CHAPTER VII 



CAUSES OF THE EVOLUTION OF 

 VIRUSES 



1. Development of Instincts Indispensable for 

 Organisms 



It has been described thus far that the viruses generated de novo 

 can evolve into fixed viruses by variations, and that they can fur- 

 ther advance into typical organisms through the stage of Rickettsiae. 

 But, little discussion has been made on the problem why they have 

 to evolve. In this chapter this problem will be fully dealt with. 



Darwin attributed the cause of the organic evolution to the sur- 

 vival of the fittest set forth by natural selection ; this likewise pos- 

 sibly folds for the evolution of viruses, although there are some 

 differences between usual organisms and viruses. 



One of the principal differences between them may lie in the fact 

 that viruses have no instinct for self- or race-preservation in contrast 

 to the indisputable organisms. Even in this respect alone it seems 

 impossible to regard viruses as organisms. This is the instinct indis- 

 pensable for organisms ; organisms cannot exist without this instinct. 



Viruses are lacking in this important instinct, so that their exis- 

 tence is of quite evanescence ; only some viruses which incidentally get 

 possession of a property favourable for their existence, can continue 

 to exist for some periods, while others are cancelled immediately 

 after their generation. 



The character most essential for organisms is their continuity in 

 existence. This is secured by the instinct, ovv-ing to which they have 

 been, and will be able to accomplish their evolution. For higher 

 organisms, however, self-preservative instinct may be only of the 

 secondary importance, merely answering the purpose of race-preserva- 

 tion the important necessity for organisms, and an individual of higher 

 organisms having already fulfilled this purpose may be entirely of no 

 use in itself ; the existence of individual may always be ready to be 

 sacrificed for the race-preservation. 



On the other hand, in case of extremely primitive organisms 

 existing at the stage of virus, there seems no distinct difference be- 

 tween the self and race-preservation. In most cases the race-preserva- 



