186 III. THE EVOLUTION OF VIRUSES 



the latter is pcm-erful. Extremely powerful viruses can remain in 

 the host not only for the host's life, but also can stick to its germ 

 cells through which they can be transmitted to progeny. 



Since such firmness of the structure must be associated with the 

 invariability of the structure, fixed viruses with strong structures 

 will hardly give rise to variants. The antigenic specificity of fixed 

 viruses is, as a rule, constant, and generally every fixed virus has its 

 own antigenic specificity. Therefore, usually various types may^bei^ 

 demonstrated in a single fi.xed virus. For example, the virus of 

 measles or of mumps has each distinct immunological specificity and 

 hence various types of measles or of mumps are never found. Also in 

 this respect fixed viruses may resemble a typical organism. 



On the other hand, newly generated viruses, as for example, those 

 of influenza are provided with an extremely weak structure so that 

 they may readily be overcome by the normal structure. Consequently 

 such weak viruses will persist only for short periods in host cells. 

 This must be the reason why newly generated viruses can confer 

 only transient immunities. 



The weakness of the structure should necessarily be associated 

 with the unusual variability. Indeed, influenza virus is noted for its 

 marked variability. Newly generated viruses may have no constant 

 immunological specificity because of this marked variability as well 

 as of the successive generation of similar viruses having varying 

 specificities. The presence of such various immunological specifici- 

 ties, therefore, must be one of the characteristics of newly generated 

 viruses. 



However, if a virus particle having a relatively rigid structure 

 is incidentally generated, it will remain longer and multiply more 

 abundantly than do other weaker itidividuals. Virus particles produced 

 by such a virus should inherit this relatively rigid structure. And 

 again a particle having the strongest structure will naturally be sele- 

 cted out of these relatively strong particles, as the strongest can 

 remain much longer and multiply much vigorously. Thus the struc- 

 ture of a virus is to' become naturally more and m^ore rigid and 

 powerful. 



The acquisition of the strong structure 'appears to answer the 

 purpose of race-preservation as well as of self-preservation of viruses. 

 In short, viruses which can give rise to prolonged immunity are more 

 fitted for existence than those which may confer only transient immu- 

 nity and those which can spread to progeny through germ cells are 

 still more fitted. As to plant viruses, the ones which can long persist 

 in insect bodies are fitter, while the ones which can be transmitted 

 through the eggs to the offspring must be still more fitter. 



