352 THE CAUSATION OF DISEASE. 



We have now to inquire into the nature of this mal-E. 

 Malignant disease is often precipitated by mere mechanical 

 irritation, such as that caused by a blow, soot, a sharp tooth, 

 and the like. The effect of mechanical irritation in causing 

 malignant disease, notably cancer, is well known. Thus we 

 explain the great frequency of this disease in the mamma?, 

 the cervix uteri, the lips and tongue, and such other parts 

 of the alimentary canal as are most exposed to injury. But 

 is this irritation alone sufficient to account for the abnormal 

 growth ? It seems on the face of it very improbable that 

 it should lead to the production of a tissue totally unlike that 

 produced by mechanical irritation in general. It is, of course, 

 open to us to suppose that the tissues have an innate tendency 

 towards malignant change, and that a mechanical irritation is 

 capable of starting it. Of starting it, I say, but not of con- 

 tinuing it. We have already seen that a specific mal-E must 

 play a large share in the causation, but mere mechanical irrita- 

 tion does not fulfil the necessary conditions. In the first 

 place, malignant tumours occur over and over again indepen- 

 dently of such irritation ; indeed, this is always the case with 

 the secondary malignant tumours, when, therefore, some other 

 mal-E must be at work. In the second place, the malignant 

 degeneration is continuous ; once started it goes on with awful 

 determination, and a continuous effect implies a continuous 

 cause. And since a peculiar cell-E plays an essential part in 

 the causation, we require a mal-E which abides during the whole 

 period of growth. Consider, for instance, the primary carcinoma, 

 which grows chiefly from the mature cells of the tissue affected ; 

 every newly affected cell requires the specific mal-E. Now, 

 inasmuch as the tumour continues growing after the removal 

 of the mechanical irritation, it is obvious that this latter 

 cannot constitute the specific mal-E. 



The form of mal-E which meets all the peculiarities of the 

 case is one consisting of a living organism, for it is an abiding 

 mal-E ; and if satisfactory proof has been given that the 

 essential portion of malignant pathogenesis belongs to some 

 continuous mal-E, then, I think, we may take it as all but 

 experimentally proved that this mal-E consists in some specific 

 bacterium, which thus comes to be a necessary element in 



