2i6 THE SCIENTIFIC MONTHLY
of the skin; aniline workers develop cancer of the bladder. But more interesting than these general chemical actions is the fact that certain chemical substances given off by organs within the body which normally have a stimulating action on the growth of certain tissues may on the strength of this stimulating action be factors favorable to the develop- ment of cancer. Thus it has been recently demonstrated that extirpa- tion of the ovaries in animals at a certain period of their life may greatly reduce the occurrence of cancer of the breast in those animals. This is^ in all probability^ due to the fact that a certain substance given off by the ovary which normally stimulates the growth of the mammary gland is removed through the extirpation of the ovaries. Apparently any external condition which causes long-continued growth processes in cells may in consequence of this stimulating action be a factor in the origin of cancer, and if we learn to eliminate such action we may to some extent prevent the development of cancer in a certain percentage of cases.
We have now learned of a number of causes for this abnormal cell proliferation which we call cancer. We know furthermore that internal and external factors must cooperate in the development of cancerous tissue. But it is very probable that the ratio between the nimiber of internal and external factors necessary to obtain definite results is not fixed. The larger the number of external factors the smaller, in all probability, need the requisite number of internal factors be. For in- stance, it is probable that very long-continued action of Roentgen rays may lead to the production of cancer in persons in whom the heredity factor may be very weak.
Stimuli, such as those which lead to the production of cancer, chemical as well as mechanical, also cause growth processes in normal tissue. We know that in lower animals parts of the body which have been lost, may be replaced; that in higher animals wounds wiU heal; that certain chemical substances call forth growth processes which are not cancer. For instance, the placenta, through which the fetus is attached to the mother, grows in response to a combined action of chem- ical and mechanical stimuli; ova can develop parthenogenetically (with- out fertilization) under the influence of various stimuli. But in all those cases the stimulus leads to only a temporary proliferative activity of the cells and tissues. Sooner or later, the tissues return to their former equilibrium in which proliferation is very limited or entirely absent. It is not so in cancer. Cells which through various agencies have once become cancerous may be destroyed; but where they are not exterminated, provided suflBcient nourishment is obtainable and other conditions necessary to their growth permit it, they prohferate eter- nally. Cancer cells, then, differ from normal cells in so far as the stimulus which in normal cells produces only a temporary growth, in the
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