336 V. THE NATURE OF EVOLUTION 



It has been known that bacteria or phages damaged by ultraviolet 

 light can be fairly reactivated by the subsequent exposure to visible 

 light (1). This fact may be explained if we assume that reactivation 

 occurs because reactivated structure is stable under the exposure to 

 visible light which may cause the proteins to return to their original 

 structure. Bawden and Kleczkowski (2) have found with Phaseolus 

 vulgaris that irradiation for two to three minutes with ultraviolet 

 light had not immediately obvious effects on the leaves, but that when 

 kept in the dark for twenty-four hours after irradiation, became se- 

 verely bronzed within the next two days ; this was prevented when 

 the irradiation was succeeded by exposure for some hours to day light. 

 In this case day light presumably acted upon the leaves to make its 

 protoplasm structure, which had been changed by the ultraviolet light, 

 return to the original structure before the symptoms came to the forth. 



It has been found that a strain of ultraviolet irradiated E. colt 

 can be reactivated also by heat (3) ; the reactivation has been reported 

 to be likewise possible by the addition of various metabolites (4). Fre- 

 quently we observed that phage inactivated by unknown cause is re- 

 activated by heating to an adequate temperature or by the addition of 

 inorganic salts (5). Heat or salts in these cases may disturb non- 

 specifically the protein structure to initiate the returning change. 

 Visible light may act in a similar way, but in addition it may exert 

 a specific effect in favour of the original structure. 



Herpes may occur following febrile diseases and poliomyelitis fol- 

 lowing tonsillectomy, and usually common cold is associated with 

 chilling. These phenomena may be based upon the reversion of virus 

 structures rather than upon the generation of the viruses themselves. 

 Vago (6) has claimed that a virus which causes polyhedron disease is 

 present in the majority of the insect, Bombyx mori, remaining latent 

 through several generations, but it is activated and accordingly the 

 disease is developed by the administration of fluorine compounds or by 

 the feeding of insects on the leaves of the Madura. 



The recovery from virus diseases is possibly ascribed to this 

 memory phenomenon. Thus, when an environmental condition, under 

 which the structure of a virus is stable, is provided in the protoplasm 

 of a certain organism, the virus will be generated or the organism 

 will be readily infected by the virus, whereas if the environment is 

 changed and a new environment is favourable for the original structure, 

 the virus structure will be expelled with the cure of the disease. 



On the one hand, as already mentioned, seasonal effect is striking 

 on the incidence of virus diseases, while on the other hand functions 

 of hormones are under the influence of seasonal factors. This sug- 

 gests strongly that the hormones are involved in the environmental 



