212 THE SCIENTIFIC MONTHLY
perimentation in the case of cancer does not only consist in the arti- ficial inoculation of cancer — which, by the way, is technically not very different from the injections of substances under the skin in human beings, and therefore not very painful — ^but also in the systematic breed- ing of certain animals and in varying the conditions of life in which animals are kept. In this way problems can be solved whose solution can hardly be attained if we restrict ourselves to observations of human cancer, because of the complicated conditions found in human society.
We have now determined what is meant by tumor and cancer, the general distribution of cancer and the methods of investigating the cancer problem. Let us then follow the threads of investigation a little farther and consider some of the results obtained by experimental means. Here we will take up in turn the three problems, namely, the conditions that lead to cancer, the reaction which takes place in the body against cancer growth, and lastly some properties of cancer cells — ^and secondarily of normal cells — ^which have been discovered as a result of cancer investigations.
In analyzing the origin of certain phenomena, we are apt to take one definite circumstance as the cause of a succeeding event. This is the procedure which we usually follow in our daily life. But a more careful analysis of any phenomenon will usually reveal to us the very much more complicated relation it bears to other phenomena. The more we search, the more apt we are to find that there are a number or rather a multitude of factors which at the very beginning work to- gether, shaping the event which interests us. Among these factors we can discern some which seems to us of greater importance, more specifi- cally related to the event under discussion and others which apply not alone to this specific event or to a relatively restricted number of events, but equally well to a large number of other experiences, not necessarily related to the one under consideration. For instance, to mention a factor belonging to the second category, the temperature which is required to sustain life processes in general and therefore also in particular cancer growth. The relation between temperature and can- cer growth is therefore not a specific one, but one which cancer growth shares with other life processes. While a certain temperature, then, is one of the conditions necessary for the development of cancer, it is not a specific condition and therefore not a cause in the strictest sense of the word.
We know that not one single factor is the cause of cancer, but we have discovered a number of conditions all more or less specifically re- lated to cancer growth. Provisionally these may be divided into two classes, namely, internal and external factors. By internal factors we mean such as are operative within the organism, apparently conditioned
�� �
