THE STUDY OF CANCER 29 
mental investigation of cancer has revealed the fallacious nature 
of all reputed analogies with known forms of infection, the 
results obtained are essentially of constructive value. The cell 
problems being attacked now were unapproachable five years 
ago, aS were also the relations existing between tumours and 
the organisms bearing them. Having recognised fully, and 
insisted from the outset of our investigations, that the repro- 
duction of the lesions of cancer in normal animals by trans- 
plantation differed from the development of the disease ab initio 
in animals spontaneously affected, | have confined myself largely 
to describing the growth of cancer in the soil provided by 
normal animals, and to drawing some inferences as to the 
conditions obtaining in the development of cancer. I have 
shown that even in normal animals the soil plays an important 
part, and a part which can be modified at will. I have indicated 
that the soil may play an important part in the spontaneous 
appearance of cancer in individuals. I have also shown that 
the primary properties of cancer cells differ. Therefore the 
circumstances associated with the development of cancer require 
to be considered with reference to the two sets of factors last 
mentioned. 
The extent to which the experimental method has already 
deepened our knowledge of the properties of cancer cells and 
of their relations to normal animals renders it probable, that 
its further application will throw light on the conditions 
underlying the association of the inception of cancer with the 
senescence of tissues, and on the increasing frequency of the 
disease as age advances, throughout the vertebrate kingdom. 
