Unquestionably the malignant tumours are the diseases most disappointing to medicine and surgery, and these sciences are as much at a loss with regard to them as in the case of infectious diseases before the discovery of pathogenic organisms. Science is perhaps even in worse case with regard to cancer than it formerly was with regard to infectious diseases, for, before the discovery of microbes, something was known of the virus which produces infection. Thus the virus of smallpox was known, and was used, by the method of inoculation, to prevent more serious attacks of the disease. Nearly a century before the discoveries of Pasteur, Jenner had been able to be of the greatest service to mankind by his discovery that the virus of cow-pox could be used as a preventive of infection by smallpox. In the case of malignant tumours, we do not even know their real nature; we are ignorant as to whether or no they are infectious, and whether they are caused by a microbe coming from without or are due to internal changes of the tissues. Our ignorance, however, affords no ground for despair. It is probable that the malignant tumours will soon come to be ranged with infectious diseases due to invasions by specific microbes. Experiments on the cancers in rats and mice have shown that these can be inoculated in the same manner as in the case of the recognised infectious diseases. Hanau has shown that this occurs in the case of epithelioma of old rats; Morau[270] has succeeded in transferring the cancers of white mice, and his results have been confirmed by Jensen[271] and Borr[272], in the Institut Pasteur. These investigations mark the beginning of a new stage in the knowledge of tumours. I am unable to see, therefore that the malignant tumours provide a satisfactory argument in favour of a pessimistic conception of the universe. Dr. Boas, of Berlin,[273] in a recent publication, has laid stress on the fact that most patients affected with cancer do not seek medical aid until the disease is far advanced. For instance, in 80 per cent. of the cases of cancer of the rectum that he had attended, the patients presented themselves too late for operation. Boas advised that the attention of the public should be drawn, by means of widespread publication, to the earliest symptoms of cancerous disease. He thought that such a course might save many lives by making possible operation in early stages.