THE SCIENTIFIC INVESTIGATION OF CANCER 211
growth; they are no longer kept in bounds through the agencies which Testrain the deyelopment of other parts of the body, and this leads to the formation of monstrosities. We may then conclude that the same laws which ultimately determine all cell and tissue growth also underlie cancerous growth. These laws are fundamentally those regu- lating the physical and chemical changes which take place in the proto- plasm. But added to these general causes of growth, which cancer has in common with embryonic deyelopment and the process of repair after the kss of parts of the body — ^the so-called regenerative growth — there must be a special factor which is responsible for the excessive growth-energy and destructive capacity of certain tissues or cells in the body which distinguishes them as cancerous growths from other kinds of growths. To find this special factor or rather this special set of factors constitutes the fundamental cancer problem.
In my introductory remarks I stated that certain experiences in life of a more or less emotional character are the starting point for investigation, and that only gradually do these investigations become dispassionate, objective, do they come into contact with neighboring fields of research and broaden into an attempt to understand reality. Hand in hand with this enlargement of the concept of the problem we find a simultaneous widening of the range of objects now consid- ered as more or less related to the original object of investigation, and a greater refinement in the methods of research. At first only human cancer was considered. It is only recently that we turned with interest to other organisms and discovered that cancer exists in all classes of vertebrates — although with unequal frequency — and that somewhat re- lated phenomena are even to be observed in plants. At first we were satisfied with the observation of human tissues with the naked eye and with statistics concerning the frequency and distribution of human cancer; later there were added microscopic investigations with gradu- ally accumulating refinements. These in turn were reinforced with experimental methods in which were recorded observations of the growth of cancers in animals which had been inoculated with tumor. And then we began to study experimentally the conditions under which cancer developed spontaneously and under which the development of spontaneous cancer can be prevented in animals, and the reactions of normal tissues under various abnormal conditions.
The experimental study of cancer is as necessary as the experi- mental study of all other problems. Mere observation confronts us usually with a considerable number of variable factors, and when we observe a sequence to a certain group of factors, observation alone does not usually permit us to decide which factors are really causative and which merely accidental. Through experiment we can at will isolate or combine various factors and thus alone find the real causes. Ex-
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