The discovery of ultramicroscopic agents of infection in the form of so-called filterable viruses came at a period in the study of certain diseases when the hope of finding a responsible micro-organism, to the existence of which all the evidence strongly pointed, began to approach the stage of despair. The long story of unsuccessful attempts to isolate and cultivate an organism responsible for yellow fever, and the more recent experiences with the causative agent in anterior poliomyelitis which

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... is now known to pass through filters that were at one time relied on to obstruct entirely the passage of pathogenic organisms, are fresh in the minds of those who have followed the progress of the study of infectious diseases. Without committing us to any etiologic theory of cancer, the preceding fragments of the history of medical science must be kept in mind whenever we are informed that micro-organisms can have no importance in the causation of malignant growths. The persistent assertions that the factors already established are sufficient to explain the origin of can¬ cer express the convictions of pathologists who have primarily been interested in the careful study of the structure of pathologic tissues. The repeated failures to substantiate the constant presence of parasitic micro¬ organisms in human cancer have led to an unduly skep¬ tical attitude toward everything that smacks of microbiotic possibilities. The recent experiments of Peyton Rous of the Rockefeller Institute for Medical Research have supplied the proof of filterable agencies which can induce body-cells in animals to proliferate in such a manner that certain kinds of animal cancer result. To the two chicken-tumors for which Rous and his collabo¬ rators have found the cause in filterable entities, a third has now been added.1 These types of spontaneous