IX. THE REJUVENATION OF VIRUSES 195 



new host, but by this process the virus will be able to get rid fully 

 of the injurious assimilizing effect of the former host and thereby 

 the rejuvenation will become quite complete. 



The concept that plant or animal viruses can multiply in an 

 insect, an entirely different organism, may seem at first sight unrea- 

 sonable, but as already pointed out, the fact that an insect is parasitic 

 on a certain animal or a plant shows the presence of common struc- 

 ture between the protein of the insect and that of the animal or of 

 the plant ; through this common structure the virus may be able to 

 affect both the insect and the animal or the plant. In other words, 

 the relationship analogous to that between an enzym^c and its substate 

 must be present between an insect and an animal or a plant on which 

 the insect is parasitic, and a similar relationship must also exist 

 between a virus and its host, and accordingly a virus can simulta- 

 neously affect both an insect and an animal or a plant. 



3. Various Means for Rejuvenation 



Whereas the host change appears to be most essential to the evo- 

 lution of viruses, it cannot be said that all fixed viruses are transmitted 

 by insects. For example, viruses of such diseases as measles, mumps, 

 or small pox seem to have no insect vectors, a fact which appears 

 contradictory to the writer's view. 



Rejuvenation is, hovv^ever, indispensable not only for viruses but 

 also for every organism without exception. Organisms are generally 

 supplied with a splendid tool to perform the rejuvenation. This tool 

 is sexual reproduction. By means of this tool a protoplasm having a 

 certain structural pattern can combine with another protoplasm having 

 different structure, thus being able to achieve the structural exchange, 

 a process which may be called mutual assimilation, and by this pro- 

 cess rejuvenation is accomplished. This appears quite analogous to 

 the rejuvenation of virus by means of the host change. 



The writer holds the opinion that above cited viruses, such as 

 those of measles and mumps which appear unable to perform any host 

 change, can achieve rejuvenation by availing themselves of the sexual 

 reproduction of the host. As fully discussed already, some fixed 

 viruses can be transmitted to offspring through the germ cells of the 

 host ; in the case of insects the transmission is said to be usually 

 established by eggs. Since egg cells are to be rejuvenated on the fer- 

 tilization by the contact with sperm cells composed of proteins with 

 somewhat different structure, the virus which has become a part of 

 the egg cell should also be rejuvenated through this process. 



