All this is indeed a great undertaking, and can only be accomplished by great effort on the part of many careful and skilled observers. But I want, in these lectures, to give you an outline of my own thought and study for many years, and, if possible, to let you see as I do the lines along which investigation should be directed. It is hoped, therefore, that this and the following lectures will throw some light on the connection of cancer with diet and mode of life, and some suggestions as to its prevention and cure. _Parasitism_ has been excluded: for while at different times many observers have reported various organisms which were thought to be the cause of malignant growths, none of these have been definitely confirmed by others, in spite of earnest endeavor; and all experimental and clinical evidence is against a parasitic etiology of cancer. It is therefore seen how improper it is to speak any longer of “_the germ of cancer_,” for, as is now widely acknowledged, there is no such germ, it is an _ignis fatuus_ which has been chased in vain. The _contagiousness_ of cancer has also been excluded, certainly in the sense in which this term is applied to other affections. For while in some animals inoculation experiments have resulted in the transmission of certain tumors, little has been determined except that such tumor material when transplanted can, in some unknown manner, multiply its cells indefinitely and form a focus of malignant disease, with disastrous effects on adjoining tissue. The same occurs in metastasis in cancer patients. But this does not at all explain the true basic nature of cancer, nor its development in those who have had no connection with other patients so afflicted. On the other hand the instances of suggested or supposed human transmission of cancer from one individual to another are so remarkably few, and so exceedingly doubtful, that a recent author, Janeway, states that “no well-authenticated cases of the transfer of a malignant tumor from one human being to another exist.” It has been found impossible to inoculate human cancer into rats, mice, and apes, nor can animal tumors be inoculated into animals of a different species.