THE NATURAL HISTORY OF TUMOURS 569 



of causing a similar effect can be produced in the body is not 

 known. (The alimentary canal, so far as concerns the forma- 

 tion of toxins, is outside the body.) There are many things 

 that suggest it, such for example as the occasional disappear- 

 ance of tumours under the influence of thyroid extract ; the 

 rapidly increasing incidence of tumours as age advances ; the 

 occurrence of multiple and of symmetrical tumours, and the 

 tendency of the same kind of tumour to occur in members of 

 the same family at about the same time of life, or in consecutive 

 generations ; but there is no evidence comparable to that 

 furnished by the action of arsenic and the other substances I 

 have mentioned. Nor is it likely there will be until we know 

 far more of the chemical changes that take place in living 

 tissues than we do at present. In the meanwhile a great deal 

 of valuable information might be obtained by the study first 

 of the chemical and developmental changes that are induced 

 in tissues by substances which it is known may lead to the 

 growth of tumours ; and then of the effects that local irrita- 

 tion produces in normal cells, and in those the nutrition and 

 development of which have been interfered with by the adminis- 

 tration of those substances. 



