164 III. THE EVOLUTION OF VIRUSES 



The reason for the increase in the virulence during the passage 

 can thus be elucidated. But in connection with this a more important 

 problem has to be considered. 



A virus can assimilize the host protoplasm, so that all the struc- 

 tures present in the protoplasm may become identical to the virus. 

 Thus, since the parent and offspring viruses share all the structures 

 in common, they can completely combine with each other, but without 

 exerting any effect on one another because there is no differences in 

 the structures. 



In like manner, if a virus propagates continuously in the same 

 kind of host until the degree of assimilation by the host protoplasm 

 reaches to such an extent that almost all the virus structures become 

 identical to those of the protoplasm, then the virus will have little 

 influence upon the host in spite of its high affinity for the host. 

 Thus, in order that a virus exerts injurious effect upon the host, the 

 virus must have a structure or structures foreign to the host proto- 

 plasm, which may be lost when the assimilation by the protoplasm is 

 advanced to an extreme. On the other hand, in order to combine with 

 the host cell a virus must share a structure in common with the host 

 protoplasm. This common structure can be increased or strengthened 

 when the virus is assimilized by the protoplasm to a certain extent 

 through the host passage. 



It will be expected from this reasoning that, although a virus may 

 increase gradually in its virulence against a certain host when passed 

 successively through the host, the increase will come to cease sooner 

 or later, and then a decrease will begin until at last the virus be- 

 comes entirely avirulent. In fact this seems actually true : An 

 epidemic, for example, of influenza usually begins gradually, at first 

 with only a slight indisposition, but in due course of time the viru- 

 lence will increase in a striking way, the disease spreading vigorously 

 and extensively ; yet, after several months or a year or two, it will 

 begin to lose vigour until entirely fades out and disappears. 



In order to avoid the reduction in the activity, vaccinia virus is 

 usually passed at times through rabbits apart from the customary 

 cattle. We shall have later many occasions to return to this pointy 

 since this is one of the most important problems concerning life 

 phenomena. 



