180 III. THE EVOLUTION OF VIRUSES 



periods. This must be a most natural result since the normal struc- 

 tural pattern should be recovered sooner or later. The action of virus 

 as already discussed is generally directly proportional to its quantity ; 

 the greater the quantity, the stronger the action. When the action 

 is stronger, replicas produced will be more distinct and firmer. Thus, 

 the above mentioned fact that the length of time for which insects 

 remain infective depends on the length of time they have fed on the 

 source of the viruses should naturally be expected, as the quantity 

 of the virus taken by the insects must be greater when they have 

 fed longer. 



The writer is of the opinion that antibodies are produced in entirely 

 the same way as the adaptive enzymes. Namely, protoplasm proteins 

 altered in their structure by antigens are the antibodies (12). Also in 

 the case of this antibody formation, it is known that both the amount 

 of antibody produced and the length of time for which the antibody 

 production persists are inclined to be directly proportional to the 

 antigen amount administered. The protoplasm structure changed by 

 antigens is likewise reversible, so that sooner or later the original 

 structure- is recovered with the cessation of the antibody production. 



Kunkel (95) has found .that a kind of the leafhopper carrying 

 aster yellows virus is unable to transmit if exposed to high tempera- 

 tures. Leafhopper exposed to 32°C. for a day lost the ability to infect 

 healthy plants, but on lowering the temperature to 24 -C. they quickly 

 regained the ability without again feeding on a source of the virus. 

 This may indicate that the structure of the virus is stable at lower 

 temperatures, while at higher temperatures the normal protoplasm 

 structure is more stable than the virus structure, so that at 32=C. 

 the virus structure may disappear but at 24''C. it may reappear. 

 Again, according to the above author the leafhopper carrying the 

 virus, if heated for one day, regained the ability to transmit in a 

 few hours at the lower temperature and those heated for a week 

 required two days or longer, whereas those heated for more than 

 12 days never regained the ability to transmit. This is analogous 

 to the above mentioned phenomenon that the length of timie for 

 which insects remain infective is determined by the length of time 

 for which they have been fed on the diseased leaves. The stronger 

 and the firmer is the change, the more prolonged period of time will 

 be required for the recovery of the original structure, no matter 

 whether the latter structure be normal or pathological. Thus the 

 reactivation of a denatured virus would occur the more readily, the 

 slighter and the shorter was the inactivation process. It may be 

 possible to consider that, in the case of reversible inactivation of a 

 virus, the inactivated structure is stable in the presence of the inacti- 



