miscellaneous: stimuli underlying tumor-formation 569 



such as the immature red leaves just emerging from their stipule 

 sheathes. See Chapter V. 



What relation, if an}^, these discoveries, especially those re- 

 lating to crown gall, may have to the question of the etiology of 

 animal tumors must be left for the oncologists to determine. 

 My own views, expressed repeatedly during the last ten years, 

 are that they have a profound relation, and Jensen of Copen- 

 hagen and Borrel of Paris, as well as several cancer specialists 

 in the United States, are of the same opinion. The moment one 

 has established that there is a cancer in plants due to an obscure 

 wound-parasite it becomes illogical and unthinkable that cancers 

 in men and animals are of non-parasitic origin. Why should 

 Nature have two such diverse ways of reaching the same end? 

 The differences between plants and animals are not sufficiently 

 fundamental to lead us to expect it. Both are equally subject 

 to parasitic diseases. Both are alike in a hundred ways, as 

 Claude Bernard first showed, and as I have elsewhere pointed 

 out {Science, I.e.). Moreover, no one has formulated a work- 

 able non-parasitic hypothesis. In plants, we see that it is easy 

 to produce short-lived tumors by means of a single short expo- 

 sure to dilute acids and alkalies, but for the production of a 

 continuously growing malignant tumor something more appears 

 to me to be necessary, to wit: a feeble commensual parasite of 

 a special type, an organism that shall continuously furnish 

 acid or alkaline substances to the cells in stimulating small 

 quantities, but is not able to destroy the cells. This we have for 

 plants in the crown-gall bacterium. Whether the cells thus 

 originated may then continue to grow in the absence of the 

 parasite, as Jensen believes, is a subject for further consideration. 



That no one has 3^et isolated a micro-organism from animal 

 cancers which will reproduce them when inoculated, does not 

 weigh heavily with me for several reasons. I recall the history 

 of syphilis, of tuberculosis, of hog-cholera, and of a dozen other 

 animal diseases, and I am no longer in awe of the animal path- 

 ologist. It may be he is as wrong about cancer as he was about 

 syphilis prior to Schaudinn, or tuberculosis prior to Villemin 

 and Koch. First, the difficulties in the way of isolation may not 

 have been overcome. Parasites are often sensitive to sHght 



