We see, then, that thus far no satisfactory cause has been established for the occurrence of cancer, much less for the steady and great increase of the disease of late years. And as far as can be learned, no measures are recognized, or at least generally adopted, to prevent its occurrence or recurrence; although, as already stated, modern surgery has seemed to improve the statistics in regard to its mortality in certain forms or locations, and the X-ray and radium have certainly also been able to remove, perhaps temporarily, some of the products of the disease. We come then to the question, what is the real nature of cancer? Alas, all scientific, experimental, and clinical investigations have failed to solve the problem, except that all “evidence points to the conclusion that cancer is to be considered as a pathological disturbance of the normal cell life,” from some unknown cause. A curious suggestion has been made by Schmidt, who found that of 241 cases of cancer of the chylopoietic system, 180 had never had any infectious disease of childhood, and 99 went through life without any infection of any kind; the figures point to the existence of a cancer diathesis—one which is resistent to germs. It would carry us too far from the practical side of our subject, even if we were at all able, to present or analyze the vast number of contributions which have been made to the pathological histology of cancer, and the changes which take place in the transformation of normal cells into those of malignant character: the amount of microscopic work which has been done along this line can hardly be imagined, and the literature relating to it is enormous. Ewing, accepting the definition that the cancer process is “atypical and destructive proliferation of epithelium,” quotes Ribert as saying that “no one has ever seen the beginnings of mammary cancer”: but he does not bring us much nearer to the solution of the cancer problem than we were before. Bainbridge rejects all possibility of a blood condition, and finds the only solution of it in the early removal of everything which is thought to lead to cancer, even the simplest benign new formations, but Ewing states that “in some cases carcinoma has developed after excision of wholly benign fibro-adenoma”: and the immense number of cases of recurrent cancer after operations shows that we must look further than surgery if we wish to stay the progress of this formidable disease. It would be useless to attempt to present the many theories which have been advanced relating to cellular metaplasia, or even to detail all the more or less accepted views in regard to the manner in which normal cells change and degenerate into those of malignant character: but some of the principal facts may be of service in understanding somewhat of the mode of development of malignant tissue from that which has been normal.