Last Work. Final Honoiis 639 



from the concurrence of two factors — an ultra microscopic microbe and 

 an unstable chemical factor derived in his experiments from propagated 

 malii;nant tumours of animals, and it would be erroneous to regard either 

 ot these factors, sinijly, as the cause of cancer. The direct evidence of this 

 dual origin of new growths has so far only been furnished for the Rous 

 towl sarcoma and for a transplantable sarcoma of the mouse. For other 

 tumours the evidence is indirect, cultures of these supplying the ultra- 

 microscopic microbe, the specific unstable chemical factor being supplied 

 by an extract from the Rous fowl sarcoma, in which species of animal the 

 two injected simultaneously gave rise to progressively growing sarcoma. 

 Injected singly, they are inert. The delicate racial and tissue specificity 

 governing the transmission of malignant new growths therefore attaches 

 to the labile chemical factor, and not to the microbe. One of the gravest 

 objections to previous forms of the parasitic hypothesis of cancer is thereby 

 met. ' 



On December 29, Dr. Gye wrote Smith expressing confidence 

 that the defenders of the " cell theory " would " in the very near 

 future . . . find it hard to hold their ground." He confided that he 

 had " a good deal of information on the subject of immunisation 

 against new growths " but the materials were " only partial " and, 

 requiring a " hundred and one qualifications," too difficult to put 

 in a letter. His next letter ^^^ told of their " new laboratories " 

 which, though small, were " as near perfection as one can ever 

 hope for in this world. Barnard and his assistants," he wrote, 



are on the ground floor; I have, with one colleague, the upper floor. It has 

 taken several months to settle down, to arrange work and so on but now 

 we are in full swing. Barnard of course is doing the microscopy and 

 happily has already made many improvements. . . . The experimental 

 work is of course in my hands and I am glad to say that we have pushed 

 along pretty satisfactorily. I have repeated my published work with newly 

 devised technique and I have nothing to withdraw or recant. If I had 

 of course I should do it at once. I am now more satisfied than I was that 

 a filterable virus is the true cause of the fundamental character of animal 

 tumours. The parasitic theory has been a little battered lately but that is 

 in the natural order of things. Scientific orthodoxy is against it and of 

 course with reason. But then in science the orthodoxy of one decade is 

 based on the best available evidence of the previous decade. And fortu- 

 nately so or there would be no stability. I am pretty confident that with 

 the better methods now devised there will be less ditriculty in repeating 

 my results and I am optimistic enough to believe that theory will change 

 slowly. In any case let truth prevail. 



^^* Nov. 7, 1926, Medical Research Council, National Institute for Medical 

 Research, Farm Laboratories, Mill Hill. 



