THEORIES AND PROBLEMS OF CANCER 105 



these two classes is that malignant tumours tend to recur after 

 removal by operation, whereas benign tumours do not. This 

 statement is very misleading. Malignant tumours usually have 

 no Well-defined margin and the cells composing them tend to 

 escape along various channels to surrounding or even distant 

 parts of the body ; therefore, at no stage can the surgeon be 

 certain that he has removed the whole of the cells which form 

 part of the malignant growth : the so-called recurrence is really 

 a multiplication of cells that have been left behind. 



Another feature of malignant growths is the formation ot 

 secondary tumours — metastases — in some other part of the body, 

 brought about by cells of the primary growth having travelled 

 and multiplied in a new position. The cells of these secondary 

 growths partake, in a marked degree, of the characters of the 

 cells of the primary growth. 



There can be but little doubt that there is sometimes an 

 insensible transition from benign to malignant tumours and that 

 it is impossible to say, at what particular time, in any given case, 

 a change from one to the other took place. 



Malignant growths produce no primary symptoms in the 

 persons in whom they occur. All the symptoms and all the 

 damage produced by them are of a secondary nature, due to 

 pressure or some other mechanical action upon surrounding 

 parts of the body. 



Having cleared up these points, I will proceed to deal with 

 the possibility of a specific parasite being the cause of cancer 

 and with the present condition of cancer research. 



The Parasitic Theory 



The discovery that so many diseases are due to micro- 

 organisms entering the body and multiplying there very 

 naturally led to a supposition that cancer was due to a similar 

 cause. The parasitic theory was most popular in the early 

 nineties but since then its adherents have diminished in numbers 

 with ever-increasing rapidity. It may be said at once that 

 very many "discoverers" of the cancer parasite have not had 

 the necessary knowledge and skill to conduct the investigations 

 they have entered upon and that a consideration of their 

 published work is neither profitable nor interesting. On the 

 other hand, men of acknowledged competence have strongly ad- 

 vocated the parasitic theory, though, as James Ewing says : " The 



