_Heredity_ has been advanced as a cause, but statistics fail to verify this in any degree whatever. While certain instances have been brought forward in which heredity seemed evident, the study of large numbers of those afflicted with cancer, in connection with others free from the disease, has shown almost the same proportion of antecedents with cancer in both classes of persons; although some recent evidence seems to show that there is some tendency in different families for different organs. Experimental studies have, it is true, seemed to demonstrate that tumors occur apparently along hereditary lines in some animals in regard to certain organs; but in these instances it is to be remembered that the animals were kept in captivity, and all fed alike, conditions which have been found to cause the development of malignant disease in wild animals when confined in Zoölogical Gardens. In former years _malaria_ was believed to have an influence in the production of cancer, and some investigators have thought to trace the prevalence of the disease to _telluric influences_, showing a preponderance of cases along certain water courses, or in certain streets or houses; but no definite proof of such connection has ever been established, and this theory is dismissed by the best authorities. _Syphilis_, in its latent effects, has also been claimed as an element in the causation of cancer, and undoubtedly the disease may develop, in suitable persons, upon old syphilitic lesions, especially about the mouth, anus, and genital region: but no one well informed in regard to cancer would regard syphilis as the real cause of the disease. All these and other etiological propositions are no longer considered to be tenable, and the very multiplicity of suggested causes shows that we are yet very far from the true etiology of cancer. _Age_ undoubtedly has a powerful influence in the development of cancer, the vast majority of cases occurring after the age of forty or fifty. But, again, this does not at all explain the true nature of the disease, for only a certain proportion of elderly people are so afflicted, and malignant tumors have been observed in those of all ages, and even in young children. The degeneration of tissue belonging to advancing years undoubtedly renders it more susceptible to malignant disease, but this does not explain why one person is affected and not another, nor why the tissues in one locality or another take on this morbid action.