II. THE GENERATION OF VIRUSES 143 



Virus-like agents seem to be produced occasionally during the 

 adaptation process of microorganisms. It cannot be considered, how- 

 ever, that the generation of a structure capable of acting as a virus 

 is necessarily associated with the adaptation. On the contrary, even 

 if a structural change takes place which is unfavourable for the or- 

 ganism, a virus-like agent will be found in the fragments of the 

 protoplasm thus changed in its structure, provided that the newly 

 formed structure is stronger than the original. 



As is generally known, viruses are liable to alter in their charac- 

 ters with the change of environmental conditions. For example, some 

 viruses undergo variations when exposed to proper amounts of N-mus- 

 tard or of ultraviolet light. Such a variation is, as will be mentioned 

 later, considered to be based upon a structural change in the virus 

 protein caused by the environmental effects. These effects capable of 

 giving rise to the change in viruses, /. e., in particles or fragments 

 of protoplasm, can, as a natural result, exert their influence upon the 

 protoplasm itself to cause changes in its structure, altering the charac- 

 ter of the cell itself. Thus the variation or mutation of organisms 

 ■will be brought about as a result of the alteration in the protoplasm 

 protein due to the change in the environmental conditions. 



In fact, microorganisms such as bacteria are known to undergo 

 variation or mutation when treated with N-mustard or wath X-ray. 

 If the newly formed structure of the protoplasm is stronger than the 

 original, its particles will be able to act as a virus. In short, it may 

 be said that the mutation of organisms is, in general, liable to be 

 associated with the generation of a virus. 



However, as will fully be discussed in Part V, there are good 

 reasons to suppose that newly produced structures are, as a rule, 

 weaker than the original, so that it cannot be expected that mutation 

 is always associated with the production of new viruses. On the con- 

 trary, it should be considered that the generation of new viruses 

 will take place only on rare occasions, since a newly produced virus 

 is always provided with a structure weaker than the original. 



2. Environmental Change and Virus Generation 



As stated in the foregoing chapter, there are many evidences sug- 

 gesting that normal, healthy cells are producing viruses. The viruses 

 appear, however, are not constantly excreted by the cells, but in most 

 cases their production seems to be connected intimately with the en- 

 vironmental conditions. For example, Theiler's virus cannot be found 

 in the feces of very young mice, though when the mice grow older to 



