272 IV. THE PRINCIPLES OF LIFE PHENOMENA 



ved with certainty. Protozoan cells tend to grow old after continued 

 asexual multiplication and lose both their youthful vitality and repro- 

 ductive power. The habitude of host change, like in viruses, is 

 common among these protozoa, and therefore it seems possible that 

 the host change can substitute the sexual reproduction in these lower 

 animals. However, the fact that the faculty for sexual reproduction 

 is fairly developed in certain parasitic protozoa, such as malaria 

 Plasmodium, indicates that the sexual process is much more advan- 

 tageous for their evolution than the host change. 



As already discussed in Chapter X in Part III, besides the re- 

 juvenation by the host change, viruses can accomplish rejuvenation 

 by taking advantage of the sexual reproduction of the host, that is, 

 viruses penetrate into germ cells of the host and are rejuvenated there 

 together with the germ cells through the fertilization. As we shall 

 see in greater detail in Part V, germ cells of higher organisms are 

 considered to have the undeveloped structure of the single celled 

 creatures from which the organisms have been evolved. In other 

 words, germ cells may be produced by the reduction of the developed 

 structure of the somatic cells to their undeveloped pattern. Mean- 

 while, the pattern of a virus itself, engraved in the mother germ cell 

 of the host, will be also altered during the reduction of the cell to 

 the germ cell, and hence, in the germ cell, the altered pattern of the 

 virus cannot behave as the virus. Hovv^ever, the pattern will recover 

 its original form with virus activity, when the germ cell gradually 

 recovers its developed original structure in the developmental process 

 following the rejuvenescence by the fertilization. Such a rejuvenation 

 of viruses may be quite perfect, because it is brought about together 

 with the complete rejuvenation of higher organisms. 



The stronger the structure of a virus, the longer period of time 

 it will continue to exist, so that the viruses will become the stronger 

 as they evolve the higher. Consequently, viruses may naturally ac- 

 quire the faculty to engrave their pattern even into germ cells as 

 they evolve to a certain extent ; thus they may be rejuvenated to 

 continue their existence still further. 



A virus, which can already be rejuvenated by a host change, may 

 still further be rejuvenated by penetrating into germ cells of the host. 

 Certain plant viruses may be able to enter the germ cells of the host 

 plant, while at the same time they may even engrave their pattern 

 into the germ cells of their insect vector. This may hold true also for 

 animal viruses. 



Viruses, unable to change the host, will likewise acquire the ability 

 to engrave their pattern into germ cells of the host, when their 

 pattern is strengthened to a certain extent. Therefore, the rejuve- 



