CHAPTER X 

 THE SECONDARY ORGANISMS 



1. Parasitism and Commensalism 



Since pathogenic filtrable agents are designated viruses, pathogeni- 

 city must be an essential feature of viruses, whereas pathogenicity is 

 by no means needed by protoplasm particles to exhibit the assimilase 

 action. Only when the particles cause an injurious effect on the pro- 

 toplasm of cells on which they exert their assimilase action they may 

 be called viruses. Their existence therefore may not be realized if no 

 injurious effect follows. Plant viruses may be called so because they 

 can cause deleterious effectes in plants, but when transmitted to 

 insect no effect is exhibited, so that despite the occurrence of their 

 multiplication, their existence is only demonstrated when they are 

 given back to the plants. In general, no difference can be found be- 

 tween the insect carrying the virus and that carrying none. In such 

 a case the virus may be called a^ latent. 



It seems highly possible as already discussed that protoplasm par- 

 ticles derived from normal, healthy cells act as a virus on some other 

 cells, but it must be of most difficulty to distinguish a genuine latent 

 virus from normal particles capable of acting as a virus. Presumably 

 the term latent virus have been used up to the present without taking 

 account of this problem. 



Assimilase action is usually involved in the protoplasm having its 

 full form, only rarely the action being preserved in the decomposed 

 particles. But, for the development of an assimilase into an indepen- 

 dent organism, it must be quite necessary for the assimilase to retain 

 its function in the form of decomposed particles. 



There is no reason to suppose, however, that pathogenicity is 

 needed for the evolution of virus. On the contrary, it seems to be rather 

 deleterious, because severe pathogenic action may have unfavourable 

 influences on the multiplication of viruses. 



If a virus possessed a virulence so violent as to infallibly kill the 

 host, it would soon be perished together with the host. It may, there- 

 fore, be supposed that less virulent or non-virulent viruses can more 

 easily pursue the course of the evolution than do more virulent. Thus 

 the number of non-virulent viruses existing may be much greater than 



