298 ON GROWTH AND OVERGROWTH 



their products affecting the cells in their new relationship, and 

 thereby leading to continued growth. This, I take it, is the 

 usual conception of those who propound the parasitic theories of 

 cancer formation. 



But herein comes the difficulty. Continuous purposeless 

 growth is characteristic of all forms of tumours proper. Never- 

 theless, as I have already stated, in a very large group of tumours 

 all the evidence we possess points surely to the fact that microbes 

 have not initiated the growth, and are not concerned in the 

 continuance of the growth. 



It would be absurd to urge that microbic agencies play a 

 constant part in the production of the group of teratomatous 

 tumours. No one would suggest that such are the cause of 

 post-operative implantation cysts, or of simple congenital 

 dermoids, such as the little hairy dermoids upon the sclerotic, 

 or of the different forms of angioma. And we can recognize 

 no sharp dividing line, histologically, between the tumours just 

 mentioned and another group including the rest of benign 

 and malignant neoplasms. Histologically all are of the same 

 type. 



Further, we can recognize no sharp dividing line between 

 the benign and the malignant forms of such tumours ; there is 

 no one stage in which it is possible to say, thus far one set of 

 conditions has been at work leading to purposeless cell growth, 

 beyond this microbic irritation enters in. In both benign and 

 malignant tumours the growth is purposeless, and the only feature 

 separating the two forms is the rate of proliferation and stage 

 in which that occurs. And certainly we encounter every transi- 

 tion between tumours of frankly benign and those of frankly 

 malignant type. 



The " Habit of Growth " in Cells 



Here let me be clearly understood. I am not arguing that 

 parasites, intracellular or extracellular, may not be regarded 

 as originating a certain number of tumours. I am pointing out 

 that certain features are common to all tumours ; that microbic 

 parasites and the irritation they may induce are not common to 

 all tumours, and that this last factor is therefore inadequate to 

 explain those characteristics which are common to tumov/rs in 



