146 III. THE EVOLUTION OF VIRUSES 



injury is made before injection of the virus, then no tumours are 

 developed (41). A similar phenomenon is also observed with papillom 

 virus : When a rabbit is injected intravenously with the papillom 

 virus after smeared repeatedly with tar on the skin, a papillom de- 

 velops only in the skin on which tar has been applied. The growth 

 of the papillom is much more vigorous than the case when the virus 

 only is applied, occasionally even developing to a cancer. Instead of 

 tar, methyl-cholanthrene can be used to cause the same effect. Thus 

 the infective faculty of the virus can be increased up to about a 

 hundred-fold (42) (43). 



These phenomena should not be compared to the above stated 

 change of provirus into virus. These may only suggest that viruses 

 can produce their replicas more readily in the cell protoplasm when 

 the protoplasm is disturbed in its structure by proper stimuli. As dis- 

 cussed already, virus particles which have been inactivated to a 

 certain degree can sometimes exert a favourable influence upon the 

 intact virus to infect the host cells, showing that the inactive parti- 

 cles, though unable to produce their replicas, can disturb the proto- 

 plasm structure of the host cells contributing to the infection. 



3. The Seasonal Change in the Virus Infection 



Generation of viruses is apparently connected with the environ- 

 mental change as above stated. Especially the connection seems in- 

 timate when the seasonal change is involved. The seasonal effect on 

 the virus production is seen in the case of chicken feces phage as 

 already mentioned. There are many other examples showing the same 

 correlation. Poliomyelitis is prevalent in early autumn, common cold 

 and influenza in winter, and smallpox from winter to early spring. 

 German measles, according to our investigation, occurs preferably in 

 early spring (44). Japanese encephalitis is epidemic only in summer. 



In addition, seasonal factors affect the symptom picture of many 

 plant virus diseases as well as the pattern of their occurrence. Ac- 

 cording to Smith (45), the bright yellow mottling characteristic of the 

 aucuba type of tobacco mosaic does not develop under winter condi- 

 tions in the glass-house but affected plants show an indistinct green 

 mottle only. If, however, inoculations are made from a plant showing 

 this indistinct mottle to other plants which are kept under artificial 

 illumination, the characteristic bright yellow mosaic will develop in 

 the illuminated plants. 



Such seasonal effects may appear at first sight to be dependent 

 upon the seasonal change in the resistance of host cells against the 



