196 III. THE EVOLUTION OF VIRUSES 



When the structure of a virus becomes the stronger, the longer 

 it may remain in the host, until it can even be transmitted to the 

 progeny. It may be said, therefore, that a virus can attain of itself 

 the property to perform the rejuvenation as it evolves to a certain 

 extent. Thus, it follows that the strong structure becomes indispen- 

 sable for a virus to continue its existence not only for the reason of 

 the necessity of the stable nature but also of the need of the ability 

 to make use of the rejuvenation of the host. 



Plant or animal viruses which can multiply in insect vectors may 

 naturally avail themselves of the sexual reproduction of the customary 

 hosts as well as of the insect vectors when the virus acquires struc- 

 ture strong enough to spread their pattern to the germ cells. Thus 

 it should be concluded that most evolved viruses are always to be 

 transmitted to the offspring through the germ cells of the host. 



In order to acquire such a strong structure the majority of viruses 

 might have attained to the habit of host change ; therefore, a virus, 

 which have become transmissible by a vector, may be said to have 

 come up to the easy course leading to a typical fixed virus. The 

 writer has already expressed the opinion that viruses such as those 

 of influenza and encephalitis may be newly generated viruses, but 

 some of them might possibly have already arrived at such a course 

 and hence it may be unreasonable to regard all of them as being new 

 viruses. Indeed, it is generally accepted that some encephalitis viru- 

 ses do posses insect vector and further that they can give rise to con- 

 siderably long lasting immunity in their host. Likewise, some strains 

 of influenza virus may be half-fixed through a host change unknown 

 to us. It has been reported that swine influenza virus is actually 

 disseminated by a lung-worm parasitic to the dog (104). The so-called 

 pneumonitis virus, highly specialized virus-group causing pneumonia, 

 may be an advanced form of influenza virus. 



It seems little doubt that insects or arthropods are generally 

 utilized for the host change, but also other organisms may possibly 

 be used for the purpose. For example, if a dog infected by rabies 

 virus, it will become wild and maniacal and try to bite other animals, 

 enabling the virus to be transmitted and at the same time to be reju- 

 venated. Some plant viruses, such as tobacco mosaic virus, known 

 to have no insect vector, may have some means of rejuvenation, if 

 they cannot use of the sexual reproduction of the host plant. It 

 is believed that various plant viruses can contaminate the soil and 

 persist in it for long periods so that many of the outbreaks in tobacco 

 and tomato crops originate from virus surviving in the soil ; in some 

 favourable soils some plant virus could be ascertained to survive for 

 more than 9 years (105). Such a fact led a number of workers to be- 



