VIII. VIRUSES AND INSECTS 187 



Now we have to discuss the nature of the rigidity of virus struc- 

 ture, the most important fector determine the virus evolution. As 

 already pointed out, protoplasm structure must be springy. A spring 

 may readily resile to its original state after a slight bend, but do so 

 only with a considerable difficulty after a heavy distortion. The 

 viruses capable of exerting on protoplasm such a heavy distortion 

 may be said to be strong and rigid. The degree of the distortion 

 may be determined by the structural pattern of the virus probably by 

 the manner of spatial arrangement of polar groups, especially of amino 

 acid residues. 



Since the structure to cause a heavy distortion can multiply in 

 the protoplasm through the infection with the virus having the struc- 

 ture, and since a heavy distortion must be the structural change 

 which cannot easily spring back to the original structure, the virus 

 which can cause a heavy distortion must be in itself stable and rigid. 

 In short, the virus having a stable, rigid structure can cause a heavy 

 distortion in the host protoplasm. The virus capable of producing a 

 strong pattern, therefore, may accomplish vigorous multiplication, and 

 at the same time the virus itself is stable and fitted for prolonged 

 existence. 



In addition to the chemical structure of the component protein,, 

 the amount and the kind of nucleic acids contained in the virus parti- 

 cles may also play an important role in the determination of the virus 

 strength. If a virus particle contains a nucleic acid in a richer amount 

 than the others, it will act as a stronger virus owing to the hardening 

 action of the acid already discussed. If, therefore, certain cells acuqire 

 the property to synthesize nucleic acid more vigorously on the infec- 

 tion with some virus, strong virus particles will be produced by such 

 cells. Thus it may be concluded that the property of a virus to make 

 the nucleic acid increase in the host cells is essential to its evolution. 



Particle size is considered to be another important factor for the 

 virus evolution. As already pointed out assimilase action is, to some 

 extent, directly proportional to the polymerization degree of the pro- 

 teins, so that larger particles can reveal greater action. 



To sum up, in addition to the specific structural pattern of the 

 component protein capable of causing marked distortion in the host 

 protoplasm, the properties to agglutinate into large particle and to 

 combine with large amount of nucleic acid appear to be essential for 

 the evolution of viruses, that is, for the increase in their template 

 strength, although these properties may be governed also by the specific 

 structural pattern of the component protein. 



Even when the infected cells of the same kind are decomposed 

 into virus particles, the particles produced may not be identical with 



