AGE- INCIDENCE 161 



from growths, or which we knew were associated 

 with growths, on individual living healthy cells. By 

 this means it was hoped that we might find some 

 exciting substance from cancerous growths which 

 might in the first place cause normal individual cells 

 to undergo a change and become similar to those 

 cells taken from the growths themselves. In the 

 event of such a substance being found, it would, of 

 course, then be necessary to try to prove the argu- 

 ment by experimentation with the substance in the 

 body itself. In other words: believing that cancer 

 might be due to a chemical agent, we proposed to try 

 to find that agent, and to test its effect, in the first 

 instance on individual cells under the microscope, and 

 lastly to test its action on groups of cells in the tissues 

 of the body. 



Malignant disease may be separated into two 

 main divisions carcinoma and sarcoma. The former 

 attacks gland-tissues and epithelial cells; the latter is 

 a disease of connective tissues. These researches are 

 almost entirely concerned with carcinoma, and the 

 term "cancer" in this book refers to that disease. 

 There is, we think, a close association between these 

 two forms of malignant disease, although there is 

 a line of demarcation in the age incidence and in 

 some of their morphological and clinical characteristics 

 which separates them. Cancer that is to say, 

 carcinoma attacks people over the age of forty, 

 although there are occasional exceptions to this rule; 

 but sarcoma may occur at any age from infancy onwards. 

 At the outset we turned our attention exclusively to the 



