152 III. THE EVOLUTION OF VIRUSES 



promptly, as the pathological structure may soon disappear with the 

 revelation of the normal. As a rule pathologically changed skin re- 

 covers its normal state in a few days when the causative agent is 

 eliminated. Such a transient nature of the change may account for 

 the failure of detecting the structure in the protoplasm particles. As 

 will be stated later in greater detail, the change is transient because 

 it is readily reversible. 



2. Cancers 



There are good many reasons that the structural change in proto- 

 plasm is generally reversible. In the case of the dermatitis above 

 mentioned the reversibility seems very manifest, whereas there occur 

 sometimes changes which appear to be irreversible, never to be expel- 

 led by the normal structures. Occasionally such changes may even 

 overcome gradually the surrounding normal structure to proliferate 

 still further at the cost of the latter. 



Tumours may be involved in such irreversible changes. The high 

 persistence of the changed structure may be ascribed to the transmis- 

 sion of the change to the genes themselves. The structural change 

 in the gene, if occurs, should give rise to new cells which may be 

 called tumour cells. The proliferation of the changed structure can 

 be achieved by the multiplication of such cells. When the multipli- 

 cability is so striknig that the cells can proliferate indefinitely, the 

 cells may be called malignant tumours, whose representative is can- 

 cer. The cancer cells multiply at the expense of normal cells so vi- 

 gorously that the animals themselves affected by cancer are ultimately 

 to be killed. 



It is generally known that cancers are induced by a variety of 

 physical or chemical agents, and it is also well realized that there is 

 the predisposition to cancers. It seems, therefore, probable that the 

 protoplasm having the predisposition must be changed in its structure 

 by some stimulus to give rise to cancer cells. If cancerous structure 

 was preserved in protoplasm particles, the cancer would be said to be 

 transmissible by a virus. Cancers are usually readily transmissible by 

 cancer cells but the transmission through the particles or a virus 

 seems difficult to occur, suggesting the lability or the weakness of the 

 structure. 



One of the best known tumours proved to be transmissible by a 

 virus is Rous tumour, a kind of cancers affecting fowls. The finding 

 that this tumour could be induced by a virus might be shocking to 

 those who regarded viruses as microorganisms like bacteria, but in 



