Modern pathology reveals that cancer is not a single disease but a multitude of different diseases and that the boundary between malignant and benign is not everywhere sharply defined. Not much can be said about that aspect of the etiology of cancer which has to do with the fundamental nature of the disease, for that is bound up with the very nature of the growth process itself, about which little is known. Therefore this paper will be restricted chiefly to a consideration of those extraneous factors which may act on cells and tissues in such a way as to produce cancer. The multiplicity of the cancer process is indicated not only by its widely varying forms but also by the great number of factors that have already been discovered to be capable of affecting tissues, apparently previously normal or predisposed by some anatomic variation, and inciting them to form malignant