X. THE SECONDARY ORGANISMS 199 



expected. However, we are unable to demonstrate their existence 

 because a virus is detected always on the basis of its pathogenicity. 



Nevertheless, when viruses have highly evolved as to arrive at 

 the state of Rickettsiae, their existence may become demonstrable on 

 account of the peculiar shape and size of their particles even if they 

 have no pathogenicity. Thus, although only six or seven species of 

 Rickettsiae have been found in association with disease of mammals, 

 over forty non-pathogenic species have been described in various in- 

 sects and other arthropods (88). In addition, isolation of various non- 

 pathogenic pleuropneumonia-like organisms from the genitourinary 

 tract has frequently been reported (107). Such organisms are con- 

 sidered to be intermediate between Rickettsiae and bacteria. It must be 

 emphasized that the majority of the bacteria thus evolved from viruses 

 are likewise non-pathogenic. 



Some bacteria are not only non-pathogenic but sometimes appear 

 to have favourable effect on the host. This may be the case also with 

 some viruses, because in doing so the viruses may produce in the host 

 more beneficial conditions for their multiplication and accordingly for 

 the continuance of their existence, though it is also conceivable that 

 some proper injurious effect upon the host may sometimes be desirable 

 for their multiplication, for it is occasionally recognized that emaciated 

 organisms are more readily affected by a virus, a phenomenon which 

 is also well confirmed in some Rickettsiae. 



Most workers have failed to find any significant differences be- 

 tween infective and non-infective individuals of insect vectors. Com- 

 monly infective individuals live as long as and breed as non-infective. 

 However, a beneficial effect from feeding on celory and aster plants 

 infected with aster yellow virus has been described by Severin 108), 

 who found that several leafhopper species multiply more freely, pass 

 through nymphal stages sooner, and live longer than those feeding 

 on healthy plants. In most cases the difference was quite clear-cut, 

 the insects dying off in a few days when put on healthy plants, but 

 building up flourishing colonies when put on diseased ones. There 

 are numerous leafhopper species which live on diseased celory and 

 asters, but when transferred to healthy plant they die. Likewise 

 with aphid and leaf-roll mosaic, Arenz (109) has found that the vector 

 multiplies substantially faster on diseased potatoes than on healthy. 

 These may show the favourable effect of the virus upon the host, but 

 at the same time they may be regarded as being only the examples 

 where injuries given rise to by virus upon the plant benefit the 

 proliferation of the insect (110). However, in discussing numerous 

 cases of an association between insects and microorganisms for mutual 

 benefit. Leach (111) has claimed that there is no reason why association 



