APPENDIX A 



The Infection Theory of Cancer 



IT was obviously impossible to discuss in the text 

 all the theories of cancer in detail, but as I under- 

 stand that some think the paper less than just to the case 

 presented by those who support the infection theory, 

 something more may be said of it. It was, perhaps, too 

 much to infer without a more special examination that 

 the parasitic hypothesis of malignancy is only explaining 

 one mystery by another. The real objection to this 

 hypothesis is that even if it were found to be a fact that 

 an infection precedes all malignant growth, it would remain 

 a mere observation and not an explanation. What we 

 want to know is why invasive cell-proliferation takes place, 

 and whether excessive cell-growth is peculiar to such 

 diseases, or can be found in others. Any bacteriological 

 explanation of malignancy must classify the phenomena 

 among those which are recognized as being symptomatic 

 of infection. So far this has not been done, and a pro- 

 visional assumption that the imagined infective agent 

 is so different from all others as to be able to produce the 

 phenomena in question exhibits many of the marks of 

 vitalism, i.e. it invests the agent with the unexplained 

 power of producing what we desire to explain. Logically 

 an unknown entity capable of causing the effects in 



question must not be propounded as a cause if any evidence 



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