174 III. THE EVOLUTION OF VIRUSES 



tion may be accomplished by the self-preservation. 



Newly formed viruses would start on the long journey of the 

 evolution as a group of particles of various individualities. Of them 

 some particles with high resistances against environmental influences 

 might be able to escape from the immediate inactivation to continue 

 their existence, and again some particles with strong assimilase action 

 might be capable of achieving extensive multiplication, so that par- 

 ticles both with strong assimilase action and high resistance would 

 be able to survive. Thus the survival of the fittest would be effec- 

 tuated by a sort of natural selection. 



Viruses thus having survived would undergo variations, with 

 the yieldance of various particles differing from one another in their 

 individual properties, and again some individual having a property 

 more favourable for their existence would be selected out of these 

 particles and would continue their existence. Variation, selection, and 

 the survival of the fittest would thus be recited over and over again. 

 There seems no doubt that the characters resembling those of higher 

 organisms are most desirable for the continued existence, so that 

 viruses that happened to possess more organism-like characters would 

 be able to survive much longer than do other viruses, and therefore 

 the characters would become continuously more and more alike to 

 those of typical organisms. 



The properties thus developed in selected viruses, resembling 

 those of higher organisms, must be the properties favouring conti- 

 nuous existence, and thus the instinct for the self- or race-preserva- 

 tion would be developed gradually in the viruses until the stage of 

 typical organisms was reached. The characters useful for the conti- 

 nuous existence, if became unusually strong and firm, should be called 

 instinct. Therefore, even if we assumed that the instinct belongs 

 only to indisputable organisms, it would be impossible to draw a line 

 between living and nonliving. 



The evolution of the shape or size of virus particles may be 

 elucidated in like manner : In the case of tumour, such as cancers, 

 newly formed structures can sometimes be transmitted to other normal 

 cells by protoplasm particles, but commonly fail to be maintained in 

 the particles, and accordingly as the structures have to share their 

 fate with organisms in which they have generated they will be lost 

 when the organisms are perished, and therefore their evolution will 

 never take place ; whereas since in the case of influenza or poliomye- 

 litis the newly generated structures can be preserved in the particles, 

 they are able to be liberated from the organisms, in which they have 

 been generated, to be transmitted to other organisms. The continuity 

 of their existence would thus be commenced, and the evolution would 



