the first to see protozoa (1675). Two hundred years later, Bütschli (1875) offered conclusive evidence of the unicellular nature of these minute forms of animal life. In the intervening period, however, owing largely to the work of O. F. Muller (1786), Ehrenberg (1833-38) and Dujardin (1835-41), many forms had been removed from the "chaos animalculæ," the name under which Cuvier had classified them and their structure had been studied by Siebold (1845) and Max Schultze (1863). In this later period also several forms now familiar to us as occasional parasites of man had been described; as the Trichomonas vaginalis (Donné in 1837), the Cercomonas hominis (Davaine, 1857), the Balantidium coli (Malmsten, 1857) and the Lamblia intestinalis (Lambl, 1859).
The first parasitic protozoon, however, to be definitely associated
