THE ORIGIN or VIRUSES 



gated afterwards by infection or by inoculation. Such diseases as 

 virus III in rabbits and perhaps Herpes zoster in man, referred to by 

 Hobiies and Pirie, may appear spontaneously. Sometimes a nutri- 

 tional defect or a bacterial infection seems to favour their appearance. 

 In these cases the virus probably results from a distortion of protein 

 metabolism in the individual in which it first appears. The new 

 proteins may arise from, or instead of, normal self-propagating cell 

 proteins which need not be transmissible as such in heredity and 

 need not, therefore, be called plasmagenes. But their replacement 

 is most easily described as mutation since the result produced is 

 indistinguishable from plasmagene mutation in a plant. 



One step further on from these intrinsic viruses are those which 

 are normal proteins in the individual of their origin, but become 

 unfriendly in an alien cell. Such is, no doubt, the virus producing 

 ascending myelitis in man after a bite by a healthy monkey, a monkey 

 which can scarcely have been bitten by a man suffering from this 

 disease. Here we have the origin of a virus by transplantation as 

 opposed to mutation. Its fullest verification is in plants. 



Plant viruses have, like tumour viruses, several orders of trans- 

 mission. A few may be conveyed by contact of stems or roots : the 

 important natural ones are carried by parasitic insects. There are 

 others, however, like those causing the yellowing o{Ahutilon, privet, 

 and laburnum, which can be passed on only by grafting. As diseases, 

 therefore, they are again artificial, and their origin must be due, 

 either to a change in a cell protein, or more probably to the grafting 

 itself, that is either to mutation or to invasion. They are not naturally 

 infecting viruses, but proviruses. Their origin has been shown, in 

 various groups, by grafting different varieties and species. Indeed, 

 though the possibility of the infection of scion by stock as opposed 

 to the incompatibility of the two has long been overlooked, it was 

 first pointed out by Patrick Blair in 1720. 



The types of abnormality resulting from the graft-creation of 

 viruses are manifold. 



Some instances of graft incompatibility are, it seems, due to the 

 scion which produces substances poisonous to the stock, the effect 

 being restricted even to particular localities as in Quick Decline or 

 Tristeza of sweet oranges on sour stock, or Graft BHght ot Lilac on 

 privet stock. 



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