TRANSMISSION AND SPREAD OF VIRUSES 17 
The best known example of a virus which, so far as we know, does 
not spread at all, is that of the paracrinkle disease of the potato plant 
(Salaman and Le Pelley, 1930). This virus, which is present in all 
plants of the King Edward variety, has never been found in nature in 
any other potato variety or plant and can only be transmitted by 
means of grafting. It is indeed quoted as evidence for the heterogenesis 
—or spontangous generation—of viruses by those who support this 
hypothesis. Similarly nothing seems to be known of the method of 
spread of the Rous sarcoma virus or if it spreads at all. 
‘‘Lighting-up’’ of Latent or “‘Silent’’ Viruses 
We have seen already a good example of the “lighting-up” of a 
latent virus when discussing swine influenza. It will be recalled that 
the virus is latent in its intermediate host, the swine lungworm, and 
to induce infection in the pig it must be rendered active by the appli- 
cation of a provocative stimulus. Multiple intramuscular injection of 
the bacterium H. influenzae suis furnishes a means of provoking 
infection. 
Serial passage of one virus through successive mice sometimes 
brings to light another virus, different from the one under study, 
which has apparently been stimulated into action by the progressive 
passages. 
It has been suggested that some such process takes place in the 
production of tumours with provocative carcinogens such as the 
tarring of the skin in rabbits and chickens. For example, Rous (1943) 
suggests that the carcinogens have, in common, an ability to play upon 
the tumour-forming potentialities of the cells, with the result that 
these become realities. The potentialities themselves may be viruses 
incapable of causing growths unless the local conditions happen to 
be right or are made right, and even then perhaps only as the result 
of virus variation. 
There is a good deal of evidence that the polyhedral viruses affecting 
caterpillars can remain in a latent condition for several generations. 
Then, when circumstances are favourable, the virus starts to multiply 
and the disease breaks out in epidemic form. 
With some latent plant viruses, an apparent exacerbation of the 
disease caused can be induced by progressive passage of the virus 
through susceptible plants. In these cases, however, the increase in 
virulence is more apparent than real and is due more to a selective 
action than to a stimulation of the virus. A case in point is the 
