178 III. THE EVOLUTION OF VIRUSES 



Furthermore, working with the virus of aster yellows and its 

 specific vector, Black (92) has confirmed that the virus could increase 

 in the insect at least one hundredfold between the second and twelfth 

 days of the inoculation. Maramorosch (93) has likewise ascertained 

 the multiplication of corn stunt virus in its insect vector, a kind of 

 leafhoppers. He has succeeded in transmitting the virus mechanically 

 from an insect to another ; the virus isolated from infected plant, 

 when injected into insects, renders the latter infective and the juices 

 of the insects can successively infect other healthy insects through 

 mechanical injection. The minimum incubation period of the virus to 

 become infective in the insects was found to be 6 weeks. A plant 

 virus not mechanically transmissible from plant to plant can be thus 

 mechanically transmitted between insect vectors. 



In spite of such numerous positive evidences, virus multiplication 

 in insect vectors are not fully admitted, because there are some facts 

 known which appear to be inconsistent with the concept. For instance, 

 if viruses multiply in insect vectors it would be reasonable to expect 

 that all the vectors, when once become infective, will remain so for 

 long periods and probably for the whole of their lives, but this does 

 not happen, for it has been found that different individuals remain 

 infective for varying periods, some soon losing infectivity and others 

 not. Moreover, it has been confirmed 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. Vectors may become infective 

 after a few minutes feeding but do not remain infective for long, 

 whilst if they have fed for hours or days they remain infective for 

 long periods, often for the remainder of their lives. Sometimes it was 

 even confirmed that both efficiency of the vectors and length of time 

 for which they remain infective increases with increased feeding time 

 on the infected plant (39). These evidences may appear at first sight 

 to indicate that a virus remains in the insect as such without multi- 

 plying, being contradictory to the above view, but in the theory of 

 the writer these are by no means inconsistent with the view as will 

 be argued in the following section. 



1. The Reversibility of Protein Structure 



The writer holds the opinion that the structure of protoplasm 

 protein is springy or elastic, so that if the factor is removed which 

 has changed to some extent the structure, the original structure may 

 be recovered sooner or later. In short, protoplasm proteins can remem- 

 ber the structure they once possessed and will resume it if possible. 



