216 III. THE EVOLUTION OF VIRUSES 



although the existence of non-pathogenic viruses cannot be demon- 

 strated so readily as in Rickettsiae. Moreover, the existence of viruses 

 exerting favourable effects upon the host is conceivable, that is, the 

 symbiosis between viruses and their hosts may be possible. 



10 



The writer designates the organisms having evolved from viruses, 

 pathogenic or non-pathogenic, as the secondary organisms. The orga- 

 nisms having served as the first scaffolding for the secondary organisms 

 are naturally to be called the primary organisms. 



Whereas the primary organisms were generated and evolved in 

 water, the secondary organisms depended on the protoplasm of senior 

 organisms both for their generation and for evolution. Therefore, 

 for the secondaty organisms, the liberation from the dependence upon 

 the senior organisms might be a matter as difficult as the release of 

 the primary organisms from the water. Accordingly parasitic, pri- 

 mitive organisms living outside the water should be regarded as the 

 secondary organisms. From this point of view, at least the majority 

 of bacteria, protozoa, and fungi, and also presumably more advanced 

 parasitic animals such as platyhelminthes and nematodes may be 

 secondary organisms. Furthermore, arthropods are considered to be 

 likewise so, most of which have already succeeded in liberating from 

 the parasitism. 



If arthropods are the secondary organisms, it should naturally 

 follow that the relationship between insects and plants or animals 

 on which they are parasitic is analogous to that between viruses and 

 their hosts ; and this latter relationship may, in turn, be regarded as 

 similar to that between enzymes and their substrates. 



Some adult arthropods enter into parasitism after free-living larval 

 stages, a fact which is to be interpreted as an atavism. It is consi- 

 dered to be impossible that free-living organisms can incidentally 

 enter into parasitism, a most specialized, and accordingly extremely 

 limited environment. Therefore, it may be proper to regard the orga- 

 nisms as the secondary ones if they reveal parasitic nature at any 

 stage in their life. In this respect, mollusks may be supposed to be 

 the secondary animals generated in the oldest age. It should be noted 

 that there is no parasitic groups in chordates and also in echinoder- 

 mates which the latter are generally believed to be intimately related 

 phylogenetically to the former, suggesting that they are the primary 

 organisms in the animal kingdom. 



It is a remarkable fact that various secondary organisms are still 



