210 III. THE EVOLUTION OF VIRUSES 



the protoplasm particles. 



If the protoplasm structure in question is preserved in decom- 

 posed particles of protoplasm, the structure will be transmitted to 

 other cells or organs through such particles, and if the cells or the 

 organs fall into a pathological state on the transmission of the struc- 

 ture, the latter particles will be regarded as a virus, and the patho- 

 logical state will be called an infectious disease. Tumour diseases 

 are commonly not infectious, as the structures cannot, as a rule, 

 remain in the protoplasm particles. Although viruses are occasionally 

 demonstrated in certain tumours, it cannot be said that the structures 

 are stable enough to make tumours regarded as infectious diseases 

 always caused by a virus. 



The structure of virus can be altered by various causes. If a 

 virus retains the faculty to act as such after a change is induced in 

 its structure, the changed structure can multiply, so that a virus 

 strain having the changed structure may be produced, because the 

 only function of viruses is to confer their own structure to the proto- 

 plasm of cells which they affect, thus the altered structure being 

 inherited to "progeny." Since the property of viruses is determined 

 by their structure, the alteration in the structure is to be accompanied 

 by the change in the property, and therefore the changed property is 

 to be inherited with the altered structure. In such a case the virus 

 is said to have undergone a mutation or variation. 



Viruses may easily be generated and each particle of them may 

 have individuality even immediately after their generation. In addi- 

 tion, they can variate in various possible directions. Among the par- 

 ticles with various properties thus having arisen, some ones with 

 suitable property for the continuity of their existence may continue 

 to exist in escaping the extinction. However, the probability of inci- 

 dentally acquiring the property to become a virus able to continue to 

 exist for prolonged periods is apparently extremely small, presumably 

 far smaller than that of spermatozoa of higher organisms to meet an 

 egg-cell to develop into a full individual. This may be attributed to 

 the lack of the property in the newly generated viruses compatible 

 with the self- or race-preservating instinct of higher organisms. 



In order to escape the extinction, high stability of the structure 

 is essential for the virus. If the structure is too unstable to be pre- 

 served in the protoplasm particles, virus may scarcely be found as in 

 the case of eczema. 



