XI. THE SUMMARY OF PART III 213 



A virus can affect different individuals if these belong to a simi- 

 lar kind of organisms, since similar organisms share some protoplasm 

 structure in common, through which the virus can affect them. A 

 virus affecting customarily a certain animal, however, is occasionally 

 able to infect a blood-sucking arthropod, parastitic on the animal, and 

 multiply in it. This is probably due to the existence of common 

 structure between the animal and the arthropod because it is con- 

 sidered that the relationship analogous to that between viruses and 

 their hosts or between enzymes and their substrates is present be- 

 tween the host animal and the arthropods. This appears likewise to 

 hold true between plants and their parasitic insects. Viruses make 

 use of such arthropods for their host change not only to be saved 

 from the extinction due to the assimilase action of the host but also 

 to be rejuvenated with the enhancement in their activity. 



Viruses having insect vectors are, as a rule, not mechanically 

 transmitted by insects, but rejuvenated in them. To evade extinction 

 newly generated viruses presumably have to acquire such means of 

 rejuvenescence ; those incapable of succeeding in the acquisition of 

 such means would soon become extinct. 



Whereas numerous viruses are transmitted by insect vectors, 

 some viruses, which are never thought to be newly generated appear 

 to possess no insect vectors. Such viruses are supposed to be capable 

 of establishing rejuvenescence without insect vectors, presumably by 

 making use of the sexual reproduction of the host. 



If a virus has evolved highly by acquiring a strong structural 

 pattern, it may become able to impart to the host protoplasm life- 

 long lasting distortion, giving rise to the immunity lasting as long. 

 It is considered that the distortion caused by an extremely strong 

 virus persists in the host for life but also may be transmitted to the 

 progeny through the germ cells of the host. Many evidences are 

 actually known indicating that some viruses are transmitted to offspring 

 through eggs or seeds. Certain plant viruses can also confer a pro- 

 found distortion of this kind on their insect vectors ; as a result the 

 viruses can be demonstrated in the offspring of the vectors. 



The rejuvenescence of organisms is generally accomplished by 

 sexual reproduction, in which a germ cell is rejuvenated by fertiliza- 

 tion through the contact with a germ cell of the other sex, just as 

 a virus is rejuvenated by a host change through the contact with 

 protoplasm of a new host. If the structural pattern of a virus is in- 



