200 III. THE EVOLUTION OF VIRUSES 



of insects and viruses may not have arisen in a similar manner. 



Sonneborn (112) has found in a kind of paramecia a virus-like 

 agent designated kappa ; the individuals of this protozoan containing 

 this agent produce a substance called paramecin which is able to kill 

 another kind of individuals having no kappa, but when a large 

 amount of this agent is present, the protozoan acquires the property 

 not to be killed by paramecin. Protozoa not containing kappa or with 

 only a few are readily killed by paramecin. The presence of kappa, 

 therefore, confers a survival advantage to the protozoan. This agent 

 is believed by some workers to be a kind of Rickettsia-like micro- 

 organisms parasitic on the protozoan. Likewise in various arthropods 

 there are found Rickettsia-like bodies, known as bacteroids ; they have 

 rarely, if ever, been grown outside the living host. It is possible, 

 however, to eliminate them in a certain insect by treatment with 

 penicillin or with certain sulfa drugs, and in this way to show that 

 bacteroid-free individuals invariably die (113). This is no doubt an ex- 

 ample of comm.ensality. It is a noticiable fact that the bacteroids are 

 said to be transmissible to progeny through the eggs like viruses. 



Anyhow, it may safely be mentioned that numberless seeds of 

 secondary organisms are still continuously being poured upon the 

 earth, but that only exceedingly small portions of them are to evolve 

 into organisms. The seeds are, of course, protoplasm particles with 

 assimilase activity having become independent of the protoplasm. They 

 are recognized as the virus when able to confer some injurious 

 effects upon certain cells, but the majority of them may silently be 

 pursuing the course of the evolution leading to the secondary organism 

 without being recognized. 



2. Important Significance of Parasitism 



The primary organisms were presumably generated and developed 

 in water. They were unable to leave the water until evolved into 

 highly developed creatures, whereas the secondary organisms were 

 produced in cells of other organisms already created, and developed 

 in the cells, so that their parasitic nature would be of most obstinacy. 

 For the second organisms the liberation from the parasitism, there- 

 fore, would be as serious and as difficult as would be the release from 

 the water for the primary organisms. 



In this respect any parasitic organisms, living outside the water 

 despite their comparatively primitive nature should be regarded as the 

 secondary. Accordingly, at least the majority of bacteria and protozoa 

 should be considered to be the secondary organisms. To be sure. 



