doubtful whether it ever disappears spontaneously. It is therefore a disease which leads without special interference to the death of the patient, sometimes very soon, sometimes many years after the first symptoms appear, according to the rapidity of the growth of the cancer. The characteristic feature of cancer as a disease may therefore be stated as follows: Cancer consists in an abnormal multiplication of cells at a certain, at first, usually well-defined place of the body. All the deleterious results are primarily produced through this continuous growth which spreads into different parts of the body. This growth as such through the pressure it exerts on neighboring organs or through its infiltration into and destruction of vital parts of the body leads to the death of the affected individual, secondarily toxic influences may be added to the primary results of the growth; but these toxic influences are as far as we know not of a specific character. It is different in the so-called infectious diseases. There the disease consists primarily in an intoxication by products given off by the invading organisms and a proliferation of the body cells plays only a subordinate role in the disease process.
While we can thus, in a rough way, define and differentiate cancer from certain other diseases, we must be well aware of the fact that a complete and satisfactory definition of a process can be given only after the completion of its scientific analysis. Usually, however, definitions are given in the beginning of the study of a certain process; and they have therefore only a provisional value. Certain apparent, often superficial features are at first used for characterization. During the progress of scientific investigation new relationships to neighboring fields are discovered, differences which at first appeared to be of a qualitative are gradually to be found to be merely of a quantitative character. Thus we must prepare for the eventuality that the sharp differentiation between cancer and infectious or toxic diseases may not be upheld through future investigations. In fact already at the present time we know of conditions which seem to be intermediate between the two sets of phenomena and it is probable that we will gradually have to add certain subdivisions defining conditions which have factors in common both with cancer and the so-called infectious diseases. At present it is of comparatively little importance to discuss whether certain intermediate phenomena are to be classed as cancer or infectious disease, while it is of the greatest importance to describe and analyze the character of these intermediate phenomena. In a similar manner, it is of relatively slight importance to justify or deny the admissibility of calling certain processes in animals and plants cancer—the definition of cancer necessarily being a provisional one—while it is of the greatest importance to discover similarities between certain tumor-like conditions in man, animals and plants.
