194 III. THE EVOLUTION OF VIRUSES 



ologically active agents such as viruses, toxins, and ferments can be 

 illustrated in the same way. Thus the nature of the reversibility of 

 protein structure discussed in the previous chapter is also made clear. 



As above discussed, some plant viruses may be activated in insect 

 bodies, a phenomenon which has been explained by taking into account 

 the second phase of the protein denaturation. This phase is considered 

 to be brought about by some denaturating or inactivating agent present 

 in the alimentary tract of insect, so that the third phase, reached by 

 passing through the second phase, should involve the inactivated state. 

 It is actually known that some agent capable of inactivating certain 

 viruses are found in insect bodies. 



However, if the inactivating action of the agent is not so strong 

 as to completely inactivate the virus, the virus will be disturbed in 

 its structure without being inactivated, and the disturbance may 

 enable the virus to recover its original structure which the virus 

 possessed before having been damaged by the host protoplasm from 

 which it is now liberated. 



Thus, if a virus which has been weakened by an unfavourable 

 condition in the host cell, is removed from the host to an insect, a 

 new host, in which no unfavourable condition is present and in which 

 the virus can regain the original active state, then the virus can be 

 said to have achieved "rejuvenation." 



For the establishment of this rejuvenation in insect, some adequate 

 stimulus to cause the shift towards the original active state may be 

 necessary. Such a stimulus is possibly given to the virus through the 

 contact of the virus with the insect protoplasm having a structure 

 somewhat different from that of the virus. If such a stimulus is too 

 strong, the virus will sooner or later be inactivated, although in the 

 second phase of the inactivation or denaturation the transient activation 

 may arise. In such a case plant viruses are transitorily activated in 

 insect bodies without multiplication. 



On the other hand, if the stimulus is mild and adequate the virus 

 will be able to regain its original active structure in the insect body 

 without inactivation, and in addition the virus thus activated is pos- 

 sibly capable of spreading its structure into the insect protoplasm, 

 that is, the virus will be able to multiply in the insect body. Since 

 different hosts must have different protoplasm consisting of proteins 

 of different structures, although they may share some structure in 

 •common as they have the property to be affected by the same virus, 

 the virus will possibly acquire some new structure through the multi- 

 plication in a new host. 



The acquisition of a new structure on the multiplication in a new 

 host means that the virus is assimilized to a certain extent by the 



