VIRUSES, PROVIUUSLS AND THE CONFLICT OF SYSTEMS 



troin the cell and even precipitated as crystals or liquid crystals. But 

 they cannot propagate outside the cells of their host. 



Now, particles transmitted in heredity as nuclear genes or plas- 

 inagenes are necessarily more or less suited to the organism of which 

 they form a part. Any organism or race carrying such inescapable 

 particles is likely to be eliminated by natural selection if they are 

 not so suited. Viruses are under no such compulsion. They infect 

 their hosts through air or water, or through the body of a specialized 

 insect carrier or vector. Their effects on a host are, therefore, within 

 a wide range inditFerent to the viruses themselves, and we find that 

 these effects indeed cover a very wide range. At one extreme, 

 perhaps, is the bacteriophage, which destroys its bacterial victim in 

 a few hours. At the other are the viruses which cause variegation 

 in Ahutilon or "breaking" in tulips. They have existed in equilibrium 

 in the cells of the host for hundreds of years. If, as sometimes 

 happens, they can be carried by the egg, they either eliminate all 

 susceptible stocks, or they will become, and may have become, 

 universal in a species and indistinguishable from its plasmagenes. 



The general effect of viruses consists in a distortion, great or small, 

 of the metabolism of the host cell. The virus may act as a phos- 

 phatase or other enzyme, which pushes the cell processes of the host 

 in a new direction. In doing this it multiplies itself and thereby 

 greatly increases, for example, the protein content of an infected 

 plant. Like the tryptophanelcss gene in Neurospora, as Rischkov 

 found, it piles up its by-products beside it as a body of inert protein. 

 Healthy processes, as we saw, are such as lead to the least accumu- 

 lation of useless or inactive product. The virus starves or deforms 

 the normal or healthy synthetic processes and replaces them by new 

 ones. A bacterial infection may consequently benefit from the 

 co-operation of a virus (as in Hog Flu). In green plants, we fmd 

 that the plastids and pigments suffer most obviously. 



With its specialized activity the virus naturally favours special 

 tissues. Just as cholera is almost harmless when injected under the 

 skin, so curly-top of tomatoes and sugar beet requires the phloem 

 for its propagation. In animals which survive the crisis ot a virus 

 attack, diffusible anti-bodies are produced which restrain its multi- 

 plication and lead to recovery. Li plants anti-bodies, perhaps owing 

 to their lack of diffusion, are knowm only in one instance. But 



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