I. THE THEORY OF MEMORY 337 



condition directing the virus structure. 



In some particular cases, however, it seems that temperature alone 

 can act as the directing factor. According to Kunkel (7) aster yellow 

 virus is lost when the leafhoppers carrying the virus are exposed to 

 32^C. The insects exposed to this temperature for a day lose the 

 virus, but on lowering the temperature to 24''C., the virus appears in 

 a few hours, whereas those heated for a week regain the virus only 

 after two days or longer at the lower temperature; the virus is never 

 regained when the insects are heated for more than 12 days. This 

 fact indicates that the structure of the virus is stable at 24"C. but 

 unstable at 32''C. at which the normal protoplasm structure is stable. 

 It is highly suggestive, however, that the prolonged exposure to the 

 high temperature renders the virus structure irreversible. This is 

 considered to be dependent upon the "oblivion" of the virus structure, 

 an extremely important phenomenon which should be discussed later 

 in great detail. 



Numerous facts indicating that the structure of plant viruses are 

 apparently unstable at higher temperatures have been shown. For ex- 

 ample, Vinca rosea and Nicotiana rustica infected with aster yellow 

 virus are freed from it after growing for two weeks at 40'^C. and V. 

 rosea is also cured by immersion in water at 45°C. for a few hours. 

 This V. rosea is also freed from potato witches' broom and cranberry 

 false blossom virus by growing for 14 days or more at 42°C. Potato 

 tubers with diameters up to 2 cm., infected with witches' brooms 

 virus, are cured by storing at 36°C. for 6 days (8). 



This seems to be the case also with animal viruses. We may 

 generally become feverish following the infection with a virus, a phe- 

 nomenon which may be of significance from the point of the expulsion 

 of virus structure. It has customarily been believed that sweating 

 treatment with a sudorific or with a hot drink is effective to common- 

 cold. It cannot be said, however, that high temperatures are always 

 effective to make virus structures unstable. Since herpes febrils is 

 generally raised accompanying with feverish diseases, the structure of 

 herpes virus must be stable at higher temperatures in contrast to 

 that of common-cold virus. 



Thus, a virus structure may be lost when the environmental con- 

 ditions become unfavourable for its existence, resulting in the cure of 

 the diseases, but as the virus structure itself is also reversible the 

 disease will return when the conditions become again favourable for 

 the virus structure in so far as the structure is not "forgotten". 



At present, chemotherapy is winning a brilliant victory over the 

 campaign against some bacterial diseases. The effect of the drugs 

 on virus diseases, however, appears to be insignificant, presumably 



