140 III. THE EVOLUTION OF VIRUSES 



more reasonable to consider that most latent viruses are protoplasm 

 particles of healthy cells stronger in the assimilase action than cer- 

 tain other cells on which they can act as viruses. 



Similarly in plants the detection of latent or silent viruses is not 

 unusual. A well known example of plant viruses, apparently belong- 

 ing to this category, is produced by paracrinkle virus, which is found 

 in every plant of the potato variety named King Edward, and which 

 is actually regarded by some workers as a normal product of the 

 metabolism of the plant with the characters of a virus when trans- 

 ferred to other potato varieties. Moreover, the so-called latent virus 

 of potato mosaic is said to occur in almost all of the potato plants 

 grown in America. It causes no damage to the potato plants, and, 

 because of this fact and because of its almost universal presence in 

 potato plants, it has come to be called the healthy potato virus. This 

 has likewise occasionally been regarded as a normal constituent of potato 

 plant. According to Smith (28), samples of apparently healthy sugar- 

 beet and mangold collected at random from different farms in England 

 and Scotland were all found to contain a virus, the presence of which 

 can be rapidly demonstrated by inoculation of the sap to the first 

 leaves of cowpea seedlings, which results in the production of charac- 

 teristic lesions. 



Whereas various plant viruses are, as is well known, transmitted 

 by insects, it is believed that viruses are latent in a high percentage 

 of apparently healthy caterpillars (29). Also there are many evidences 

 that neurotropic viruses are present in normal insects which transmit 

 commonly various animal virus diseases. Since the relation between 

 insects and animals or plants, upon whose blood or sap they thrive, 

 may be analogous to the relation between bacteria and animals, on 

 which the bacteria are parasitic, there should be a common protoplasm 

 structure between them. Therefore it seems a natural result that the 

 protoplasm particles of some normal insects behave as viruses towards 

 some animals or plants. The reason why the protoplasm particles of 

 animal cells can act as a phage upon some bacteria can be explained 

 in a similar way as already discussed. 



The various facts above cited have naturally led a number of 

 workers to the theory that viruses are originated from normal cell 

 components. For example, Darlington (30) has postulated viruses ori- 

 ginating from specified cytoplasmic units such as plasmagenes. Again, 

 de Buy and Woods (31) have expressed the opinion that viruses are 

 produced from plastids and mitochondria which have been altered in 

 the form and function. 



Since these cytoplasmic particles are known to contain nucleic 

 acids in large quantities, also from the writer's theory it appears 



