Cancer Its Course and Its Causes 9 



spontaneous cancer. 3) Injection of the agent leads ap- 

 parently not only to the production of the same kind of 

 tumor, but also to other kinds of tumors; this is due to 

 the fact that the agents can affect certain other kinds of 

 cells. 



In these respects then this particular kind of tumor 

 differs from the typical mammalian cancer. We must 

 assume that in this case the stimulus which calls forth 

 the transformation of ordinary cells into cancer cells con- 

 sists in the presence and continuous action of a microor- 

 ganism which presumably acts through the medium of a 

 chemical substance which it gives off. 



To summarize then briefly what we know about the 

 causes of cancer, we found an inherited predisposition, 

 combined with the action of external or inner chemical 

 or mechanical stimuli, to be responsible for the typical 

 mammalian cancer. These factors have been analysed 

 experimentally to such an extent that in certain cancers 

 it is possible not only to predict, how many cases of can- 

 cer of a certain kind will originate in a given family or 

 strain, but also to prevent the appearance of the cancer 

 or to reduce its frequency at will. It has furthermore 

 been possible to produce experimentally through the ap- 

 plication of these stimuli various types of cancer. Con- 

 ditions which favor cell growth in general favor the or- 

 igin of cancer. Certain mechanical and chemical stimuli, 

 an embryonic condition of tissues act in this way and 

 the inherited factor in cancer acts presumably in a similar 

 way. In a certain type of avian cancer a filterable virus 

 causes the cancerous growth of cells. The same may ap- 

 ply to certain other kinds of atypical cancer. It is, how- 

 ever, not probable that microorganisms play a part in the 

 typical mammalian cancer. But even if later it should 

 be found that here, too, microorganisms are involved, the 

 latter would have access equally to all individuals of a 

 certain species; they would therefore not explain, why on- 



