144 III. THE EVOLUTION OF VIRUSES 



a certain age the virus is constantly excreted. The same is true for 

 submaxillary-gland virus of guinea-pigs. These facts suggest that 

 the animal cells may undergo a change in their protoplasm structure 

 with the growth of the animal to yield the virus at certain periods 

 of age. Such physiological change of the protoplasm structure to be- 

 come capable of acting as a virus may be regarded as another example 

 of virus generation. 



The fact that phage can be detected in chicken feces in warm 

 seasons, especially in summer, may also be explained by the assump- 

 tion that the protoplasm structure of some chicken cells capable of 

 acting as a phage can be provided only in warm seasons, in winter 

 the structure being lost. 



As emphasized by Bawden (39) the flowering-stimulating agent 

 called florigen, present in the leaves of some plants, is remarkable in 

 its virus-like properties; thus, like a virus, it moves from the cells in 

 which it occurs initially and can be transmitted between plants by 

 grafting. It is a fact worthy of note that this agent can be produced 

 in the leaves of some plants when the latter are exposed to day-light 

 of an appropriate duration. This may indicate that a certain struc- 

 tural change may be brought about in the protoplasm of the leaves on 

 the exposure to day-light, the change being transmitted by a virus- 

 like agent to another plant to stimulate the flowering. In this con- 

 nection, it is of interest to note that Japanese encephalitis is believed 

 to occur frequently when one's head is exposed to the violent day- 

 light of summer. Again lysogenic bacteria which usually fail to 

 liberate phage may be lysed with phage production on the irradiation 

 with ultraviolet light (7). 



These facts have led the writer to consider that the environmental 

 effect may cause a change in protoplasm structure of some cells, the 

 fragments or the particles of which are subsequently enabled to trans- 

 mit the changed structure to other cells as a newly produced virus. 



It should, however, be emphasized that for the production of a 

 structure capable of acting as viruses, there should exist the cell pro- 

 toplasm having the character to be altered into the virus structure by 

 the environmental effect. In other words, for the generation of viruses 

 certain cells are necessary possessing the character to generate the 

 viruses ; namely, certain cells with the predisposition to yield the 

 virus are needed in addition to appropriate environmental factors. 

 Viruses are yielded by such cells when the cells are exposed to certain 

 stimuli involving environmental changes. Thus, a certain protoplasm 

 having a certain predisposition and a proper simulus are essential for 

 the generation of a virus. 



In the case of Theiler's virus, for instance, the mice having the 



