THE NATURE OF TUMORS 337 



of the cancerous process, although we cannot rule out the possibility that a 

 virus or virus-like substance may be a hidden cause of all cancers. 



Given these fargoing changes in the reactions which normal tissues un- 

 dergo in becoming cancerous, the problem arises whether these remarkable 

 changes at the same time induce alterations in the character of the individu- 

 ality differentials of the tissues or in the reactions of the host against the 

 abnormal transplants. To answer these questions, we present in the follow- 

 ing chapters the data which are relevant in this regard. 



